Hyperopia and Presbyopia - part 7 potx

Crossed monovision occurs when the nondominant eye is corrected for distance and thedominant eye for near.. Crossedmonovision may be the intended goal when, for example, a contact lens m

The most commonly used approach, however, is determining which eye is the nant eye and correcting that eye for the most commonly used viewing distance (11), which

domi-is generally considered to be the far ddomi-istance The dominant eye has been shown to besuperior for spatial-locomotor tasks such as walking, running, or driving a car (2,20) Blursuppression appears to be greater when the dominant eye is corrected for the most com-monly used distance (i.e., far) (3) Correcting the dominant eye for distance also producesless esophoric shifts (21)

The dominant eye is generally identified by use of sighting dominance tests (22).One of the more common tests is the hole test (8), for which the patient is asked to frame

an object that lies at an intermediate distance from him or her with a triangle created byhis or her outstretched arms while keeping both eyes open The eye that is in alignmentwith the object and the hole is considered the dominant eye

Crossed monovision occurs when the nondominant eye is corrected for distance and thedominant eye for near This can happen either accidentally or intentionally (1) Crossedmonovision may be the intended goal when, for example, a contact lens monovision trialdemonstrates better visual function if the nondominant eye is corrected for distance Apatient may also change his or her mind regarding monovision versus full distance correc-tion for both eyes after the nondominant eye has already been treated for distance and thedominant eye has not yet been treated Patients who wish to have only one eye treatedand who are markedly more myopic in the nondominant eye may elect to have the nondomi-nant eye corrected for distance (1,3)

Unintentional crossed monovision can occur when correction in the dominant eye

is less than expected in patients requesting full distance correction for both eyes versely, in patients desire equivalent undercorrection in both eyes, an overcorrection inthe nondominant eye can produce crossed monovision Unintentional crossed monovision

Con-is a result of the fact that refractive surgery Con-is not a completely predictable procedure (1)

Of our 97 LASIK patients, 69 (71.1%) had uncrossed monovision, and 28 (28.9%) hadcrossed monovision The average age was 51.7Ⳳ 0.5 years for patients with uncrossedmonovision and 49.7Ⳳ 0.7 years for patients with crossed monovision (p⳱0.04) The

two groups were similar with regard to distribution of gender, dominance, myopia, type

of laser used, and type of microkeratome used (␹2⳱NS) The average anisometropia ofuncrossed monovision patients was 1.28 D, and of crossed monovision patients 0.98 D

(p⳱0.03).

Of the 69 uncrossed monovision patients 56 (81.2%) were satisfied after LASIK,

as were 22 (78.6%) of the 28 crossed monovision patients Two (33.3%) of the 6 patientsdissatisfied with crossed monovision and 2 (15.4%) of the 13 patients dissatisfied withuncrossed monovision were unhappy for reasons unrelated to monovision, such as dry-eye symptoms and floaters One patient each (16.7%) complained of poor distance vision,poor near vision, imbalance, and poor overall quality of vision in the crossed monovisiongroup Of the 13 dissatisfied uncrossed monovision patients, 3 (23.1%) complained ofpoor distance vision, 2 (15.4%) were unhappy with near vision, 5 (38.5%) felt imbalanced,and 1 (7.7%) was unhappy with overall quality of vision

Among minimonovision patients, 19 of the 24 (79.2%) patients with uncrossedmonovision were satisfied, as were 10 of the 13 (76.9%) with crossed monovision Ofthe monovision patients, 37 of the 45 (82.2%) patients with uncrossed monovision weresatisfied, as were 12 of the 15 (80.0%) crossed monovision patients Of the 3 patientswho were monovision failures, 2 had crossed monovision One patient with crossed monov-ision was retreated to uncrossed monovision, while the other two patients did not have achange in crossed monovision status The patient with uncrossed monovision before andafter retreatment remained dissatisfied.

All 13 patients dissatisfied with monovision outcome were offered retreatment, but only

3 (23.1%) elected to undergo a second procedure Most patients chose to defer retreatmentfor one of three reasons (1) the patient was unwilling to sacrifice near and intermediatevision for sharper distance vision, (2) symptoms were not bothersome enough to meritrisks of additional surgery, and (3) the patient was willing to give additional time toadjust to monovision Patients were then prescribed glasses for distance vision or reading,depending on the complaint, or were to be re-evaluated for retreatment following someperiod of adjustment Many patients were then lost to follow-up This suggests that thedegree of dissatisfaction was relatively mild and that many patients eventually adjust tomonovision or wear glasses on occasion for specific activities

Overall patient satisfaction with monovision after LASIK was 80.4%, compared to80.6% in contact lens wearers after exclusion of contact lens intolerance Furthermore,the satisfaction among strict monovision, minimonovision, uncrossed monovision, andcrossed monovision groups did not differ significantly from satisfaction in contact lenswearers

Due to the retrospective nature of most monovision refractive surgical studies, therewas no standardized instrument to measure patient satisfaction Rather, patient dissatisfac-tion was noted if the patient had any complaints or negative comments about vision atthe last follow-up visit In addition, near vision was not documented in many charts andcould not be analyzed as an outcome It was also difficult to determine from charts whethermonovision was intended or whether regression of one or both eyes led to a monovisionresult These factors may be better analyzed through a prospective study

Two tests used to measure the ability to suppress interocular blur are the anisometropicblur-suppression test and the American Optical vectographic test The anisometropic blur-suppression test indicates that the interocular suppression of blur is greater for smallerdegrees of anisometropia (2) Both testing modalities indicate that blur suppression isgreater when the dominant eye is corrected for distance

Monovision success is dependent on interocular blur suppression In successful ers of monovision lenses, the interocular suppression of blur was found to be approximatelytwo orders of magnitude greater than in unsuccessful wearers of monovision lenses (2)

wear-Of note, interocular blur suppression becomes less effective under dim illumination tions (2), which accounts for the well-known poorer visual performance of monovisionpatients under night driving circumstances

condi-3 Binocular Visual Acuity

Jain and colleagues (1,2) reviewed six articles addressing the effect of monovision onbinocular visual acuity and found the effect to be mild High-contrast and low-contrastvisual acuities at standard room illumination were found to be reduced by 0.04 to 0.08logMAR unit and 0.04 to 0.09 logMAR unit, respectively This reduction was slightlyhigher (0.10 logMAR unit) under low illumination conditions The effect on visual acuitywas particularly pronounced when the dominant, distance-corrected eye had a residualastigmatic error at an oblique axis (23)

Reduced stereoacuity is considered to be the major disadvantage associated with sion (24) Jain and coworkers reviewed twelve articles that examined the effect of monovi-sion on stereoacuity (1,2) When near stereoacuity under monovision conditions was com-pared to stereoacuity under binocular viewing conditions, a mean decrease of 37 arcseconds (from 87 to 124 arc seconds) was found The average normal value for stereopsis

monovi-is 20 arc seconds and, for persons over 40 years of age, 58 arc seconds (13,25) A morerecent paper by Kirschen and coworkers found that near stereoacuity decreased from amedian of 50 arc seconds with bifocal contact lenses to 200 arc seconds with monovision(26)

Patients in whom monovision is successful exhibit a lower reduction in stereoacuitythan do unsuccessful monovision patients Patients in whom monovision was unsuccessfulwere found to have a 50 to 62 arc seconds greater reduction in stereoacuity as compared

to successful monovision patients (1)

When two eyes are used instead of one, visual performance, and especially contrast

sensi-tivity, greatly improves (binocular summation) Contrast sensitivity increases by a factor

of兹2 when the stimulus is viewed binocularly; therefore, binocular contrast sensitivity

is 42% greater than monocular contrast sensitivity With increasing monocular defocus,the binocular contrast sensitivity decreases steadily until it is actually worse than monocular

contrast sensitivity (binocular inhibition) (27) If the defocus is increased beyondⳭ2.50

D the binocular contrast sensitivity reverts back to the monocular level, indicating sion of the defocused eye Because monovision results in loss of binocular summation,

suppres-or may even result in binocular inhibition, monovision results in a significant reduction

in contrast sensitivity, especially at higher spatial frequencies (greater than 4 cycles perdegree)

Monovision appears to have no significant effect on peripheral visual acuity and only aminimal effect on binocular visual field width (14)

The binocular depth of focus is the range in which an image may move without noticeableblur under binocular viewing conditions (without changing accommodation) In patients

in whom neither eye is clearly dominant (i.e., in whom there is no sighting preference),

the binocular depth of focus is approximately equal to the sum of the monocular depths

of focus However, in patients with a strong sighting preference, the image becomes blurred

as the object moves from the monocular clear range of the dominant eye to the monocularclear range of the nondominant eye Therefore, in patients with a strong sighting preference,the depth of focus under monovision conditions is considerably less than the sum of themonocular depths of focus (3)

Patients using monovision tend to exhibit a small-angle esophoric shift At distance, thismanifests as an esophoria At near, the effect is offset by the fact that presbyopes generallyexhibit a moderate to large exophoria at near The magnitude of the esophoric shift isbelieved to correlate with the degree of binocular stress created by monovision The eso-phoric shift at distance in successful monovision contact lens (0 to 0.6 prism diopters)was found to be less than the shift in unsuccessful monovision wearers (2.1 to 2.2 prismdiopters) (7,28) Interestingly, the magnitude of esophoric shift is less when the dominanteye is corrected for distance, thus lending support to the generally accepted custom ofcorrecting the dominant eye for distance (21)

Monovision appears to be associated with adverse effects on, in particular, stereoacuityand contrast sensitivity in particular The question is whether these effects have clinicalsignificance The effect of monovision on the performance of various visually orientednear tasks can be assessed by comparing an individual’s performance of these tasks undermonovision conditions, under monocular viewing conditions (i.e., with one eye covered),and under binocular viewing conditions (i.e., with full near correction for both eyes) Use

of this method revealed that monovision reduced performance of the tasks by 2 to 6%when compared to performance of the tasks under binocular viewing conditions However,this reduction was quite minimal when compared with the 30% reduction seen undermonocular viewing conditions with near tasks requiring high stereopsis (29)

On the basis of the above-mentioned findings, poor candidates for monovision are patientswho exhibit minimal interocular suppression of blur, patients with large esophoric shiftswith monovision, and patients with a significant reduction in stereoacuity with monovision.Certain psychological and personality factors also appear to play a role in determiningthe success of monovision (30)

An additional consideration is sighting preference The inputs from the two eyesare not identical in their relative influence on cortical cells: the dominant eye produces agreater response to a given stimulus than does the input from the other eye Those individu-als who do not have a strong sighting preference (i.e., who have alternating dominance)appear to have constant interocular blur suppression and therefore tend to be more success-ful with monovision Furthermore, the choice of eye that is corrected for distance, whetherthe dominant or the nondominant eye, appears to have an effect on monovision success

In 16 articles reviewed by Jain and coworkers, the average age of successful sion users ranged from 48 to 55 years (1,2) No articles were found that compared thesuccess rate in younger versus older presbyopes Two articles examined the difference

monovi-between the average age of successful versus unsuccessful monovision patients but failed

to find any statistically significant difference in age between the two groups (5,31)

Monovision has been evaluated extensively in contact lens users, but few studies comment

on its success in refractive surgery Furthermore, the impact of the magnitude of tropia created in monovision patients has not been fully characterized We have introduced

anisome-a new term, minimonovision, to chanisome-aranisome-acterize monovision panisome-atients with anisome-a lesser degree ofnear vision correction than full monovision (7) The inclusion criteria for strict monovisionand minimonovision are mutually exclusive We found that strict monovision and minimo-novision groups had comparable satisfaction rates We also found that crossed monovisionpatients overall were as satisfied as uncrossed monovision patients, and that within theminimonovision and monovision subgroups, crossed monovision did not affect satisfac-tion The rate of satisfaction for monovision after LASIK was similar to the rate in contactlens wearers

Monovision is associated with some compromises of visual function, the extent ofwhich depend on the particular individual and the requirements imposed by differentviewing conditions However, for those refractive surgery patients willing and able toadapt, these compromises constitute reasonable a trade-off for reducing dependence onnear-vision correction

Refractive surgery may be used to take advantage of the monovision option inpresbyopic refractive surgery patients However, this option should be pursued only aftercareful preoperative screening and counseling of the patient Creating a monovision situa-tion with refractive surgery constitutes a practical alternative to other surgical treatmentmodalities for presbyopia, such as scleral expansion/relaxation and multifocal cornealtreatment

4 Schor C, Erickson P Patterns of binocular suppression and accommodation in monovision

Am J Optom Physiol Opt 1988; 65:853–861

5 Schor C, Landsman L, Erickson P Ocular dominance and the interocular suppression of blur

in monovision Am J Optom Physiol Opt 1987; 64:723–730

6 Fonda G Presbyopia corrected with single vision spectacles or corneal lenses in preference

to bifocal corneal lenses Trans Ophthalmol Soc Aust 1966; 25:78–80

7 Chang MA, Kloek CE, Zafar S, Jain S, Azar DT Analysis of strict monovision and monovision LASIK surgery in presbyopes Arch Ophthalmol 2002 Submitted

mini-8 Maguen E, Nesburn AB, Salz JJ Bilateral photorefractive keratectomy with intentional eral undercorrection in an aircraft pilot J Cataract Refract Surg 1997; 23:294–296

unilat-9 Wright KW, Guemes A, Kapadia MS, Wilson SE Binocular function and patient satisfactionafter monovision induced by myopic photorefractive keratectomy J Cataract Refract Surg1999; 25:177–182

10 Wilson SE, Klyce SD, McDonald MB, Liu JC, Kaufman HE Changes in corneal topographyafter excimer laser photorefractive keratectomy for myopia Ophthalmology 1991; 98:1338–1347.

11 Moreira H, Garbus JJ, Fasano A, Lee M, Clapham TN, McDonnell PJ Multifocal cornealtopographic changes with excimer laser photorefractive keratectomy Arch Ophthalmol 1992;110:994–999

12 Harris MG, Classe JG Clinicolegal considerations of monovision J Am Optom Assoc 1988;5:491–495

13 Emmes AB A statistical study of clinical scores obtained in the Wirt steropsis test Arch AmAcad Optom 1961; 38:398

14 Collins MJ, Brown B, Verney SJ, Makras M, Bowman KJ Peripheral visual acuity withmonovision and other contact lens corrections for presbyopia Optom Vis Sci 1989; 66:370–374

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

16 Bennett ES, Henry VA Bifocal contact lenses In: ES Bennett, VA Henry, eds Clinical Manual

of Contact Lenses Philadelphia: Lippincott, 1994; 362–398

17 McLendon JH, Burcham JL, Pheiffer CH Presbyopic patterns and single vision contact lenses

II South J Optom 1968; 10:33–36

18 Sanchez FJ Monovision: which eye for near? Contact Lens Forum 1988; 13:57

19 Lebow K, Goldberg J Characteristics of binocular vision found for presbyopic patients wearingsingle vision contact lenses J Am Optom Assoc 1975; 48:1116–1123

20 Trevarthan CB Two mechanisms of vision in primates Psychol Forsch 1968; 31:299–348

21 Rigel L Which modality works best? When monovision makes sense Rev Optometry 1998;13:90

22 Coren S, Kaplan CP Patterns of ocular dominance Am j Optom Arch Acad Optom 1973;50:283–292

23 Collins M, Goode A, Brown B Distance visual acuity and monovision Optom Vis Sci 1993;70:723–728

24 Erickson P, Schor C Visual function with presbyopic contact lens correction Optom Vis Sci1990; 67:22–28

25 Wirt SE A new near-point stereopsis test Optom Weekly 1947; 38:647–649

26 Kirschen DG, Hung CC, Nakano TR Comparison of suppression, stereoacuity, and interoculardifferences in visual acuity in monovision and Acuvue bifocal contact lenses Optom Vis Sci1999:832–837

27 Pardhan S, Gilchrist J The effect of monocular defocus on binocular contrast sensitivity.Ophthal Physiol Opt 1990; 10:33–36

28 McGill EC, Erickson P Sighting dominance and monovision distance binocular fusionalranges J Am Optom Assoc 1991; 62:738–742

29 Sheedy JE, Harris MG, Busby L, Chan E, Koga I Monovision contact lens wear and tional task performance Am J Optom Physiol Opt 1988; 65:14–18

occupa-30 Du Toit R, Ferreira JT, Nel ZJ Visual and nonvisual variables implicated in monovision wear.Optom Vis Sci 1998; 75:119–125

31 Koetting RA Stereopsis in presbyopes fitted with single vision contact lenses Am J OptomArch Am Acad Optom 1970; 47:557–561

Multifocal Corneal Approach to Treat

Refractive surgery to correct presbyopia continues to be at an experimental stage despite

a decade of investigation The dilemma in presbyopia is the need for differing refractivepowers of the optical media for near versus distance vision In this chapter, we reviewseveral studies using a multifocal corneal approach to treating presbyopia The techniquesand results of the studies are presented, as well as a discussion of comparative conclusionsand areas in need of further investigation

For several decades, refractive surgery has been successfully employed in treating patientswith myopia and, later, hyperopia Nevertheless, the presbyope continues to pose a chal-lenge to refractive surgeons, owing to the need for differing optical powers for near anddistance vision In the late 1980s, investigators observed a phenomenon wherein radialkeratotomy patients achieved excellent uncorrected visual acuity at near and distance;however, this was inconsistent with the measured change in spherical equivalent (1,2).Further topographical analysis demonstrated an unintended multifocal lens effect of the

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cornea, enabling optimization of both near and distance vision through a range of opticalzones These studies offered the theoretical basis for refractive surgery to treat presbyopiausing a multifocal corneal approach The multifocal approach espouses the concept ofpseudoaccomodation, or the ability to process multiple simultaneous images at the retina(3).

Anschutz performed studies with photorefractive keratectomy (PRK) on polymethylmethacrylate (PMMA) lenses and porcine eyes to intentionally create multifocal cornealsurfaces A sectoral near zone as part of a concentric zone for distance was favored Hesubsequently applied the models in clinical trials testing the effectiveness of PRK intreating myopia-presbyopia and hyperopia-presbyopia, described below (4) In addition,Moreira et al investigated several modalities for achieving a multifocal surface (5) Fourconfigurations of ablation were compared: monofocal ablation, two concentric ablations,two ablations with the smaller diameter ablation decentered inferiorly, and a single progres-sive ablation The group concluded that a corneal surface with multiple refractive powerscould be achieved in PMMA hemispheres and blocks, as well as, in rabbit corneas Theybelieved that the single progressive ablation would be the most effective, created with aniris diaphragm initially fully open to 6 mm and progressively closing until 3 mm, leaving acentral zone with the preoperative refractive power (5) This technique would theoreticallyfunction in concert with pupillary miosis during near accommodation to decrease thepercentage of light rays traversing the flattened zone of the cornea Thus, degradation ofthe retinal image as a result of the multifocal lens effect may be reduced

In March 1999, Anschutz began human clinical trials of multifocal PRK to treat presbyopia A 193-nm Aesculap-Meditec laser was used with an iris diaphragm to bifocallysculpt the cornea (4) The investigation involved two techniques for creating zones fornear and distance vision: (1) an inferior pie-shaped sectoral near zone within a concentricdistance zone and (2) a central near zone within a concentric distance zone Both techniquesinvolved an initial circular ablation of 2 or 3 D less than the myopic baseline refraction.For technique 1, this was followed by a second ablation to the full myopic correctionusing a sectoral template Technique 2 used a central nonrotating template in a similarfashion Figure 1 demonstrates the two techniques

myopia-In treating hyperopia-myopia, Anschutz investigated the technique of an inferiorsectoral steepening of the cornea, to create active myopization for near vision (4) For theinitial hyperopic PRK ablation, a spiral eye mask (or double-heart mask) was used alongwith a rotating spiral template to correct for the complete hyperopic refraction Next, anonrotating presbyopic template with an oval aperture was inserted for the second ablation,

to create an inferior sectoral zone of an additional 2.0 to 3.0 D presbyopic correction (Fig.2)

The hyperopia-myopia study also included a subgroup of emmetropic presbyopes.Their ablations were performed with the hyperopic spiral mask and an oval template, tocreate an inferior zone of steepening (3.0 D) within a transition zone of 0.5 D (4) Theconfiguration is similar to the inferior sectoral ablation for near vision shown in Figure1

Figure 1 Multifocal myopia-presbyopia PRK Left, central near zone; right, sectoral near zone.(Adapted from Ref 4.)

In 1998, Vinciguerra et al published their study involving zonal PRK for treatingpresbyopia The group used a 193-nm Aesculap-Meditec Mel 60 excimer laser with amask consisting of a mobile diaphragm formed by a blunt concave blade and a bluntconvex blade (6) An inferior semilunar region was ablated for a presbyopic correction

of 3.00 D Within this region, the depth of cut was progressively reduced from the cornealcenter to periphery as the blades of the diaphragm progressively closed upon each other

Figure 3 PRK for presbyopia; inferior semilunar zone of ablation (Adapted from Ref 6.)

Thus, the superior pole of the ablated area served as the optical zone for near vision.Figure 3 shows the ablation zone

Multifocal laser-assisted in situ keratomileusis (LASIK) has also been employed totreat hyperopia-presbyopia Bauerberg used a 193-nm Coherent/Schwind Keratom 2 exci-mer laser with an 8.2-mm-diameter, 160-␮m-thickness corneal flap (7) Ablation depthwas calculated by adding 10% to the preoperative spherical equivalent A centrally locatedablation was tested as well as an inferiorly decentered ablation (by 1 mm)

The myopia-presbyopia PRK study by Anschutz involved 46 eyes of 23 patients withfollow-up of 21⁄2to 3 years (4) The preoperative refractions ranged fromⳮ2.0 to ⳮ12.0D; patients were divided into three groups: group 1 (ⳮ2.0 to ⳮ6.0 D), group 2 (ⳮ6.25

toⳮ10.0 D), and group 3 (ⳮ10.25 to ⳮ15.0 D) The goal was a presbyopic correction

of 2.0 to 4.0 D At 30 months, group 1 had a mean regression of ⳮ0.75 D, with anuncorrected near visual acuity (VA) of 20/22, which was three lines better than for themonofocally treated eyes Group 2 had a mean regression ofⳮ1.5 D and an uncorrectednear VA of 20/25 Group 3 had a mean regression ofⳮ4.0 D, with uncorrected near VAonly 11⁄2lines better than with monofocal ablation Overall, greater regression occurredwith greater preoperative myopic refraction, and multifocal PRK results were identical tothose of monofocal PRK in patients with preoperative refraction greater thanⳮ6.0 D Interms of postoperative complications, the investigators found 2 cases of loss of best cor-rected VA due to decentration; 1 case of wound-healing difficulties; and 2 cases of dimin-ished near VA due to pupil sizes less than 2 mm Glare and halo effects were also present

in patients for only the first 6 months postoperatively In addition, 20% of patients enced “ghost pictures” and double contours for the initial 3 to 4 months postoperatively.Frequent complaints of monocular diplopia occurred in patients who received a centralnear zone ablation

experi-A total of 18 eyes with follow-up of 16 to 20 months were studied by experi-Anschutzfor PRK treatment of hyperopia-presbyopia At 18 months, patients with preoperativerefractions ofⳭ1.0 to Ⳮ4.75 D showed a mean regression of Ⳮ1.5 D and a mean near

VA of 20/30 (4) Mean regression wasⳭ3.5 D for patients with preoperative refractions

ofⳭ5.0 to Ⳮ8.0 D, with a postoperative mean near VA of 20/50 The investigators found

a small loss of contrast sensitivity over 7 months In addition, haze was greater in patientswith higher degrees of hyperopia Some 30% of patients complained of ghost images and10% complained of double contours; with all complaints resolved by 6 months postopera-tively Two cases of decentration occurred during the study Last, the improvement in VAoccurred very slowly in the higher-diopter group secondary to a small optical zone (4mm)

Four emmetropic eyes were also treated for presbyopia during the same study rected near VA at 18 months follow-up was 20/30, with postoperative spherical equivalentchange ofⳮ0.25 to ⳮ0.75 D (4)

Uncor-The Vinciguerra et al study of PRK for presbyopia treated three patients with afollow-up period of 24 months A regression of 1.00 D occurred, followed by stabilization

of the presbyopic correction (6) The patients read Jaeger 3 at 35 cm without near correctionand were also able to read with their preoperative presbyopic correction using the 85%

of the pupillary area that was not treated by PRK A mild haze was reported in the firsttwo postoperative months Loss of contrast sensitivity only occurred with the 11% Reganchart Videokeratography of a treated eye is shown in Figure 4

preoperative; upper right, 3 days postoperative; lower left, 1 month postoperative; lower right, 1year postoperative Immediate postoperative corneal steepening was almost 6.00 D By 1 month,the presbyopic correction was within 0.25 D of the planned 3.0-D correction, remaining stable at

1 year At 1 year, there is also a slight nasal decentration of 0.63 mm (From Ref 6)

The Bauerberg LASIK study for hyperopia-presbyopia involved 16 eyes of 8 patients(8 eyes with centered ablation and 8 eyes with off-center ablation) with maximum follow-

up of 22 months (7) Preoperative refractions ranged fromⳭ2.0 to Ⳮ6.0 D At 12 months,the off-centered ablation eyes achieved uncorrected near VA of 20/30 or better with noloss of Snellen lines The centered ablation eyes had uncorrected near VA of 20/40 orbetter with two eyes experiencing loss of 1 Snellen line No glare was reported; however,

2 eyes had induced astigmatism Subjectively, 6 patients preferred the eccentric inferiorablation for near vision, while 2 noticed no difference

A summary of results is presented in Table 1

The multifocal corneal approach to treating presbyopia remains experimental, althoughthe technology and techniques of multifocal corneal sculpting have been well investigated.The four studies reviewed had the drawback of low patient numbers; however, follow-uptime was considerably meaningful A most important reason for studies of greater magni-tude is the need to delineate the incidence of postoperative side effects such as glare,halos, ghost images, and monocular diplopia The reviewed studies reported that symptomssuch as halos, glare, and ghost images were present for a short period of time in the limitedpatient groups The presence of these symptoms was perhaps due to a temporary transitionperiod during which wound healing and adjustment to multifocal images occurred Stability

of the presbyopic corrections and their relationship with the natural progression of opia should also be followed long-term

presby-Each of the studies revealed certain limitations for presbyopic multifocal refractivesurgery as well as benefits for particular techniques For myopia-presbyopia, Anschutzconcluded that their inferior sectoral near zone is appropriate for patients with pupilsgreater than 2 mm diameter (4) Additionally, for patients with pupils larger than 3 mm,the central near zone may be advantageous in the case of dominant near vision, particularly

in the presence of high myopia (⬎ⳮ10.0 D) The investigators also concluded that currentmultifocal hyperopia-presbyopia PRK is effective only for patients with a baseline refrac-tion of less thanⳭ5.0 D due to the high degree of regression and the small optical zonefor those with refractions greater thanⳭ5.0 D (4) Anschutz cites a need for improvement

of the transition zone and aspherical reprofiling of the cornea in addition to simplifyingthe technique to require only one template

Vinciguerra et al found that the zonal presbyopic correction was effective for pupils

up to 6 mm (6) Advantages of their technique include a pupillary center with intactepithelium acting as a protective shield against decentration during photoablation and theneed for a very superficial (10 to 15␮m) and small ablation zone Because only 15% ofthe light entering a 3 mm pupil traverses the treated zone, contrast sensitivity was notsignificantly reduced Nevertheless, Vinciguerra notes that the technique requires extremeprecision to avoid erroneously aligned ablation and suboptimal presbyopic correction Inaddition, for pupils greater than 6 mm, the technique may lead to an inadequate presbyopiccorrection

LASIK for presbyopia was shown to have stable postoperative results with minimalrecovery time and complications in a small study by Bauerberg (7) However, he notesthat longer follow-up is needed He concluded that the preferred orientation of the presby-opic ablation should be inferior off-center It may be worth considering the implicationsthat an asymmetrical corneal contour has for flap orientation and healing To our knowl-

edge, no studies have been undertaken to compare multifocal PRK and LASIK in thetreatment of presbyopia.

Last, pupillary size appears to play a significant role in the effectiveness of multifocalrefractive surgery, as shown by the aforementioned studies Another aspect that may re-quire investigation is the positioning of a individual’s upper and lower lid margins withrespect to the treated zone during distance and near vision Could a raised lower lid positioncover up a zone of inferior corneal steepening such that the presbyopic correction has nobenefit? Conversely, could the same positioning protect the patient’s vision from visualdistortion by covering up a rough transition zone?

3 Talamo J, Krueger R eds The Excimer Manual Boston: Little, Brown, 1997:106

4 Anschutz T Laser correction of hyperopia and presbyopia Int Ophthalmol Clin 1994; 34(4):107–137

5 Moreira H, Garbus JJ, Fasano A, Lee M, Clapham TN, McDonnell PJ Multifocal cornealtopographic changes with excimer laser photorefractive keratectomy Arch Ophthalmol 1992;110(7):994–999

6 Vinciguerra P, Nizzola GM, Bailo G, Nizzola F, Ascari A, Epstein D Excimer laser tive keratectomy for presbyopia: 24-month follow-up in three eyes J Refract Surg 1998; 14(1):31–37

photorefrac-7 Bauerberg JM Centered vs inferior off-center ablation to correct hyperopia and presbyopia JRefract Surg 1999; 15(1):66–69

Scleral Relaxation to Treat Presbyopia

in the anterior and posterior zonules The result is an active elongation of the lens diameterwith peripheral thinning and central thickening due to dynamic internal volume changes(Fig 2) The net result is increased plus refracting power of the eye

The important difference between the Helmholtz model and the Schachar model isthat the latter suggests a more active interaction between the ciliary muscle and the lens/zonule complex, positing an interaction in which active effort by the ciliary muscle leadsnot only to passive relaxation of the lens/zonule complex but also a more complicatedactive differential response of different zonular types resulting in morphological changes

in the lens

If this recent model of accommodation is correct, then presbyopia may not be plained by simple sclerosis of the lens fibers and capsule as previously understood Rather,the decline in accommodative power of the eye may be due to the inability of the lensequator to expand into the posterior chamber Thornton (5) has described this as “a crowd-ing” of the lens in the posterior chamber as the lens grows

ex-209

Figure 1 Helmholtz’s model of accommodation.

The lens is ectodermal in origin and grows throughout life, increasing in size in alldimensions The sclera, on the other hand, is mesodermal in origin and ceases growth inand around puberty There is a discontinuity in growth between the ectodermal lens andmesodermal scleral shell that begins around puberty The result is an increase in thediameter of the lens and a gradual, progressive narrowing of the space between the lens