British Journal of Ophthalmology 1996;80:794-798

Visual acuity in unilateral cataract

D A Thompson, H M0ller, I Russell-Eggitt, A Kriss

AbstractBackground-Patching the fellow eye ininfancy is a well recognised therapy toencourage visual development in thelensectomised eye in cases of unilateralcongenital cataract. The possibility ofiatrogenic deficits of the fellow eye wasinvestigated by comparing the vision ofthese patients with untreated unilateralpatients and binocularly normal controls.Methods-Sweep visual evoked potentials(VEPs) offer a rapid and objective methodfor estimating grating acuity. Sweep VEPswere used to estimate acuity in 12 childrenaged between 4 and 16 years who had hada congenital cataract removed in the first13 weeks of life. The acuities of aphakicand fellow phakic eye were compared withthe monocular acuities of similarly agedchildren who have good binocular vision,and with children with severe untreateduniocular visual impairment. Recognitionlinear acuities were measured with alinear Bailey-Lovie logMAR chart andcompared with the sweep VEP estimates.Results-A significant difference wasfound between Bailey-Lovie acuity of thefellow eye of the patient group and theright eye of binocular controls, and thegood eye of uniocular impaired patients(one way ANOVA, p<0.01). However, thiswas not evident for a similar comparisonwith sweep VEP estimates. There was nosignificant difference between the rightand left eye acuities in binocular controlsmeasured by the two techniques (paired ttest).Conclusion-A loss of recognition acuityin the fellow phakic eye of patients treatedfor unilateral congenital cataract has beendemonstrated with a logMAR chart. Thisloss was not apparent in children who havesevere untreated uniocular visual impair-ment and may therefore be an iatrogeniceffect of occlusion. An acuity loss was notapparent in the patient group using thesweep VEP method. Sweep VEP tech-niques have a place for objectively study-ing acuity in infants and in those whosecommunication difficulties preclude otherforms of behavioural test. The meansweep VEP acuity for the control groups is20 cpd-that is, about 619. When acuitieshigher than this are under investigation-for example, in older children, slowertransient VEP recording may be moreappropriate, because higher spatial fre-quency patterns are not as visible athigher temporal rates (for example, 8 Hzused in sweep VEP recordings).(BrJ Ophthalmol 1996;80:794-798)

Visual acuity is the most commonly used clini-cal measure of visual function. Most ophthal-mological centres assess linear letter recogni-tion acuity in adults, and single letter or picturerecognition acuity in preschool children. Reso-lution acuity is assessed in younger, preverbalinfants with forced choice preferential lookingtechniques using gratings or vanishingoptotypes.'- These tests depend upon theidentification of looking behaviour which canbe particularly difficult to determine when achild is active or has an oculomotordisturbance-for example, oculomotor apraxiaor nystagmus. The possibility of an objectiveand rapid assessment of vision using visualevoked potentials (VEPs), therefore, is particu-larly attractive for the paediatric population.

It is known that resolution and recognitionacuity can be differentially affected by patho-logical visual degradation. By stimulatingperipheral retina, preferential looking acuitycan underestimate the depth of amblyopia in astrabismic eye, or in a case of foveal abnormal-ity.45 The VEP predominantly reflects activa-tion of the macular pathway and the fovealrepresentation at the occipital pole. It can pro-vide an objective indication of visual pathwayfunction and theoretically could provide amore accurate indication of interocular acuitydifferences. Transient VEPs to pattern reversalor onset stimulation have been used in the pastto assess the quality of vision."' Subtleties oftransient waveform interaction'2 are hidden inthe steady state VEP as the responses mergeand appear like a sine wave. However, thesteady state VEP offers the strong advantageover transient recording in that the much fasterresponse acquisition time suits the limitedattention span of an infant. The sweep spatialfrequency VEP is a steady state response to arange of spatial frequencies sequentially pre-sented with fast counterphase, typically be-tween 8 to 16 reversals per second." 4 Anextrapolation to zero response amplitude givesan estimate of visual resolution. We have com-pared the swept spatial frequency VEP acuityestimates, made in older children with unilat-eral aphakia, with those of normally sightedcontrols.There are few patients with unilateral congeni-

tal cataract in the literature who have achieved'good' (6/6-6/24) visual acuity in the aphakiceye."'MThe success in these cases appears todepend upon 'early' cataract extraction and opti-cal correction-that is, within 12 weeks, andupon the degree of compliance patching thefellow eye. Patching of the good, phakic eye is anessential part of the amblyopia treatment in uni-lateral aphakia. Occlusion per se is amblyogenicand it is not clear at present what long term effectit is having on the fellow eye.

Department ofOphthalmology, GreatOrmond StreetHospital for ChildrenNHS Trust, LondonD A ThompsonH MollerI Russell-EggittA Kriss

Correspondence to:Dr D A Thompson.

Accepted for publication24 May 1996

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Visual acuity in unilateral cataract

n = 21

I n=s n=9

nI=5--

ity estimates obtained by a sweep spatialfrequency VEP technique. These children hadfollowed an occlusion regime of patching thephakic eye for 50% of waking hours in the firstyear of life, irrespective of interocular acuitydifferences. This study has enabled us to com-pare sweep VEP acuity with recognition acuityin older children and to assess its utility in aclinical population who may have amblyopiaand may have latent nystagmus.

v0 2

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3

Corrected age (years)

Figure 1 Comparison of binocular acuities measured in a group of clinically normallysighted infants using Keelerforced choice preferential looking and sweep visual evokedpotential (VEP) techniques. The sweep VEP method overestimates acuity in the first year,the agreement is closer over the next 2 years.20 (There is a significant correlation betweenthe two measures accountingfor 60% of the variance.)

We currently use interocular resolution acu-

ity differences measured by forced choice pref-erential looking (FCPL) techniques to modu-late the patching regime in the clinical followup of preverbal infants with unilateral apha-kia."' We established the relation between theacuity estimates made with FCPL and sweep

VEP acuity estimates, obtained with a vENusNeuroscientific program, in a group of 35 oph-thalmically normal infants aged 4 weeks to 3years,20 (Fig 1). In agreement with otherauthors we found that the sweep estimateexceeded the PL estimate in infants under 1year of age, but became closely correlated afterthis time.2' In older children and adults prefer-ential looking acuity is correlated with opto-type acuity, but overestimates typically by 1-2lines of Snellen (about 1 octave).22 SweepVEP estimates of acuity have also beencorrelated with recognition optotype acui-ties.2324 In a retrospective study Gottlob et al 25suggested that the sweep VEP estimates madein infancy were predictive of single optotypeacuity in early childhood.We have become interested in the visual

function of the phakic eye in infants followingunilateral cataract removal.26 Patching duringthe developmentally sensitive period places thefellow eye at risk of occlusion amblyopia. Thisbecomes particularly relevant as surgical inter-vention may now occur within days of birth.Deleterious effects of patching on visualperformance of the fellow eye, however, are

considered rare and to be associated with fulltime occlusion.27 28 Such effects, measured witheither VEP techniques2930 or preferential look-ing,3' have been claimed to be reversible.Indeed there is some debate as to whetherthere is an acuity deficit in the fellow eye

following unilateral cataract removal andwhether this is iatrogenic.1826 32-35

To address this issue we have measuredlinear recognition acuity, with a Bailey-LovielogMAR chart, in a group of older childrenwho had unilateral congenital cataracts re-

moved as infants, and compared this with acu-

MethodsThe letters on a Bailey-Lovie linear letter rec-

ognition test,36 have approximately equivalentlegibility, and the letter size and line spacingvary systematically in a logarithmic progres-sion. Visual acuity can be recorded as a

logMAR score (minimum angle of resolution):each letter read has a score of 0.02 logMARwhich contributes to the line score of 0.1. Weused a matching template for two of thechildren in our sample-both were 4 years old.The template was large enough and the letterspacing such that the ceiling for acuity was thewall chart and not the match template.

Swept spatial frequency VEPs were meas-

ured from an active electrode placed at Ozreferred to Cz; an electrode at Fz was

connected to the earth. This montage gaveVEPs of good size and compared with a refer-ence sited at Fz, reduced the contamination ofraw data by eye movements and periocularmyogenic activity. The amplifier bandpass was

set at 0.1-100 Hz. Horizontal black and whitesine wave gratings, of 80% contrast, were

counterphases at 8Hz (16 reversals per sec-

ond). Closed circuit television was used tomonitor fixation throughout the recording. Asweep was stored only if fixation was deemedto have occurred throughout the 8 secondpresentation period. If fixation deviated, atten-tion was drawn again to the stimulus and thesweep restarted. Individual sweeps were storedand at least four sweeps vector averaged forsubsequent analysis. A horizontal gratingorientation was selected in order to minimisethe adverse effects of nystagmus." Everysecond the pattern size became smaller andtypically eight pattern sizes were presentedwithin the range 0.5-36 cpd. The second har-monic of the response was extracted bydiscrete Fourier transform, and its amplitudeand phase displayed as a function of spatial fre-quency. Linear regression was used to extrapo-late from the highest amplitude response to thezero amplitude baseline. This intersection withthe baseline determined the sweep VEPestimate of acuity. Phase data are used todistinguish reliable amplitude data. The phaseis expected to show a progressive lag as spatialfrequency increases and to have narrow

confidence limits. Figure 2 is an example of thegraphical display of the data and the regressionin cpd (cpd/30 gives the Snellen equivalent).The VEP amplitude tends to be large in younginfants and to diminish towards adulthood,with pathological factors such as amblyopiaand nystagmus. Extrapolation to a zero ampli-tude is a relative measure of amplitude change

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6Thompson, M0ller, Russell-Eggitt, Kriss

Aphakic eye.5

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Spatial frequency

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Figure 2 An example of the sweep data to illustrate the regression estimate of acuity in a 6-year-old patient who had a leftcataract extraction aged 8 weeks. His Bailey-Lovie linear acuity is right eye 0.26, left eye 0.56, approx 6110 right eye and6/24 left eye.

and the acuity estimate is mostly independentof absolute VEP amplitude.

SubjectsTwelve children, aged between 4 and 16 yearsof age, who had been treated for unilateralcongenital cataract were recalled. The childrenhad been operated on in the first 13 weeks oflife and the fellow phakic eye was patched for atleast 50% of waking hours in the following9-12 months. The fellow eye was deemedclinically normal on ophthalmic examination.All of the patients had squints and 50% hadlatent nystagmus which was evident on electro-oculogram and video recording. The patientgroup was compared with two groups of simi-larly aged children. The first were 12 binocu-larly normal children (mean age 9.3 years),'the binocular control group', the secondgroup was comprised of nine children, (meanage 8.5 years), who had a severe untreated uni-lateral condition (including six with persistenthyperplastic primary vitreous (PHPV) one

dense cataract, one unilateral retinoblastoma,and one unilateral optic atrophy, 'the uniocularcontrol group'.

ResultsSweep VEP acuity estimates were expressed as

logMAR (minimum angle of resolution) fordirect comparison with the Bailey-Lovie data(Snellen = d/D, MAR = D/d, cpd 30/MAR). Inthe binocular control group there was no

significant difference between the acuities ofright and left eyes, measured by eithertechnique and the right eye estimates were

compared with the fellow eye of the othergroups. Phakic eye acuity was significantly bet-ter that aphakic eye acuity in the patient group(paired t test p<0.00l for both sweep VEP esti-mates and Bailey-Lovie acuity). Mean recogni-tion acuity for the fellow eye was 0.26 logMAR

6/10 and aphakic eye acuity was 1.3 logMAR= 6/120. Sweep fellow eye acuity was 0.24 log-MAR _ 6/10 and aphakic eye acuity was 0.64logMAR _ 6/30. A comparison of Bailey-Lovieestimates from phakic, fellow eyes with controleyes demonstrated a significant difference (oneway ANOVA, p<0.01). This was not evidentfor similar comparisons using the sweep VEPestimates of acuity. Figure 3 highlights themean acuity difference for patient and controlgroups. This also demonstrates that latent nys-tagmus was not confounding the results,because removal of these data from the patientgroup reveals similar group mean differences.Sweep VEP acuity estimates and Bailey-

Lovie acuity were correlated significantly whendata from all groups were considered; R'=0.4,the gradient of the regression was 1.5 (Fig 4).Thus when Bailey-Lovie acuity is high thesweep acuity estimate is lower. This discrep-ancy between the two forms of testing is alsoseen when the effects of defocus are assessed.That the sweep VEP is vulnerable to defocusindicates that pattern/contrast sensitive mecha-nisms are involved in producing the electro-physiological response, but the rate of acuityfall off with blur is less than for subjective test-ing with recognition acuity, suggesting thatsweep acuity is less sensitive to high spatial fre-quencies."8 Figure 5 illustrates representativedata from one experienced adult observer anddemonstrates the effects of defocus on bothforms of acuity test.

DiscussionOur findings demonstrate that fellow, phakiceye acuity measured with a linear Bailey-Loviechart is reduced significantly compared withcontrol eyes of binocular subjects and un-

treated monocular subjects, most ofwhom hadsimilar conditions; the majority had PHPV.The chart design tends to diminish effects of

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797Visual acuity in unilateral cataract

T

IFellow Fellow (no nystagmus)

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B control

Figure 3 A graphical display of the mean Bailey-Lovie letter anpotential (VEP) acuity (+1 SEM) for all groups treated. This illt

difference between Bailey-Lovie letter acuity in the fellow eyes ofamonocular acuities of the control groups. The sweep VEP estimatesdifferent between the groups. The groups are referred to as follows:.all patients;fellow (no nystagmus) (n=6) excludes datafrom pati

nystagmus;B control (n=12) right eye estimatesfrom binocularlycontrol (n=9) visual acuity from the good eye of children with sev

dysfunction.

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The gradient of the regression is 1

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measures when the effects of defomexperienced adult observer.

test scoring criteria (for e)of acuity if the majoritywhich could hide a subtlerithmic progression of le

sampling gaps inherent iBailey-Lovie acuity is a rerequires the perception (

frequencies. In contrast, the sweep VEP useshigh temporal frequencies which are subopti-mal for the highest spatial resolution, but allow

ailey-Lovie easier detection of low spatial frequencies.39 Aweep VEP high spatial frequency deficit therefore may not

be apparent in the sweep VEP estimatesdespite its predominantly macularrepresentation at the striate cortex. The lessereffect of defocus on sweep VEP acuity is read-ily understandable in this context.

It is valuable to know the extent and robust-ness of the relation between objective and sub-jective acuity testing particularly when testingpaediatric populations, or older children andadults who cannot respond conventionally. Asignificant correlation was found between

U control sweep VEP estimate and Bailey-Lovie acuity

Id sweep visual evoked accounting for 40% of the variance in the data.4strates the significant The sweep VEP estimates tend to be loweriphakic patients and the than Bailey-Lovie acuity when the latter isare not significantly

fellow (n=12) datafrom high, but tend to be higher at lower Bailey-ents exhibiting latent Lovie acuities. This implies that the sweep esti-normal controls; U mate is slightly more resistant to amblyogenic,ere unilatral ocular factors-that is, an amblyopic deficit is not

revealed or present at high temporal rates inolder children."4 ' Crowding occurs in normalindividuals, but is more pronounced in am-blyopia and a higher recognition acuity may bemeasured with single letter or symbol presenta-tion. This may account for the greater correla-tion reported when single letter recognition

/°D acuity has been compared with sweep VEPestimates.25 In younger (under 3 years), clini-

o cally normally sighted children a greater corre-

R2 0.4 lation is obtained when sweep VEP estimate is|I i compared with spatially similar minimal re-

1 2 3 solvable grating PL acuity."icuity (logMAR) It is possible that the good progress initially

n the two acuity measures reported in the early years following removal ofdfor 40% of the variance. unilateral congenital cataract may be a conse-.*5. quence of the relatively low behavioural acuity

of infants, coupled with the methods of acuityassessment and the wide range of 'normal'acuity, sometimes spanning 2 octaves. The fall

m / s off in success, or asymptotic visual develop-ment,4' as the infant matures may be attributedin part to the occurrence of strabismus and

o latent nystagmus at a time when acuity testsD / which are more sensitive to the high spatial fre-

quency deficits associated with amblyopia canbe used.

Latent nystagmus is not normally apparentunder binocular viewing conditions, but devel-

* Bailey-Lovie ops during monocular viewing. It appears to beo Sweep VEP characterised by an absence of binocular vision

development during the sensitive period ofX I binocular function.4 Latent nystagmus was2 3 4 noted in 50% of the clinical population

tric blur studied, but removal of these data did not alter

lissociation of acuity the difference in mean acuity between fellowcus are studied in an eyes (Fig 3) In the uniocular control group

those who had profound uniocular visual dep-rivation from birth were noted to have

Kample, scoring a line symmetrical optokinetic nystagmus (OKN). Inof letters are read), contradistinction, OKN was asymmetrical fordeficit and the loga- the uniocular aphakic group; retaining the

tter size removes the immature characteristic of a weaker responsein the Snellen range. in the nasotemporal direction. It appears that aicognition task which lack of early interocular rivalry may determineAf static, high spatial symmetrical OKN."

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7Thompson, Moiler, Russeil-Eggitt, Kriss

ConclusionBailey-Lovie recognition acuity is reduced inthe fellow eye of patients operated on foruniocular congenital cataract compared withcontrols. When compared with sweep VEPacuity estimates a significant correlation isfound, but the gradient of the regressionindicates that at lower Bailey-Lovie acuity thesweep acuity tends to be higher. Sweep VEPsacuities for the right and left eyes of a normallysighted individual show a close correspond-ence, within 1 octave. This lends support forthe use of the technique as an objective moni-tor of interocular acuity difference and am-

blyopia therapy, particularly in the preverbalinfant and child. In older children the hightemporal rate used in sweep VEP recordingmay diminish its sensitivity at detecting highspatial frequency losses.

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