Hyperopia and Presbyopia - part 5 potx

It should also be noted that in myopic PRK, significant decrements in the characterand magnitude of corneal optical aberrations have been found with larger optical andtransition zones..

Table 2 Safety of Hyperopic Phakic Intraocular Lenses

Three pupillaryblock glaucomas

Two pupillaryblock glaucomasOne lens opacityNone

2 STAAR Collamer Phakic IOL

Four studies on hyperopic correction using the STAAR Collamer phakic lens have beenpublished (11–14)

These studies included 9, 24, 15, and 10 hyperopes respectively, the latter study(14) being a phase I clinical trial sponsored by the U.S Food and Drug Administration

In total, 58 patients underwent STAAR Collamer phakic IOL implantation

Cumulative data show that preoperative SE ranged fromⳭ2.25 to Ⳮ11.75 D Meanfollow-up ranged from 3 to 12 months Postoperative SE ranged fromⳮ3.88 to Ⳮ1.50D; 58 to 80% of eyes were within 0.50 D of emmetropia and 79 to 92.3% of eyes werewithin 1.00 D of emmetropia In Rosen’s study (11), the efficacy index was 0.98, whichwas superior to the index for myopic patients implanted with the same type of phakic IOL

in series published by the same authors Davidorf et al (12) also compared their resultsfavorably to the predictability in their series of high myopic eyes

Seven of 24 eyes (29%) (12) and one of 15 eyes (7.69%) (13) lost one or more lines

of postoperative BCVA Conversely, only 8% of hyperopic eyes operated on by Davidorf et

al (12) demonstrated a gain in postoperative spectacle BCVA compared to the preoperativespectacle BCVA This is explained by the loss of magnification induced by the surgery

In Fechner’s series of the artisan lens, one patient had glaucoma and corneal edema inboth eyes (10) In our study, no complications occurred and no change in endothelial celldensity was noted after a follow-up of 1 year (personal communication)

For the ICL, postoperative pupillary block glaucoma occurred in 3 of 24 eyes and

in 2 of the 15 eyes in the series of Davidorf et al (12) and Pesando et al (13), respectively.This complication was due to iridotomies that were too small

Two types of phakic IOLs are available to correct hyperopia: the Artisan iris-claw lensand the STAAR Collamer PC IOL These represent the only surgical refractive procedurescapable of correcting hyperopia ofⳭ4 D or more There have been very few publications,but the results are encouraging The predictibility, efficacy, stability, and safety of theseprocedures are excellent, as well as the quality of the resultant vision The time of recovery

is short and the surgeries are reversible Long-term follow-up is, however, mandatory withrespect to delayed complication such as iris atrophy at the fixation sites and progressiveendothelial cell loss (iris-claw lens), and cataract and pigmentary dispersion (PC phakiclens)

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refrazione Ann Ottamol Clin Oculist Parma 1954; 80:75–82

HE, Stulting RD Complications of hexagonal keratotomy Am J Ophthalmol 1994; 117:37–49

Surg 1989; 15:661–666

results Arch Ophthalmol 1998; 116:425–428

noncon-tact holmium:YAG laser thermal keratoplasty; United States phase IIA clinical study with a1-year follow-up Ophthalmology 1996; 103:1525–1536

low hyperopia An 18-month assessment of safety and efficacy Ophthalmology 1998; 105:1727–1738

astigma-tism J Refract Surg 1999; 15:406–414

intraocu-lar lens implantation for hyperopia J Refract Surg 1999; 15:316–323

eyes with two piggyback posterior chamber intraocular lenses Ophthalmology 1996; 103:1118–1123

study J Cataract Refract Surg 1998; 24:48–56

hyperopia J Cataract Refract Surg 1998; 24:596–606

lens for myopia and hyperopia J Refract Surg 1999; 15:415–423

phakic intraocular lens for hyperopia J Refract Surg 1999; 15:309–315

negativen Intraokularlinse bei myopen Patienten Klin Mbl Augenheilk 1988; 193:99–102

Hyperopia and Presbyopia

Techniques for refractive surgery have made tremendous strides since the pioneering work

of Jose Barraquer and the introduction of radial keratotomy in the late 1970s (1) Traditionaloutcome measures for the efficacy of specific refractive surgeries are primarily uncorrectedand best-corrected visual acuities and cycloplegic and manifest refractions Corneal topog-raphy analysis has not been considered a primary outcome measure for clinical trials inthe United States—this despite the fact that corneal topography is now the standard ofcare for preoperative screening of refractive surgical candidates and analysis of postopera-tive results and is a mainstay of anterior segment practice Direct analysis by cornealtopography has clearly shown the causes of visual loss after eventful refractive surgery.The best examples include the formation of central islands and peninsulas after surfaceablation with the excimer laser (2) and induced generalized irregular astigmatism afterautomated lamellar keratectomy (3) In this chapter, the topographic characteristics of thepresbyope and the current modalities for the correction of hyperopia are reviewed

HYPEROPIA—TOPOGRAPHICAL CORRELATES

Kohnen et al (4) used computed videokeratography to demonstrate peripheral cornealflattening and central corneal steepening following noncontact Ho:YAG laser thermalkeratoplasty (LTK) for the correction of hyperopia Greater changes in corneal curva-

129

ture and smaller amounts of topographical regression were noted when a two-ring lasertreatment pattern was applied When the topography was analyzed, several forms of in-duced astigmatism were observed: bowtie (both symmetrical and asymmetrical), irregu-larly irregular, and semicircular patterns Only one eye in the entire study group wasobserved to have a homogeneous pattern At present, noncontact LTK appears to be mostpromising for low hyperopia up to approximately 2 D Regression of the effect appears

to limit the procedure’s usefulness for refractive errors higher than 2 D Furthermore,factors such as younger age (less than age 30) and increased preoperative corneal thicknessmay also contribute to faster rates of regression (5)

Early hyperopic photorefractive keratoplasty (H-PRK) ablations consisted of smalloptical zones (approximately 4.0 mm) with small transition zones, creating an overalltreatment zone diameter of 7 to 8 mm Small optical zones increase the patient’s sensitivity

to small decentrations Likewise, small transition zones produce abrupt topographical andrefractive changes between treated and untreated tissue This “lack of smoothness” pro-motes more aggressive stromal and epithelial regeneration and thus refractive regression(6) It should also be noted that in myopic PRK, significant decrements in the characterand magnitude of corneal optical aberrations have been found with larger optical andtransition zones Larger optical and transition zones result in a more natural physiologicalpattern of measured aberrations in myopic PRK (6), and a similar result would be expected

in approaches to correct hyperopia These considerations have led to larger optical zones

of 6.0 mm, with overall hyperopic ablations now reaching 9.0 mm

With these considerations, induced aberrations after H-PRK have been carefullyevaluated (7) Corneal topography after H-PRK showed a change from positive to negativespherical aberration on the order of 3 D It is known that the positive spherical aberration

of the cornea and the spherical aberration of the crystalline lens act in concert to decreasethe overall aberrations of the eye However, if hyperopic procedures over correct forcorneal spherical aberration, a negative impact on visual function is expected This effectcan be seen in Figure 1

Even with larger ablation sizes, difficulties remain By the nature of the procedure,the functional optical zone becomes smaller as the attempted correction increases in size.This is undoubtedly one of the most significant factors contributing to the poor successrate of both H-PRK and hyperopic laser assisted in situ keratomileusis (H-LASIK) for thecorrection ofⳭ5.00 D or greater Moreover, Choi et al (8a) report an increased risk ofirregular astigmatism based on topographic analyses when corrections above this levelare attempted The comfort level in this respect seems to be surgeon-related; thereforesome surgeons limit attempted corrections toⳭ4.0 D or less

In reference to H-LASIK, a 9.0-mm ablation size requires the creation of a 9.5-mmflap Although modern microkeratomes may provide for this flap size, some patients withsmall eyes or thin corneas are unsuitable candidates for this treatment Larger flap diame-ters and larger amounts of correction increase the chances of striae formation, which cantranslate to irregular astigmatism on corneal topography

H-LASIK is gaining widespread use as a procedure to correct primary hyperopia aswell as to modify consecutive hyperopia after overcorrection from LASIK for myopia; it

is said to be safe and effective (8) Two typical case reports are given below to illustratethe topography obtained Each patient underwent hyperopic LASIK with the VISX, Inc.,Star Excimer Laser System The diameter of the optical zone was 5.00 mm, with a totaltreatment zone of 9.00 mm OU

Figure 1 H-LASIK effect on corneal topography and total eye aberration measured with NIDEKOPD-Scan (1) (top left panel) standard corneal topography showing off-center treatment; (2) skias-copic (pointwise refraction) map: in the postoperative period, corneal aberrations for this eye accountfor the bulk of the total ocular aberrations; (3) placido image; (4) wavefront map showing induction

of excess negative spherical aberration and coma

Case 1 A 66-year-old woman with no prior history of ocular surgery underwent H-LASIK

for monovision Her preoperative K-readings were 44.3/44.5 at 118 OD and 44.4/44.8 at 163

Ⳮ 1.50 ⳯ 180 The total ablation depth was 20 ␮m OD and 38 ␮m OS Optical zone diameterwas 5.00 mm Her visual acuity without correction on postoperative day 1 was 20/200 ODand 20/80 OS Two weeks postoperatively, her visual acuity without correction was 20/70(ⳮ1) OD and 20/200 OS and her BSCVA was 20/25 OD and 20/40 OS The manifest

postoperatively, her visual acuity without correction was 20/30 (ⳮ2) OD and 20/25 (ⳮ2)

B

Figure 2 H-LASIK 1-month postoperative topography for 66-year-old requesting monovision

⳯ 180

showed the extent of induced cylinder, revealed a steepening of the central 5 mm of thecornea, and produced simulated keratometry readings (SimKs) of 46.13/44.17 at 96 with apotential visual acuity (PVA) of 20/25 to 20/30 OD and 47.41/46.68 at 94 with a PVA of20/20 to 20/30 OS (Figure 2).

Case 2 A 26-year-old woman presented for refractive surgery evaluation She had a refractive

OU Her keratometry readings were 44.1/45.6 at 091 OD and 44.1/46.0 at 099 OS The desired

postoperative day 1 her uncorrected visual acuity was 20/30 OD and 20/40 OS Six months

BSCVA OS was 20/25 (Ⳮ1) with a manifest refraction of Ⳮ1.00 Ⳮ0.75 ⳯ 165 There wassome evidence of regression OS Postoperative keratometry readings were 48.70/49.18 at 058

OD and 47.24/48.07 at 051 OS (Figure 3)

Hence, H-LASIK seems a good choice of procedures at least for the temporarycorrection of hyperopia Long-term stability will need to be demonstrated for this approach,

as for others discussed in this chapter

Conductive keratoplasty (CK) is being developed as an alternative procedure fortreating hyperopia It is argued that if the collagen is heated to a carefully controlledcritical temperature, the shrinkage and changes in corneal shape might be more permanent

Figure 3 Six-month postoperative corneal topography of H-LASIK patient showing good tion OU (Central green irregularities OS are temporary, from tear film breakup.)

centra-Figure 4 Preoperative and postoperative topographies after CK Note large uniform area of creased power.

in-Conductive keratoplasty uses radiofrequency energy to generate heat in the corneal ery As with LTK, the shrinkage of the collagen occurs from the production of a ringpattern of treatment spots around the corneal periphery This shrinkage creates a purse-string effect to steepen the central cornea One of the immediately appreciated benefits

periph-of CK over H-LASIK is the larger functional optical zone (Figure 4)

As the number of patients undergoing refractive surgery expands, the curious phenomenon

of presbyopic patients presenting with functional near and far vision after refractive surgery

is being more frequently reported for both myopic and hyperopic corrections Described

as a “multifocal” effect, this side effect of the surgery deserves scrutiny

It was Benjamin Franklin who conceived the first bifocal spectacle in 1874, initiatingwhat is perceived to be a sequence of developments (Figure 5) Deliberate multifocalitywas introduced to the contact lens field prior to 1967 (9) and to the intraocular lens(IOL) arena before 1987 (10) While early models of IOLs and contact lenses exhibitedpronounced aberrations that reduced contrast sensitivity, current renditions have enjoyed

a measure of patient acceptance, at least with contact lenses Unintentional iatrogenicmultifocality was first identified with corneal topography in 11 eyes after radial kerato-

Figure 5 Historical use of multifocality in vision correction: Ben Franklin’s bifocal spectacles,bifocal contact lenses, bifocal IOLs, multifocality in radial keratotomy (11) and in photorefractivekeratectomy for myopia (13).

tomy, and although the possibility of complications from degradation of contrast sensitivity

as well as monocular diplopia was anticipated, no patient complaints of this type were infact reported (11) However, shortly after this report, additional analysis showed thatcertain patients with the multifocal effect after radial keratotomy could experience visuallydebilitating irregular astigmatism This should be considered a complication of surgery(12) Multifocal effects have also been found following PRK (13) for myopia and contrib-ute to a form of artificial accommodation in pseudophakic eyes (14)

It is well known that patients with an extreme amount of irregular corneal tism often refract over a large range of powers This is the basis for the so-called multifocaleffect; in spectacles, distinctly separate areas of the lens are prepared with different specificpowers, whereas the power distributions of the multifocal cornea are more continuouslygraded and are analogous to gradations of refractive powers of the Varilux contact lenssystem It might therefore be more accurate to describe the multifocal property as one of

astigma-varifocality.

A topographical multifocal effect can be assessed by direct examination of the bution of corneal powers over the entrance pupil The standard statistical metric for measur-ing the width of such distributions is the coefficient of variation; hence, an appropriatetopographic definition of corneal multifocality is the coefficient of variation of cornealpower (CVP) (15) The increase in the range or width of the distribution of central corneal

distri-Figure 6 Corneal power distribution in the central 3 mm before and after conductive keratoplasty.Note the broader distribution of corneal powers after surgery, which will enhance the multifocaleffect This is an analysis of the topography shown in Figure 3.

powers is illustrated in Figure 6 It can be noted that the power distribution is broad and

without distinct peaks; hence the appellation varifocal Conversely, Benjamin Franklin’s

bifocals would produce a bimodal distribution: two peaks whose widths directly relate tothe precision of manufacture

Despite the promising aspects of artificially inducing accommodation with controlledcorneal multifocality, significant levels of uncontrolled multifocality can lead to a reduction

Figure 7 The effect of irregular astigmatism on vision can be simulated by placing the measuredsurface into a model eye and doing ray tracing CTView V3.12 (Sarver and Associates, MerrittIsland, FL) was used for this calculation

Figure 8 After correction for distance vision with conductive keratoplasty for hyperopia,

CA)

in contrast sensitivity and symptomatic vision The effect of varifocality in corneal surgerycan be evaluated mathematically by fitting the surface with Zernike polynomials andcalculating from this the modulation transfer function This will give the global opticalcharacteristics of the corneal surface and allow the simulation of multifocal effects onvision, as shown in Figure 7

With hyperopic keratorefractive surgery, there is another effect that comes into playunder the guise of multifocality In one cohort of patients undergoing the conductivekeratoplasty procedure for the correction of low hyperopia, postoperative near vision eitherremained constant or was enhanced at 1 month for every eye in the study (Figure 8) The

average improvement was statistically significant (p⬍ 0.001) This is a striking effectthat generally contrasts with myopic keratorefractive surgeries, where functional nearvision typically worsens in the presbyopic patient population (16) This effect can beexplained Presbyopes who are mildly myopic often have excellent near vision withoutcorrection With keratorefractive surgery, near vision is sacrificed for improved distancevision On the other hand, presbyopic hyperopes have very poor uncorrected near vision,and when keratorefractive surgery is used to correct their distance vision, this brings theirnear focal point closer to the eye and improves vision at the near reading distances.Multifocality and better than expected near vision after keratorefractive surgery forthe correction of hyperopia are due to a combination of factors Focus over a range ofdistances is made possible by the varifocal nature of some postoperative corneal topo-graphies Residual accommodation in younger patients can enable uncorrected near vision.Use of the pinhole effect and bright illumination make a contribution as well Finally,improvements in uncorrected near vision can be expected after hyperopic correctionsbecause the near focal plane is brought closer to the eye, whereas with myopic corrections,

it is moved further away

Several approaches have been developed to provide for the automatic interpretation ofcorneal topography (17) Among these, neural networks appear to have the greatest poten-

tial for success (18–20) A principal consideration in developing a strategy for the training

of such a network is data collection Generally, 20 to 30 examples of each class of cornealtopography are collected to provide a broad range of “experience” for the neural network

In this way the network can “learn” the hallmarks of each corneal condition and then beable to classify new corneal maps accordingly With the widespread success of refractivesurgery, there is concern that donor corneas for transplantation might be compromised byprevious surgery As a result, a class of corneas was developed that are referred to ashaving “myopic refractive correction.” There appear to be no consistent features amongthe various myopic refractive corrections that persist to allow differentiation between thevarious types This even includes radial keratotomy, because the lower power over theincision sites tends to be erased with time Fortunately, no other corneal condition ordisorder is known that has the principal feature of uniform central corneal flattening Withhyperopia, central corneal steepening is the principal characterizing feature, and again,differentiation among the several corneal surgical approaches may not be possible How-ever, the central corneal steepening after hyperopic correction, unlike correction for my-opia, outwardly mimics the characteristics of keratoconus, with a centralized cone (Figure9) This may confound the clinician, as well as the automated classification algorithmsthat detect keratoconus Patient history and corneal pachymetry may be required for differ-entiation between hyperopia-corrected corneas and keratoconus unless some distinguishingtopographical metric is found

Figure 9 Postoperative topography of H-LASIK Note the fairly typical appearance of keratoconus

as a consequence of the surgery Corneal topography classification programs will need to be retrained

to determine whether it is possible to automatically differentiate H-LASIK from keratoconus

E SUMMARY

Corneal topographic analysis is helpful in elucidating the strengths and weaknesses ofrefractive surgical procedures, and surgery for hyperopia is no exception Centration iscritical, and a large treatment zone size is technically difficult to achieve A hyperopicprocedure’s stability can be objectively and precisely measured with corneal topography.However, the results of stability measurements may be confounded by the fact that people

in this age group (50–65 years) are undergoing progressive hyperopia naturally; this must

be taken into account Several factors, including varifocality of corneal topography, tribute to better than expected near visual function after the surgery With advancing age,qualities of the tear film diminish, and this leads to fine surface irregularities, while theinduction of coma results from global asymmetrical changes in shape

con-REFERENCES

keratec-tomy for myopia J Cataract Refract Surg 1993; 19(suppl):149

Practice of Ophthalmology Philadelphia: Saunders, 2000: 668–694

YAG laser thermal keratoplasty to correct hyperopia J Cataract Refract Surg 1995; 22:427–435

keratoplasty J Refract Surg 1997; 13:17–22

HC Effect of larger ablation zone and transition zone on corneal optical aberrations afterPRK Arch Ophthalmol 2001; 119:1159–1164

corneal optical aberrations induced by photorefractive keratectomy for hyperopia J RefractSurg 2001; 17:406–413

8a Choi RY, Wilson SE Hyperopic laser in situ keratomileusis: primary and secondary treatmentsare safe and effective Cornea 2001; 20:388–393

Surg 1987; 13:557–560

corneal tissue, Curr Eye Res 1981; 1:123–129

kerato-tomy Am J Ophthalmol 1988; 106:692–695

Refract Corneal Surg 1989; 5:394–399

keratectomy: multifocal corneal effects J Cataract Refract Surg 1997; 23:1029–1033

accommoda-tion and corneal multifocality in pseudophakic eyes Ophthalmology 1999; 106:1178–1181

Pallikaris IG, Siganos DS, eds LASIK Thorofare, NJ: Slack, 1997:339–357

after monovision induced by myopic photorefractive keratectomy J Cataract Refract Surg1999; 25:177–182

17 Maeda N, Klyce SD, Smolek MK, Thompson HW Automated keratoconus screening withcorneal topography analysis Invest Ophthalmol Vis Sci 1994; 35:2749–2757.

corneal topography: preliminary demonstration Invest Ophthalmol Vis Sci 1995; 36:1327–1335

videokeratography Arch Ophthalmol 1995; 113:870–874

net-work approach Invest Ophthalmol Vis Sci 1997; 38:2290–2299

Corneal Surface Profile After

Hyperopia Surgery

DAMIEN GATINEL

Fondation Ophthalomogique Adolphe de Rothschild and Bichat Claude Bernard

Hospital, Paris, France

The desired change in corneal curvature to correct for hyperopia with current excimerlaser systems is based on principles of geometric optics and the precise interaction of theexcimer radiation with the corneal tissue In comparison to myopic correction in whichthe goal is to flatten the central cornea, in hyperopia the central corneal area must besteepened to increase its optical power This central steepening makes the planned correc-tion of the hyperopic eye more difficult because the steepened central corneal portion has

to join the peripheral unablated area of lower curvature via a transition area These representthe important special features of the correction of hyperopic errors, which are emphasized

in this chapter

The profile of ablation to correct for spherical hyperopia is radially symmetrical andpredominates in the periphery in an annular fashion A subtraction shape model based ongeometric optics allowed Munnerlyn et al., in 1988, to announce the principles of laser-guided photoablation in the central corneal area (effective optical zone) (1) The modifica-tions of the corneal profile are analyzed separately below for the optical zone and for thetransition zone

1 Optical Zone Design

Conforming to the pioneering work of Munnerlyn et al., the change in paraxial cornealpower can be predicted by considering the initial unablated and the final ablated corneal

141

Figure 1 Schematic representation of the lenticule ablated for the correction of spherical opia The profile of ablation is outlined along two perpendicular meridians (green) The thickness

hyper-of the lenticule is maximal at its edges and null at its center

surface as two spherical surfaces with a single but different radius of curvature Theremoval of tissue is equivalent to adding a thin lens of equal but opposite power Thispermits the calculation of the ablation profile over the optical zone for a spherical hyperopicerror (see Appendix 1)

To generate a three-dimensional graphic representation of the theoretical shapes ofthe lenticules ablated during laser-assisted in situ keratomileusis (LASIK) of similaramounts of spherical and cylindrical ablation, we used a digital modeling software thatallow to visualize the results of Boolean operation on orientated three-dimensional surfaces(see Appendix 2)

The difference between each of the radii of curvature was exaggerated as compared

to the surgical range so as to facilitate the spatial visualization of the contour features ofthe generated lenticules

Spherical hyperopic ablation results in the ablation of a concave lenticule withinthe optical zone, which is represented on Figure 1 Its thickness is null in the center andincreases progressively toward the periphery, where it reaches its maximum at the edge

of the optical zone In first-order approximation, the maximum thickness of the edge ofthe ablated lenticule over the optical zone is proportional to the magnitude of the hyperopictreatment and to the square of the chosen optical zone diameter The volume of tissueablation needed to steepen the cornea is thus delimited by the initial anterior surface andthe final postoperative steeper spherical surface over a circular optical zone

2 Transition Zone Design

For necessary geometric feature, Any cornea that has had tissue removed centrally tosteepen its curvature (optical zone) while leaving the periphery untouched must undergo

an additional ablation to sculpt a smooth blending zone (transition zone)

This flatter area, commonly referred to as the transition zone, thus represents a

constant feature that ideally would have no undesirable optical effects and would ensurethe stability of the induced refractive changes in the optical zone by limiting unwantedbiological and biomechanical changes