LASIK Fundamentals, Surgical Techniques, and Complications - part 3 pps

Patients who have unstable refractive errors or sig-nificant pre-existing ocular pathology of the cornea or anterior segment including but notlimited to scarring, severe dry eye syndrome

36 PA Bloom, D Papakostopoulos, Y Gogolitsyn, JA Leenderz, S Papakostopoulos, RH Grey Clinical and infrared pupillometry in central retinal vein occlusion Br J Ophthalmol 1993; 77:75–80.

37 BS Wachler, RR Krueger Agreement and repeatability of infrared pupillometry and the parison method Ophthalmology 1999;106:319–323.

com-38 M Colvard Preoperative measurement of scotopic pupil dilation using an office pupillometer.

J Cataract Refract Surg 1998;24:1594–1597.

39 EM Schnitzler, M Baumeister, T Kohnen Scotopic measurement of normal pupils: Colvard versus Video Vision Analyzer infrared pupillometer J Cataract Refract Surg 2000;26:859–866.

40 FW Price, DL Koller, MO Price Central corneal pachymetry in patients undergoing laser in situ keratomileusis Ophthalmology 1999;106:2216–2220.

41 AC Snyder Optical pachometry measurements: reliability and variability Am J Optom iol Opt 1984;61:408–413.

Phys-42 RB Mandell, KA Polse Keratoconus: spatial variation of corneal thickness as a diagnostic test Arch Ophthalmol 1969;82:182–188.

43 PS Binder, JA Kohler, DA Rorabaugh Evaluation of an electronic pachometer Invest thalmol Vis Sci 1977;16:855–858.

Oph-44 C Giasson, D Forthomme Comparison of central corneal thickness measurements between tical and ultrasound pachymeters Optom Vis Sci 1992;69:236–241.

op-45 JJ Salz, SP Azen, J Berstein, P Caroline, RA Villasenor, DJ Schanzlin Evaluation and parison of sources of variability in the measurement of corneal thickness with ultrasonic and op- tical pachymeters Ophthalmic Surg 1983;14:750–754.

com-46 S Mishima, BO Hebdys Measurement of corneal thickness with the Haag Streit pachymeter Arch Ophthalmol 1981;80:710–713.

47 DZ Coleman, FL Lizzi, RL Jack Ultrasonography of the eye and orbit Philadelphia: Lea and Febiger, 1977, pp 113–114.

48 V Yaylali, SC Kaufman, HW Thompson Corneal thickness measurements with the Orbscan Topography System and ultrasonic pachymetry J Cataract Refract Surg 1997;23:1345–1350.

49 LE Probst LASIK instrumentation In: J Machat, S Glade, L Probst, eds The Art of LASIK Thorofare, NJ: Slack, 1999, pp 73–78.

50 L Buratto, S Brint, M Ferrari Surgical instruments In: L Buratto, SF Brint, eds LASIK ciples and techniques Thorofare, NJ: Slack, 1998, pp 35–68.

LASIK Indications, Contraindications,

and Preoperative Evaluation

RICHARD E BRAUNSTEIN and MARC WINNICK

Columbia University College of Physicians and Surgeons, Harkness Eye Institute, New York, New York, U.S.A.

KENNETH A GREENBERG

Columbia University College of Physicians and Surgeons, New York,

New York, U.S.A.

Corneal refractive procedures are currently widely applied to correct ametropia A cessful refractive procedure is gauged by many criteria: safety, efficacy, predictability, andlong-term stability Laser-assisted in-situ keratomileusis (LASIK) is presently the mostwidely performed refractive procedure, but it is not appropriate for all patients Optimal re-sults are achieved through proper patient selection, education, examination, and consent Acomplete understanding of the risks of the procedure and the effects of pre-existing ocularconditions are critical in selecting patients for surgery The ability to recognize subtle ocu-lar conditions through careful patient examination will reduce the likelihood of complica-tions Finally, helping patients have realistic expectations and the ability to say no to pa-tients who clearly are poor surgical candidates are the keys to building a successful LASIKpractice This chapter will outline the present refractive indications for LASIK with specialattention given to the preoperative evaluation and patient preparation prior to refractivesurgery Clinical tips for maximizing refractive outcomes and contraindications to LASIKwill be discussed

The excimer laser is currently approved for a wide range of myopia, myopic matism, and hyperopia Many studies indicate that the predictability of LASIK decreaseswith increasing preoperative myopia and astigmatism (1) Hyperopia ranging up to 6diopters is correctible, but at higher levels of attempted correction, predictability is againreduced (2) For hyperopia greater than 6 diopters, loss of spectacle-corrected visual acu-ity occurred in a significant number of eyes in some studies, and accuracy was sufficientlypoor to advise against LASIK in these eyes (3) Approval for treatment of hyperopic astig-matism and mixed astigmatism is expected in the near future.

Contraindications for LASIK include patients with systemic collagen vascular disease, munodeficiency, autoimmune disease, severe atopy, and diabetes mellitus, diseases alllikely to affect corneal wound healing Patients who have unstable refractive errors or sig-nificant pre-existing ocular pathology of the cornea or anterior segment including but notlimited to scarring, severe dry eye syndrome, uncontrolled blepharitis, uveitis, or earlycataract should also not have laser vision correction Surgery should not be performed onwomen who are pregnant or nursing or on patients taking Amiodarone or Acutane LASIK

im-is contraindicated in eyes with a him-istory of herpes simplex keratitim-is or herpes zoster thalmicus Special concern should be given to eyes with corneal neovascularization within

oph-1 mm of the ablation zone and those patients with difficult anatomy including small orbitalaperture, narrow interpalpebral fissure or deep-set eyes Patients with keratoconus or kera-toconus suspects should not have LASIK surgery Finally, patients with signs of anteriorbasement membrane dystrophy may be better served with photorefractive keratectomy(surface ablation) rather than LASIK, as the likelihood of an intraoperative epithelial de-fect may create numerous postoperative management difficulties

The cornea should not be flattened to less than 33 D or steepened to greater than 52

D, as refractive outcomes in this range are less predictable (4) To determine the ative corneal curvature, the preoperative keratometry is reduced or increased by the amount

postoper-of desired correction at the corneal plane All candidates for LASIK procedures shouldhave a stable refraction for at least 12 months prior to the procedure differing by no morethan 0.50 diopter in manifest sphere or manifest cylinder For hyperopic patients, manifestand cycloplegic refraction should not differ more than 0.75 diopter

D IATROGENIC KERATECTASIA

LASIK alters both the shape and the structural integrity of the cornea Preoperative ation involves determining what the resultant corneal curvature and residual stromal thick-ness will be, prior to proceeding with surgery Preoperative corneal thickness is of particu-lar concern in preventing iatrogenic keratectasia Post LASIK keratectasia results in aprogressive central corneal steepening and myopic shift causing irreversible damage by oneyear It occurs in approximately 1 in 1000 eyes (5) Preoperative assessment of cornealthickness, flap and resultant stromal bed thickness, and amount of desired ablation are es-sential to prevent such ectasias The average thickness of the human cornea is 520 microns,and an average flap created by a microkeratome is from 130 to 160 microns The keratec-tomy depth of the excimer laser ablation on average must not exceed 45 to 120 micronswith 325 (conservative case) to 250 microns, respectively, left in the stromal bed (6) Thissparks questions regarding the minimal thickness of residual stroma, which needs to bemaintained to prevent keratectasia Many LASIK surgeons currently employ a 250 micronlimit to the residual stromal bed following LASIK, but this is only an average value, andthe biomechanical constants of the human cornea vary over a wide range, so that the range

evalu-of residual corneal thickness that would prevent keratectasia is unknown Some advocateusing a percentage of the corneal thickness as a minimal residual stromal thickness ratherthan an absolute number, given ultrasound pachymetry measurement errors and biome-chanical considerations, such as the deeper stroma having less tensile strength compared tothe anterior layers (6) This dilemma can ultimately be resolved when we are betterequipped to measure the biomechanical constants of the cornea in vivo Promising ap-proaches such as mechanical spectroscopy and measurement of birefringence of the corneacan assist in future determination of preoperative stromal bed thickness, which would benecessary to avoid iatrogenic keratectasia The residual stromal bed following a LASIKprocedure should be calculated prior to surgery based on a nonnomogram adjusted treat-ment of the refractive error to be corrected

Preoperatively, each patient must have a complete evaluation including a medical, surgical,and ocular history as well as an ocular examination A general medical history with em-phasis on the above-mentioned systemic diseases should be discussed and a medication listobtained Past ocular surgery and any previous or existing ophthalmic conditions, such asglaucoma, dry eye, amblyopia, and past contact lens use, should be reviewed

An initial preoperative evaluation for LASIK in contact lens wearers should be formed at least two weeks after discontinuation of soft contact lenses or at least three weeksafter discontinuation of soft toric, hard, or rigid gas permeable lenses Patients wearingrigid lenses must demonstrate keratometric and refractive stability prior to treatment Itmay be necessary to discontinue contact lenses in these patients at least one month for ev-ery decade of contact lens use Previous glasses and contact lens prescriptions should becompared to the manifest refraction

Uncorrected and best corrected visual acuity should be assessed A careful refraction is ical to maximizing refractive surgery outcomes Different refraction techniques are appli-

crit-cable to different refractive errors When refracting the myope, a resolution-based tion should be performed to avoid overcorrections A resolution-based end point involvesusing the least minus lens to visualize the most letters The refraction is not terminated at20/20 if an additional 0.25 diopters yields several letters on the 20/15 line The JacksonCross cylinder is used to determine the maximal amount of cylinder The correct axis of thecylinder is easier to determine in eyes with higher degrees of astigmatism Occasionally,autorefraction may be helpful in finding the astigmatic axis Care should be taken to ensurethat the trial lens frame or phoropter is appropriately positioned and level to define the axismost accurately.

refrac-Hyperopic manifest refraction should emphasize a “push plus” technique Patientsare encouraged to accept the most plus sphere to see the most letters Cycloplegic refrac-tion with 1% cyclopentolate should be performed on all patients because it eliminates ac-commodation This is essential to identify the overminused myope and to uncover latenthyperopia For patients with hyperopia, manifest and cycloplegic examination should dif-fer by 0.75 diopter or less If a large amount of latent hyperopia is identified or if there is asignificant discrepancy between the manifest and cycloplegic refraction in a myope, a postcycloplegic manifest refraction should be performed with an emphasis on pushing addi-tional plus power Occasionally, glasses may be prescribed temporarily to help the patientaccept the additional plus power prior to performing surgical correction

A clinical workup should include manual keratometry, a pupillary examination, and

a slit lamp examination with emphasis on any lid margin inflammation, corneal epithelialdisease, basement membrane dystrophy, or stromal scars consistent with prior keratitis.Tonometry should be performed on all patients and gonioscopic examination on all hyper-opes Central corneal pachymetry readings should be performed on all patients A careful,dilated fundus examination is performed to analyze the optic nerve and retina for anypathology with careful attention to the peripheral retina in highly myopic eyes that may be

at risk for lattice degeneration Identified retinal tears or large areas of lattice may requirelaser photocoagulation prior to performing LASIK (7)

Special attention should be given to the pupillary examination in myopic and ularly astigmatic patients Pupil size should be measured in dim and in bright lighting con-ditions and recorded Measurement can be made with a pupil gauge or with an infraredpupillometer Patients with higher degrees of refractive error and larger pupils may be atgreater risk for postoperative night vision disturbances, although this remains a subject ofgreat controversy (3)

Corneal topography is essential in all patients prior to refractive surgery Topography is used

to identify patients with corneal curvature abnormalities that are not apparent on slit lampexamination True keratoconus is often easy to detect by clinical history and examination,but subclinical cases may only be apparent by corneal topography Corneal topography isnecessary to determine whether patients have contact lens related warpage and to help de-termine when a cornea is stable following contact lens discontinuation We recommend thatany patient who wears contact lenses have corneal topography repeated 1 week apart to as-certain stability prior to surgery Topography is also used to verify postoperative results andcomplications such as decentrations, central islands, and irregular astigmatism

H DISCUSSION OF SPECIFIC CONDITIONS

1 The Keratoconus Suspect

Keratoconus is a noninflammatory bilateral corneal ectasia that produces irregular matism and leads to marked refractive error Clinical signs differ depending on the sever-ity of the disease External signs include Munson’s sign and the Rizzuti phenomenon Slitlamp findings include a Fleischer’s ring, Vogt’s striae, stromal thinning and scarring,prominent corneal nerves, and epithelial nebulae Retroillumination signs include scissor-ing on retinoscopy, which is often the first evidence of early keratoconus, and an oil dropletsign Early in the disease, many corneas appear normal on slip lamp biomicroscopy Sev-eral devices are currently available for detecting early keratoconus Simple, inexpensivedevices such as a handheld keratoscope can show egg-shaped or inferocentral compression

astig-of mires, which may be indicative astig-of early keratoconus Ultrasonic pachymetry to strate central and peripheral corneal thickness has been studied, and although it is highlyaccurate and reproducible for measuring corneal thickness, its use has failed to identify alarge percentage of patients with clinically obvious keratoconus, and it should only be used

demon-as corroborative evidence for the diagnosis of keratoconus (8)

Computer-assisted videokeratoscopes, which generate color-coded maps and graphic indices, are an excellent tool for the diagnosis of keratoconus even when signs ofthe disease are not obviously apparent at the slit lamp Three features of keratoconus arecommon to video keratography; a localized area of increased surface power, inferior–su-perior power asymmetry, and skewed steep radial axes above and below the horizontalmeridian depicting irregular astigmatism Much work has been done to quantify the mini-mal topographic criteria for diagnosing keratoconus One method is pattern recognition andone specific topographic pattern, asymmetic bow tie with skewing of the radial axis aboveand below the horizontal meridian, was found by Rabinowitz in virtually 100% of patientswith clinical keratoconus This pattern could represent the earliest sign of irregular astig-matism and might be a reasonable cutoff point in the transition from normal topography tokeratoconus (9) The second method, the use of quantitative videokeratography derived in-dices, may represent a more reproducible way to quantify keratoconus and its early pheno-types Maeda and coauthors devised an expert system classifier, utilizing an analysis ofeight topographic indices derived from the TMS videokeratoscope A linear determinantfunction is used to determine a composite discriminant value for each map: the KPI A KPIvalue greater than the optimum cutoff is classified as keratoconus (10) Rabinowitz de-scribed a KISA% index, a quantitative videokeratographic algorithm embodying minimaltopographic criteria for diagnosing keratoconus Using a combination of four indices: cen-tral K, an expression of central corneal steepening; the I-S value, an expression of infe-rior–superior dioptric asymmetry; the AST index, which quantifies the degree of regularcorneal astigmatism; and the SRAX index, the skewed radial axial index, the KISA index

topo-is derived Ktopo-isa%  K I-S AST SRAX 100 (11) This has been shown to haveexcellent preliminary clinical correlation and can be used by surgeons to determine a pa-tient’s chance of having early keratoconus A single index with excellent clinical correla-tion would be optimal to depict early keratoconus and warn a refractive surgeon of poten-tial intra- and postoperative risks and complications The formulation of these indices arestill being altered to improve accuracy and at present should be used with appropriate clin-ical correlation including slit lamp biomicroscopy, pachymetry, retinoscopy, and keratom-etry to determine the relative risk of laser refractive surgery Patients with keratoconus or

those who are likely keratoconus suspects are not candidates for LASIK using present gorithms and treatment profiles and should be considered to have a progressive corneal dis-ease Patients with asymmetric bow tie astigmatism and inferior steepening who do not ap-pear to have keratoconus may be treated surgically with appropriate informed consent.

al-2 The Incipient Cataract

The incipient cataract patient requires special consideration Cataract extraction should beviewed as both a media clearing surgery and a refractive surgery Progression of central vi-sual axis opacity will decrease the best corrected visual acuity post laser and result in a pa-tient unhappy with LASIK and in need of a second surgical procedure It is therefore rec-ommended that the best corrected visual acuity be assessed preoperatively and the effect ofthe incipient cataract on visual acuity be determined If it is believed that the cataract is sig-nificant or is progressing rapidly, laser vision correction should not be performed, and con-sideration may be given to cataract surgery, if appropriate Although phacoemulsificationcan be performed on an eye that has had LASIK, the intraocular lens power calculation can

be problematic, and a more reliable refractive outcome may be achieved with lens tion and intraocular lens implantation (12)

extrac-3 The Dry Eye Patient

Patients with a history of dry eye symptoms and contact lens intolerance frequently seeklaser vision correction Dry eye symptoms and corneal epithelial staining are commonproblems following LASIK The mechanism of this disorder following surgery is not wellunderstood, although alteration of corneal innervation following LASIK is the most likelycause Evaluation of these patients includes assessment of tear film quality and breakup aswell as corneal epithelial integrity Patients with evidence of corneal staining should betreated preoperatively Management may include topical lubricants, punctal occlusion, andoccasionally oral doxycycline if the patient has evidence of meibomian gland dysfunction.Schirmers’ testing has not been shown to be predictive of postoperative dry eye problemsfollowing LASIK

4 The Glaucoma Suspect

If preoperative screening identifies a patient with elevated intraocular pressure or opticnerve head cupping, focal ischemia, or hemorrhage, a 24-2 Humphrey visual field is rec-ommended, and further testing or consultation is necessary to determine if indeed the pa-tient has glaucoma If the patient has glaucoma, LASIK should not be performed Patientswho are glaucoma suspects by family history, borderline elevated intraocular pressure, ormild disc asymmetry must be treated cautiously If the patient does not have glaucomatousoptic neuropathy, and a diagnosis of glaucoma is considered unlikely, they may be consid-ered for laser vision correction However, informed consent discussing the potential re-duced value to tonometry measurement must be discussed with the patient

In general, the validity of applanation tonometry following laser vision correction isless accurate and ultimately may limit the ability to treat glaucoma in selected patients.Falsely low intraocular pressure measurements are obtained by applanation due to easycompressibility of the fluid-filled space between the corneal flap and the stromal bed andthe direct relationship between corneal thickness and Goldman applanation tonometry (13)

In addition, the intraoperative increase in intraocular pressure following application of the

suction ring during LASIK will further decrease optic nerve head perfusion pressure andcan cause additional damage to an already susceptible glaucomatous optic nerve.

For peripresbyopic and presbyopic lower myopes (1.00 through 3.50 diopters), we quently treat the dominant eye first and postoperatively determine the correction for the non-dominant eye, if any Some patients happily tolerate large amounts of anisometropia, whileothers are intolerant of as little as 0.50 diopters The refractive goal needs to be tailored tothe requirements of the individual patient Patients who spend most of their time doing nearwork without glasses preoperatively need to understand that correcting their distance visionwill necessitate the use of reading glasses Often, patients have a preconceived notion thatsurgery will allow them to throw away their glasses Occasionally, patients misunderstand

fre-or are misinffre-ormed and are disappointed in their postoperative spectacle dependence ffre-ornear work when they did not require glasses preoperatively

7 The Very High Myope

Patients with high degrees of myopia are usually very motivated as they have greater visualdisability A preoperative evaluation in high myopes must include a careful dilated retinalexamination using indirect ophthalmoscopy to search for peripheral retinal pathology

as well as a careful macular exam Preoperative retinal consultation is of value to determine

if existing retinal pathology requires any treatment prior to surgery Additionally, withhigher degrees of myopia, there is an increase in the loss of BSCVA as well as reduced ac-curacy Patients need appropriate preoperative counseling to help understand these risks

8 The Hyperope

LASIK, at present, shows great promise for effective and stable correction for mild to erate hyperopia A gonioscopic examination should be performed prior to LASIK in all hy-peropic patients and a prophylactic peripheral iridotomy may be indicated for an occlud-able angle Although there is no cause and effect relation between hyperopic LASIK andacute angle closure glaucoma, patients with hyperopia and narrow angles should be fol-lowed for the possibility of angle closure at a later time (15) The ablation profile for hy-peropia extends out to 9 mm requiring a large corneal flap Patients with a history of con-tact lens wear and corneal neovascularization may have significant bleeding with LASIKsurgery, and potential complications of bleeding should be discussed and included in theconsent prior to surgery

Selecting the appropriate candidate for refractive surgery requires an understanding of thepatient’s expectations, desires, and disposition in addition to a clinical ocular examination.

A candidate must have the ability to understand the risks and benefits of LASIK and be able

to give informed consent The patient must be able to tolerate the procedure and have theability to lie flat without difficulty, to tolerate topical anesthesia, and to fixate steadily andaccurately for the duration of the procedure Patients must be targeted toward their differingvisual needs A myope prior to laser vision correction enjoyed close and clear visual spaceand will be disappointed if overcorrected postoperatively Some presbyopes desire mono-vision, while others would rather wear spectacle correction Patients who demand spectacleindependence all the time and have unrealistic expectations of laser vision correction should

be avoided in addition to all patients with the above-mentioned systemic diseases or ocularpathology that otherwise jeopardize the efficacy, safety, and stability of LASIK

A patient’s understanding of the possible postoperative complications and adverse tomatology is an essential component of an informed consent Patients should be given acopy of the FDA brochure provided from the excimer laser manufacturer citing the results

symp-of clinical trials using the specific laser Additional statistics may be provided regarding thedoctor’s own data and experience

A detailed informed consent document is often helpful in highlighting many of theknown side effects and complications of LASIK surgery Undercorrection, overcorrection,and induced astigmatism and the possibility of additional surgery should be discussed.Complications that could lead to a loss of best corrected visual acuity include but are notlimited to irregular flaps, irregular astigmatism, haze, scarring, infection, central islands,striae, and epithelial ingrowth; these must be explained to the patient in a manner that theycan understand Many patients do not appreciate that they do risk a loss in best correctedvisual acuity and possibly blindness due to an infection or retinal vascular event Patientswho require excellent night vision (e.g., truck drivers) should be cautioned prior to LASIKsurgery regarding their risk of night vision impairment Night vision impairment has beenreported by Guell and Muller in 23% of patients at 6 months postoperatively, and other re-ports of night halos occurred in up to 30% of eyes at 6 months (16) Halos around lights andnight vision impairment appear to decrease with larger ablation zones and smaller pupilsize and often improve with time (17) Finally, patients should be advised of the possibil-ity of dry eye symptoms, which may affect vision following laser vision correction Prior

to surgery, the patient should be given an opportunity to read all of the materials providedand to have all of their questions answered by their surgeon

1 T Salah, GO Waring III, A El-Maghraby, K Moadel, SB Grimm Excimer laser in situ atomileusis under a corneal flap for myopia of 2 to 20 diopters Am J Ophthalmol 1996;121: 143–155.

ker-2 S Esquenazi, A Mendoza Two year follow-up of laser in situ keratomileusis for hyperopia J Refract Surg 1999;15:648–652.

3 MC Arbelaez, MC Knorz Laser in situ keratomileusis for hyperopia and hyperopic tism J Refract Surg 1999;15:406–414.

astigma-4 DJ Salchow, ME Zirm, C Stieldorf, A Parisi Comparison of objective and subjective refraction before and after laser in situ keratomileusis J Cataract Refract Surg 1999;25:827–835.

5 T Seiler Iatrogenic keratectasia: academic anxiety or serious risk? J Cataract Refract Surg 1999;25:1307–1308.

6 T Seiler, K Koufala, G Richter Iatrogenic keratectasia after laser in situ keratomileusis J fract Surg 1998;14:312–317.

Re-7 TP Werblin Barraquer Lecture 1998 Why should refractive surgeons be looking beyond the cornea? J Refract Surg 1999;15:357–376.

8 YS Rabinowitz, K Rasheed, H Yang, J Elashoff Accuracy of ultrasonic pachymetry and videokeratography in detecting keratoconus J Cataract Refract Surg 1998;24:196–201.

9 YS Rabinowitz Keratoconus Surv Ophthalmol 1998;42:297–319.

10 N Maeda, SD Klyce, MK Smolek Comparison of methods for detecting keratoconus using videokeratography Arch Ophthalmol 1995;113(7):870–874.

11 YS Rabinowitz, K Rasheed KISA% index: a quantitative videokeratography algorithm bodying minimal topographic criteria for diagnosing keratoconus J Cataract Refract Surg 1999;25:1327–1335.

em-12 B Seitz, A Langenbucher Intraocular lens calculations status after corneal refractive surgery Curr Opin Ophthalmol 2000;11:35–46.

13 J Najman-Vainer, RJ Smith, RK Maloney Interface fluid after LASIK: misleading tonometry can lead to end-stage glaucoma J Cataract Refract Surg 2000;26:471–472.

14 MM Hom Monovision and LASIK J Am Optom Assoc 1999;70:117–122.

15 M Paciuc, CF Velasco, R Naranjo Acute angle closure glaucoma after hyperopic laser in situ keratomileusis J Cataract Refract Surg 2000;26:620–623.

16 JL Guell, A Muller Laser in situ keratomileusis (LASIK) for myopia from 7 to 18 diopters.

J Refract Surg 1996;12:222–228.

17 SG Farah, DT Azar, C Gurdal, J Wong Laser in situ keratomileusis: literature review of a veloping technique J Cataract Refract Surg 1998;24:989–1006.

Preoperative Optical Considerations in

LASIK Refractive Errors, Monovision, and Contrast

Sensitivity

BALAMURALI K AMBATI

Massachusetts Eye and Ear Infirmary and Harvard Medical School,

Boston, Massachusetts, U.S.A.

LEON STRAUSS

The Wilmer Eye Institute, Johns Hopkins University School of Medicine,

Baltimore, Maryland, U.S.A.

DIMITRI T AZAR

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

and Harvard Medical School, Boston, Massachusetts, U.S.A.

Preoperative optical considerations in LASIK have gained increased importance, for theyallow interpretation and anticipation of postoperative aberrations and provide a basis forpatient education prior to surgery This chapter will focus on refractive error evaluation,monovision, and contrast sensitivity Subsequent chapters will cover the topics of cornealtopography, wavefront analysis, and corneal biomechanics

metry usually of the cornea and sometimes of the lens Regular astigmatism is correctablewith a spherocylindrical lens, whereas irregular astigmatism is not correctable with such alens Regular astigmatism is called with the rule when the steepest (most refracting) merid-ian lies near 90 degrees It is termed against the rule when the steepest meridian lies near

180 degrees When regular astigmatism is neither with nor against the rule, it is termedoblique

1 Spectacle-Correction of Ametropias

Spectacle lenses placed in front of the eye must have their focal point coinciding with thefar point of the eye, so that parallel rays are focused onto the retina Drawbacks of this sys-tem include image distortion, prism effects, anisometropic effects, and field tilt Minuslenses minify images by approximately 2% per diopter, while plus lenses magnify the im-age but create a peripheral scotoma between what is viewed inside and outside the specta-cle frame Astigmatic correction produces meridional minification or magnification, dis-torting the image Off-axis viewing and lens tilt alter the effective spherocylindrical power

of the lens, while viewing away from the center of the lens produces prism, leading to thewell-known pincushion and barrel distortions of hyperopic and myopic lenses, respec-tively Disparity between astigmatic correction of the two eyes produces a perceived tilt ofthe object This can be minimized by decreasing the cylinder power and/or rotating the axis

of the cylinder, albeit at the expense of clarity All of these effects are heightened in higherpower lenses

What should the refractive surgeon aim for in patients whose spectacle usage has entailedsuch compromises? The surgeon should still aim for full correction of astigmatism, as cor-rection at the cornea minimizes distortion effects Further, surgery may relieve distortiondue to anisometropia Indeed, patients who are undercorrected with surgery often reportdouble images not eliminated by monocular patching and other distortion effects not due tomeridional magnification Some patients who have adapted to spectacle distortion and tiltmay take some time to readapt their binocular spatial sense and appreciate the lack of op-tical distortion Refractive surgery in the myope, by removing the minification of the spec-tacle lens, produces a larger retinal image of objects, improving visual acuity even whenclarity, i.e., optic resolving power, is the same or decreased

For the same reason that a presbyopic myope has a more remote near point whenwearing contact lenses than when wearing glasses, refractive surgery giving full correction

of myopia may make reading without reading glasses more difficult than it was with glassesbefore surgery

1 Contact Lens Wear

Preoperative evaluation for refractive surgery is influenced by several considerations tact lens wearers should discontinue soft contacts for at least 3 days and rigid lenses for atleast 3 weeks so that the cornea can assume its natural shape prior to evaluation Vertex dis-tance considerations become significant for refractive error over approximately 5 D; ver-tex distance should be measured from the rear surface of a corrective lens to calculate therefractive power at the cornea

Con-2 High Myopia

In patients with unilateral high myopia, placement of a corrective contact lens allows a operative prediction of the degree of aniseikonia they may experience after refractivesurgery Patients with bilateral high myopia should be appraised of the possibility of dis-turbing aniseikonia after one eye has undergone surgery before the other eye: the possibil-ity of unequal photoreceptor spacing is considered in cases of high refractive errors, inwhich case aniseikonia testing should be performed

pre-3 Cycloplegic Refraction

Cycloplegic refraction allows the examiner to discern the degree of myopia in the manifestrefraction due to accommodative tone; however, pupil dilation leads to a mild myopic shiftdue to spherical aberration of the lens Cycloplegic refractions of hyperopes enables the de-termination of latent, manifest, and therefore total hyperopia; the surgeon must considerthat the latent portion will gradually become manifest as the patient ages

4 Diabetes

Diabetic fluctuations in blood sugar can change lens size and curvature Stability of tive error is essential prior to refractive surgery Further, diabetes is a relative contraindi-cation to elective corneal surgery, as diabetic corneal epithelium is more prone to persis-tent epithelial defects, duplication of basement membrane, and recurrent erosions

refrac-5 Pupil Size

Larger pupils allow light rays peripheral to the optical zone to be transmitted to the fovea.The Styles–Crawford effect dampens this somewhat, as photoreceptors are oriented to-wards reception of light passing through the central cornea

6 Ocular Motility

Motility examination with measurements of convergence and divergence amplitudes is part

of the preoperative evaluation for refractive surgery, which can increase or decrease commodative requirements in various circumstances As mentioned above, the myope pre-viously corrected with spectacles will lose the near effectivity of distance-corrective minuslenses Undercorrection of myopia with surgery relieves the demand for accommodation atnear but increases the need for nonaccommodative convergence at near Patients with lowreserves of fusional divergence may become symptomatic if surgery overcorrects myopia,

ac-as the resultant hyperopia increac-ases the demand for accomodation and attendant modative convergence Thus measuring with prisms the amplitudes of convergence and di-vergence (far and near, and with or without accomodation) helps predict whether a change

acco-of accomodative demand may create problems acco-of convergence or divergence insufficiency

Accomodative amplitude with the correction mode at the cornea should be measured fore surgery to plan targeting for near vision, i.e., equal correction vs monovision, full cor-rection vs partial The amplitude of accomodation can be measured several ways, e.g., thedifference in diopters between the least and most spheres accepted with clear vision whilegazing at a distant target Low amplitudes may be due to medication, oculomotor nerve

be-paresis, trauma, lack of effort, incorrect distance refraction, or accomodative spasm surement of the near point while wearing myopic spectacles will overestimate the ampli-tude of accomodation amplitude, because of the so-called near-effectivity of the myopicspectacle lens Correcting the same eye for emmetropia by contact lens or refractivesurgery will yield a more remote near point and smaller amplitude of accomodation.Myopes who have been “overminused” with presbyopic symptoms may become lesssymptomatic once the extra minus is eliminated Determination that a patient is overmi-nused can be performed with cycloplegia or possibly with prolonged fogging with pluslenses If the cycloplegic manifest refraction is accepted in new glasses, surgery should bebased on those values However, it should be borne in mind that some patients who havehad overcorrected myopia for years will not be able to relax completely their accomodativetone soon after surgery The surgeon and patient must then be aware of the prospect ofblurred vision in either the short or the long term.

Making the central cornea flatter and the peripheral cornea steeper with LASIK producesblur, which is more pronounced when the pupil is large, as it is in dim light When the pupildilates, the peripheral lens is also more exposed; this part of the lens has greater plus power.The clinical relevance of peripheral corneal irregularity, scars, decentration, and centralcorneal haze are topics of active investigation The impact of postoperative irregular astig-matism of the central cornea on uncorrected and spectacle-corrected vision is unclear Pre-dictive factors for good spectacle correction remain to be identified

1 Astigmatism

An objective for surgical correction of astigmatism is a relatively spherical central cornealzone The outline of this region is oval, the narrower aspect being the meridian with themaximum difference in curvature between central and peripheral cornea When the pupildilates, the peripheral cornea becomes more relevant, resulting in blur or monoculardiplopia, especially noticeable when details vary with torsion of the eye or observed object

As these patients have a clear image from the central zone, the blurred regions of an objectstand in contrast to the sharper portions: this can be more disturbing than preoperative dif-fuse blur

2 Retinoscopy

Streak retinoscopy is performed in the standard fashion with attention to the central reflex.Retinoscopy after refractive surgery may be more easily performed using minus cylindertechniques to neutralize against motion The endpoint of retinoscopy is less influenced bythe steeper peripheral cornea if the target is neutralization of any part of the against motion,generally seen first in the flattest central cornea A novice depending on complete neutral-ization of against motion or any part of with motion may overcorrect patients significantly.Further, when against motion is observed, the patient is fogged and thus less likely to ac-comodate during retinoscopy

During retinoscopy, skew, break, and straddling are useful to refine the axis of rective cylinder Skew is streak motion not paralleling the motion of the retinoscope and ishelpful in patients with postoperative irregular astigmatism, when break (when the streakfalls on iris, indicating imperfect alignment of the retinoscopic reflex) is not easily ob-

cor-served Straddling involves observing reflexes 45 degrees on either side of the presumedaxis With motion should be observed with this technique and can be produced by addingminus sphere or moving closer to the eye.

When using plus cylinders, the correct axis is approached turning towards the ner, brighter reflex When using minus cylinders, the correct axis is approached by turningaway from the thinner, brighter reflex Retinoscopy after dilation may be confused due toscissoring of the reflex; this is commonly seen after refractive surgery In this situation, oneshould focus on the reflex of the central three millimeters Further, it should be rememberedthat off-axis retinoscopy will give a false measurement of astigmatism

thin-3 Confirmation of Retinoscopic Findings

Other tools used in refraction include automated refractors, wavefront analyzers, graphic maps, astigmatic dials, and stenopeic slits In the final analysis, subjective refrac-tion, based on these objective methods, is the most important test used for planning refrac-tive surgery Automated refractors have little utility after refractive surgery Topographymay be of use in detecting lenticular astigmatism, as it can be detected preoperatively by adisagreement between the axis of astigmatism found in topography and manifest refraction.The astigmatic dial is useful to determine axis and power of cylinder when retinoscopy andthe Jackson cross-cylinder fail With fogging to 20/40 acuity, the patient is asked to iden-tify the lines that appear blackest and sharpest The minus cylinder axis is determined bymultiplying the smaller “hour” number by 30 Minus cylinder is then added until the linesare equally blurred The stenopeic slit, essentially an elongated pinhole, can be used inmanifest refraction of patients with irregular astigmatism with unhelpful retinoscopy Thepatient is first refracted with spheres The stenopeic slit is then placed in front of the eyeand rotated to the position with best acuity Sphere is again adjusted, indicating the powerneeded at the axis parallel to the slit The slit is rotated 90 degrees away, and the sphereonce again adjusted, providing an estimate of the power of cylindrical correction At theconclusion of these objective methods of refraction, subjective refraction should be per-formed with relaxed accomodation and/or cycloplegia

topo-When patients still need spectacles after refractive surgery, the spectacle prescription

of young patients may be overminused 0.25 or 0.40 D to blunt postoperative diurnal tuation and aid the patient in tasks requiring sharpest acuity Cycloplegic refraction can bedone with an aperture blocking peripheral light rays, for reasons noted above

fluc-4 Anisometropia

During the preoperative evaluation, a discussion of the possibility of anisometropia countered postoperatively is valuable The management of these cases is similar to that ofanisometropes in general, depending on what sensorimotor adaptations the patient has de-veloped, the presence of amblyopia, etc A contact lens worn in the unoperated eye can al-leviate aniseikonia and spectacle-induced vertical prism effects on up and downgaze Par-tial correction or a balance lens can be used in spectacle correction in cases where deepamblyopia is present or symptoms of aniseikonia and anisophoria are severe Spectaclesmay be attempted in the postoperative period to protect a dominant eye when the other eye

en-is amblyopic

Monovision (one eye targeted for distance, the other for near) may be given withoutcompromise when there is good vision but little stereopsis in both eyes A preoperative trialwith contact lenses can be used to predict whether significant anisophoria or aniseikonia

would occur in patients after correction for monovision In patients with high myopia, existing strabismus and suppression may be present Surgical overcorrection may lead toaccommodative spasm and esodeviations In this situation, contact lenses or enhancementsurgery may correct the consecutive hyperopia.

Convergence and accommodation issues are also encountered postoperatively rected hyperopes, astigmatics, and overcorrected myopes can develop spasm of accommo-dation, causing asthenopia, headache, or blurred vision Exophoria, stress, convergence in-sufficiency, iridocyclitis, and certain medications (especially anticholinesterase agents) canworsen this phenomenon Accommodative spasm can be addressed with reading glasses,adding plus to spectacles, bifocals, or chronic cycloplegia Subclinical convergence insuf-ficiency may be unmasked after refractive surgery for hyperopia, as accommodation-re-lated convergence is lessened Spectacle-induced hyperopia usually solves this problem

Uncor-6 Optical Requirements for Near Vision

As noted above, the borderline presbyopic myope may lose the near effectivity of minusspectacles and have increased difficulty at near after refractive surgery In general, patientsare better served by choosing a lesser add to give a larger range of accommodation and tonot blur middle distances High-riding progressive adds can be useful in patients with diur-nal fluctuations in refractive error Very anisometropic patients may have unequal ampli-tudes of accommodation, and may need unequal adds The more myopic eye will need lessplus in its bifocal segment Horizontal phorias can be aggravated by reading adds; trial frameevaluation is very useful in this circumstance to judge comfort of focusing and fusion.Stronger adds will minimize accommodative convergence in esophoric patients, whileweaker adds will stimulate it in exophoric patients Those with large cylindrical errors mayneed cross-cylinder refraction at near, as torsion of the globe can occur with convergenceand downgaze When prescribing bifocals, surgically undercorrected myopes benefit fromflattop bifocal segments, which minimize image jump (as the optical center of the add isclose to that of the far correction segment) and prism displacement (counterbalanced by theopposite effect of the underlying minus lens) The overcorrected myope must choose be-tween the greater image displacement of flattop segments and the greater image jump ofroundtop segments Vertical prism effects can be minimized by slabbing off prism from themore myopic lens, lowering the optical centers of the distance correction, or using differentsegment types for the two eyes These problems of off-axis viewing can be avoided by sin-gle-vision reading glasses Options for patients with minimal amplitudes of accommodationinclude trifocals, progressive-add bifocals, and separate intermediate-zone glasses

An often successful strategy for presbyopia is monovision, which is refractive correction

of one eye for distance and the other for near In myopes, the dominant eye is generally rected for distance and the nondominant eye is undercorrected (2) Ideally, such a patientshould see clearly at all distances, without significant functional impairment Monovisionacceptance rates for contact lens wearers usually range from 60 to 80%

cor-Definitions of monovision success vary One commonly used set of criteria is quate adaptation to 1 to 2 diopters of monocular blur after 3 weeks The mean success rate

ade-of patients reported in 19 articles was 76% (434 out ade-of 573 patients) Failure was due tocontact lens intolerance or poor visual adaptation If previously contact-lens intolerant pa-tients are excluded, monovision success rates are approximately 86% (2).

There are several factors determining monovision success: ocular dominance, ing preference, interocular blur suppression, stereo acuity, and phorias When the dominanteye was corrected for distance, overall monovision success rate was 75% (3) These pa-tients performed better at visual locomotor tasks requiring directional prediction (walking,driving) and also had lesser esophoric shifts at distance Patients with alternating domi-nance (no sighting preference) have interocular blur suppression, another factor predictive

sight-of monovision success Those with strong sighting preferences had reduced blur sion, decreased binocular depth of focus (relative to others), and higher monovision failurerates; they frequently reported ghosting at near or distance (secondary images that can bedistracting and sometimes affect balance) (4) After monovision correction, unsuccessfulmonovision patients had a mean of 50 to 62 seconds of arc less in stereopsis than success-ful patients (5–6) Postoperative esophoric shifts were less in successful monovision pa-tients than in unsuccessful patients, but no differences in fusional vergence ranges havebeen documented Age has yet to be shown to be a factor affecting monovision success.Monovision affects several visual functions (7–13) Monovision correction generallyproduces a small reduction in high and low-contrast visual acuity, especially in conditions

suppres-of low lighting (11) The average visual acuity reduction was 0.05  0.02 logMAR units.The decrease in high-contrast binocular visual acuity is quite variable and worsens withlarger pupils and greater astigmatic errors Monovision has no significant effect on binoc-ular peripheral visual acuity or visual field width (6) Contrast sensitivity is 42% less withmonovision than with binocular vision (12) Thus monovision is not preferable in thosewhose occupations require fine, detailed work Task performance is reduced by less than6% in activities requiring moderate stereopsis, e.g., card-filing (13) In those with alternat-ing dominance (no sighting preference), the binocular depth of focus is almost equal to thesum of monocular depths of focus (9) In those with strong sighting preferences, the binoc-ular image becomes blurred as the object moves from the dominant eye’s clear range to thenondominant eye’s clear range, and thus the binocular depth of focus in these patients ismuch less than the sum of the monocular depths of focus After 3 weeks of monovisionadaptation, a significant recovery of stereo acuity occurs (8) Increasing the ocular blur in-creases the stereoscopic threshold In presbyopes, the stereo acuity decreased greatly whenthe blur was more than
1.75 D Monovision reduced secondary fusion in 10 to 20% of pa-tients but did not affect simultaneous perception or gross stereopsis The binocular visualstress created by monovision is thought to cause esophoric shifts These shifts are greaterwhen the nondominant eye is corrected for distance Divergence and convergence ranges

at distance are reduced with monovision, but reductions in fusional vergence ranges are nificant only when the nondominant eye is corrected for distance Monovision reduces thetypical exophoria seen in presbyopes at near by 2.5 to 5.2 prism diopters (6) The diver-gence range is significantly less with monovision at near viewing, but not the convergencerange

sig-Monovision should be avoided in patients with strong sighting preference, significantloss of stereoacuity with monocular correction, large esophoric shifts, minimal interocularsuppression, or occupations requiring fine work Patients must be informed of the reduc-tion in visual acuity and stereopsis Spectacle lenses may need to be prescribed for tasks re-quiring sharp distance vision (aniseikonia is generally acceptable in these circumstances).Monovision is clearly not a panacea for presbyopia and should be used only in patients who

have been carefully screened and who are willing to tolerate its attendant visual mises Preoperative use of monovision contact lenses may identify those who are likely to

compro-do well with monovision refractive surgery; these should be conducted for at least 3 weeks

If patients do not improve significantly in their adaptation to monovision over that time riod, they may not be good candidates Lastly, it should be noted that there are presently nopublished reports of LASIK being used to produce monovision

Contrast and glare sensitivity are two important parameters of visual function that can beaffected by LASIK (14) Refractive surgery can cause light scatter (due to corneal haze)and aberrations of the corneal curvature, both of which can affect contrast and glare sensi-tivity Light scatter can cause starburst phenomena, while spherical aberration can causehaloes to appear around bright objects Pupil size and the size of the treated area are im-portant factors in the prominence of the starburst and halo effects

Contrast measures of sensitivity assess how much a pattern must vary in luminance

to be seen; this generally parallels visual acuity in normal, healthy eyes In various ders, including cataracts, amblyopia, glaucoma, optic neuritis, cerebral lesions, and dia-betic retinopathy, the two may not correlate well Loss of contrast sensitivity can make theworld appear hazy Loss of low spatial frequency contrast sensitivity hampers face recog-nition, navigation in unfamiliar environments, and reading of low-contrast text (15).Contrast sensitivity is traditionally measured by using sine wave gratings that vary inspatial frequency (bar width) and contrast; a contrast sensitivity function is generated bymeasuring the lowest detectable contrast across a range of spatial frequencies Rubin andLegge (17) have shown that global changes in contrast sensitivity and changes near thepeak of the function curve are more clinically relevant than subtle variations in the curve.The Pelli-Robson Letter Sensitivity Chart is a commercially available test that provides asingle, global measure of contrast sensitivity (16) Letters consisting of contrasting lighterand darker bars are arranged in triplets of decreasing contrast and are equivalent to a 20/720letter with respect to visual angle subtended They reliably measure contrast sensitivity forpatients with visual acuity of 20/400 or better

disor-One of the most common visual side effects of refractive surgery is glare Disabilityglare occurs when light sources in the visual field reduce visibility of a target, and it occurswhen light from the glare source is scattered by the ocular media, forming a veiling illumi-nance that dampens the contrast and thus the visibility of the target (17) Glare testing usescontrast or acuity tests in the presence of a glare source (18) The most commonly availableglare test is the Brightness Acuity Test (BAT), a brightly illuminated dome held in front ofthe eye through which a standard eye chart is viewed (19)

There have been few studies of the effects of LASIK on contrast and glare ity Holladay et al found that contrast threshold worsened the first postoperative day by amean of 0.6  1.0 lines in darkness and 0.8  0.7 lines at high BAT (20) Contrast sensi-tivity in light conditions recovered by 1 week, but contrast threshold in darkness did not re-turn to baseline through 6 months Larger pupil sizes were associated with worse contrastoutcomes This study speculated that the conversion by LASIK of corneal asphericity to anoblate shape accounted for this result El Danasoury compared night glare after usingLASIK in two different ways: a single ablation zone of 5.5 mm in one eye, and an ablationzone of 5.5 mm with a transition zone of 1.0 mm diameter larger on the other eye (21) Use

sensitiv-of the transition zone significantly decreased night glare in this study Carr et al compared

single-zone vs multizone LASIK (22) Multizone eyes were found to have a greater crease in log contrast sensitivity at 12 cycles/degree under undilated conditions Perez-San-tonja et al found that contrast sensitivity 1 month after LASIK decreased significantly only

de-at low and intermedide-ate spde-atial frequencies (3 and 6 cycles/degree) (23) By 3 months, therewere no significant differences in contrast sensitivity at all spatial frequencies compared tobaseline Wang et al found that LASIK eyes recovered contrast sensitivity by 3 months aswell, a significant difference from their control group, PRK eyes, in whom recovery took

6 to 12 months (24)

Several preoperative optical considerations are important when evaluating a patient sidering LASIK surgery In addition to understanding the basics of refractive error evalua-tion, anticipation of postoperative outcomes and of optical aberrations after surgery are es-sential for patient education prior to surgery The use of corneal topography and wavefrontanalysis is valuable to achieve this goal An understanding of certain fundamental princi-ples of optics will facilitate and optimize clinical application of LASIK Refractive sur-geons must be aware of more than spherocylindrical thin-lens first-order optics in order tomake informed decisions The following chapters will cover additional preoperative con-siderations that are necessary to optimize LASIK outcomes and improve patient satisfac-tion

14 GS Rubin Contrast sensitivity and glare testing in keratorefractive surgery In: D Azar, ed fractive Surgery Stamford, CT: Appleton & Lange, 1997, pp 143–151.

Re-15 JA Marron, IL Bailey Visual factors and orientation-mobility performance Am J Optom iol Opt 1982;59:413–426.

Phys-16 DG Pelli, JG Robson, AJ Wilkins The design of a new letter chart for measuring contrast sitivity Clin Vis Sci 1988;2:169–177.

sen-17 GS Rubin, GE Legge Psychophysics of reading—the role of contrast in low vision Vis Res 1989;29:79–91.

18 DB Elliott, MA Bullimore Assessing the reliability, discriminative ability, and validity of ability glare tests Inv Ophthalmol Vis Sci 1993;34:108–119.

dis-19 JT Holladay, TC Prager, J Trujillo, RS Ruis Brightness acuity test and outdoor visual acuity in cataract patients J Cataract Refract Surg 1987;13:67–69.

20 JT Holladay, DR Dudeja, J Chang Functional vision and corneal changes after laser in situ atomileusis determined by contrast sensitivity, glare testing, and corneal topography J Cataract Refract Surg 1999;25:663–669.

ker-21 MA El Danasoury Prospective bilateral study of night glare after laser in situ keratomileusis with single zone and transition zone ablation J Refract Surg 1998;14:512–516.

22 JD Carr, RD Stulting, Y Sano, KP Thompson, W Wiley, GO Waring III Prospective son of single zone and multizone laser in situ keratomileusis for the correction of low myopia Ophthalmology 1998;105:1504–1511.

compari-23 JJ Perez-Santonja, HF Sakla, JL Alio Contrast sensitivity after laser in situ keratomileusis J Cataract Refract Surg 1998;24:183–189.

24 Z Wang, J Chen, B Yang Comparison of laser in situ keratomileusis and photorefraactive atectomy to correct myopia from 1.25 to 6.00 diopters J Refract Surg 1997;13:528–534.

Corneal Topography and LASIK

Applications

LI WANG and DOUGLAS D KOCH

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, U.S.A.

DIMITRI T AZAR

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

and Harvard Medical School, Boston, Massachusetts, U.S.A.

ROBERT T ANG

Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A., and Asian Eye Institute, Makati, The Philippines

RENGIN YILDIRIM

Cerrahpasa Medical School, University of Istanbul, Istanbul, Turkey

The use of computerized videokeratography (CVK) for the evaluation of the corneal face has become widespread CVK is an indispensable tool for refractive surgeons for pre-operative screening, surgical planning, assessment of surgical outcomes, detection andmanagement of complications, and refinement and development of surgical techniques.This chapter reviews the basic principles of CVK, the recognition of corneal topographicpatterns, and the role of corneal topography in refractive surgery

1 Terminology

It is important to introduce some terminology first The corneal light reflex is in fact an age reflected off the tear film and is the basis for placido-based topographic measurements

im-The tear film of the anterior corneal surface acts like a convex mirror to form a virtual, erectimage of reflected light This image is called the corneal light reflex or the first Purkinjeimage.

The line of sight is a line connecting a fixation point at optical infinity with the ter of the entrance pupil (1) The pupillary axis is a line normal to the corneal surface pass-ing through the center of the entrance pupil, which is usually temporal to the line of sight,and the angle between them is known as angle lambda and is in the range of 3° to 6°.The topography of the cornea can be arbitrarily broken down into four zones Thecentral zone generally refers to the central 3 to 4 mm of the cornea The paracentral zone is

cen-an cen-annulus with inner cen-and outer diameters of 4 cen-and 7 mm, respectively The peripheral zone

is an annular region with inner and outer diameters of 7 and 11 mm, respectively The bal zone is the border, about 0.5 mm wide, between the cornea and sclera

A meridian is a line that spans the diameter of the cornea from one point on the bus to a point on the opposing limbus Meridians are located by their angular position, in-creasing counterclockwise from 0° at the 3 o’clock position to 180° at the 9 o’clock posi-tion for both the right and left eyes (2) A semimeridian is a radial line on the cornealsurface from its center, and it is located by its angular position from 0° at the 3 o’clock po-sition, increasing counterclockwise around the full 360° for both the right and the left eyes

lim-In the literature, there is confusion over the terms apex and vertex as applied to thecornea Waring defined the apex as the high spot of the corneal (2) Maloney suggested de-noting the high point of the cornea as the corneal vertex and the apex as the region of great-est curvature (3) According to Webster, however, both terms refer to a point on a shapefurthest from its base (4) A vertex sometimes also refers to the point where the axis of acurve intersects the curve itself It is important to understand that the high point and the re-gion of greatest curvature often do not coincide In this chapter, we will use Maloney’s def-initions, referring to the vertex as the high point on the cornea relative to the imaging sys-tem (and hence the center of the placido disk rings), apex referring to the region of steepestcurvature

2 Placido Disk System

a Data Acquisition

Placido disk technology has been most widely accepted, used, and understood The ent units share certain components but can differ in data acquisition, processing, and per-formance (5) All systems contain a transilluminated disc or cone (modified Placido disc),

differ-an imaging system consisting of differ-an objective lens, a black differ-and white (B&W) camera noMed utilizes a color camera and color ring system), a video frame grabber, and a com-puter system The number, thickness, color, and position of the rings relative to each othervary from system to system Most systems can be divided into “near design” (Tomey TMS,TechnoMed C-Scan, and Keratron) and “distant design” (EyeSys Corneal Analysis Sys-tem-2000, Humphrey Atlas, Alcon EyeMap EH-290, and Dicon CT-200) The near designunits typically image a greater portion of the cornea and require lower levels of illumina-tion, but they may be more susceptible to focusing error (unless they are software cor-rected), and at times the patient’s brow may interfere with the positioning of the cone Thedistant design systems are less susceptible to focusing error and typically are not affected

(Tech-by the patient’s anatomy, but they require brighter illumination and have less corneal erage Recently, the additions of new cone designs, more sensitive video cameras, and cor-recting algorithms have decreased the clinical distinction between near and distant units

cov-The placido target is projected onto the cornea and reflected off the tear film to form

a smaller upright image that is in focus near the iris plane This two-dimensional digital age is captured by a CCD camera and analyzed to reconstruct three-dimensional cornealshapes, using the distances between rings or ring edges as the basis for calculating the ra-dius of curvature or refractive power A larger spacing between ring edges indicates a flat-ter cornea with a greater radius of curvature (Fig 8.1)

im-b Data Processing and Map Display

Three basic formulas exist for calculating corneal power: axial radius of curvature, taneous (or tangential) radius of curvature, and refractive Clinically significant differencesexist in the corneal power values calculated by the three formulas (6) The topographic datamay then be represented in a variety of ways Clinically, the most useful representation tothe clinician is the color-coded map The warm colors, red and orange, represent relativelyhigher powers (steeper curvatures), green and yellow are used for powers associated withnormal corneas, and cool colors, hues of blue, denote relatively lower powers (flatter cur-vatures) Types of topographic displays include

instan-1 Axial radius of curvature maps The axial radius of curvature maps are the

orig-inal and most commonly used CVK maps and are derived from the axial radius of ture This formula is used by the keratometer and is based on the distance from the cornealvertex to the center or edge of the ring This distance is then used to calculate the length

curva-of the radial line connecting this point to the optical (or sagittal) axis This approach simplifies the optical principles of the cornea by assuming that the rays of light strikingthe cornea are paraxial, i.e., the angles of incidence are small The simplified paraxial formula is

in-Figure 8.1 The principles of Placido disk technology The larger the distances between the rings, the flatter the cornea, and the less dioptric power it has The reverse is true with a steeper cornea, which shows smaller distances between the rings and a higher corneal power.

The axial radius of curvature map is the standard that may be used by most clinicians.However, it is less accurate than instantaneous radius of curvature maps in providing detailregarding corneal curvature, and it is less accurate than refractive maps in portraying therefractive power of the anterior corneal surface As a result, it may be less valid than theseother two maps, despite its popularity.

2 Instantaneous radius of curvature maps The instantaneous radius of curvature

varies from the axial radius of curvature in that it calculates a true radius of curvature dependent of the sagittal axis (Fig 8.2B):

be-at the sagittal axis Instantaneous radius of curvbe-ature maps typically show more markedchanges in dioptric power over smaller regions and provide more accurate measurements

of corneal curvature and certainly better representation of local irregularity (7)

Figure 8.2 Two approaches for the calculation of the corneal radius of the curvature: (A) axial dius, (B) instantaneous radius.

ra-Both of these maps, however, have marked limitations in predicting corneal tive power For the central paraxial rays, these formulas can be used to estimate corneal re-fractive power Outside of the central region, however, the assumptions and their equationsare not necessarily valid, and in particular they ignore the incident angle of the incominglight, thereby underestimating the refractive effect of peripheral rays (spherical aberration)(8).

refrac-3 Refractive maps The appropriate equation for the refractive power (secondary

focal point power) for the incoming parallel rays was first described by Gullstrand (9) andmodified by Klein (10) as

P n

x/tan (ni r) (3)where ƒ is the focal length, defined as the distance from the corneal vertex to the intersec-

tion of the refracted ray with the optical axis, x the corneal zone, i the angle of incidence,andr the angle of refraction Maps calculated using this formula attempt to estimate therefractive power of the anterior corneal surface

This map best characterizes the image-forming properties of the anterior corneal face

sur-4 Elevation maps Elevation data, which describe the difference between the

height of the cornea and a reference surface, are provided by many of the Placido-basedunits These elevation data are derived from the placido ring measurements, which is an ap-proach that obviously differs from the projection principle used by elevation-based systems

to obtain direct height measurements (see below)

As will be described below, elevation maps have particular value in assessing operative visual problems, such as central islands

post-5 Difference maps These are typically calculated using axial radius of curvature

maps They are useful in characterizing the change that has occurred during any interval

As with any color-coded map, it is important to note the scale for the difference map In theauthors’ view, a 0.5 D and 0.25 D interval is often most useful

6 Other comparative maps Different devices have a number of color-coded maps

that compare corneal curvature or refractive power to some standard surface An larity map may compare local changes in curvature to an idealized spherical cylindrical sur-face A profile map may compare corneal curvature to a standard aspheric corneal surface

irregu-as a way of evaluating the irregu-asphericity of the cornea Corneal acuity maps display the mated visual acuity potential for any given region of the cornea and for the cornea as awhole

esti-In viewing a color-coded dioptric map, it is critical first to ascertain the dioptric scale

A number of different dioptric intervals have been recommended for standardized scales,but the authors prefer a 0.5 D scale Scales with intervals greater than 0.5 D are useful forcorneas with large dioptric ranges, such as advanced keratoconus However, for refractivesurgery, a 0.5 D scale is required to obtain sufficient detail regarding nuances that affect vi-sual performance Fortunately, most topographers now offer some type of standardized ab-solute scale and adjustable scales that allow the clinician to customize the information formaximal clinical value

c Indices

CVK devices provide a number of quantitative indices that can tremendously enhance ysis of topographic maps Simulated Keratometry (Sim K), which is provided by all de-