BCOVS and AVA Abstracts 2002:Defective Vision
British Congress of Optometry and Vision Science (BCOVS) and Applied Vision Association (AVA)
Joint Conference, Anglia Polytechnic University, East Road, Cambridge, UK, 910 July 2002
Steady-state VEP and behavioural measures of visualacuity in infants and children with Down syndrome
Ff. M. John,a N. R. Bromham,a T. R. Candyb andJ. M. Woodhousea
aDepartment of Optometry and Vision Sciences,Cardiff University, Cardiff CF10 3NB, UKbSchool of Optometry, Indiana University,Bloomington, IN 47405, USA
Purpose: Children with Down syndrome (DS) have reduced beha-vioural visual acuity (VA). This may reect sensory decits, oralternatively, a loss of performance in later mechanisms responsible forbehavioural responses. This study compared acuity measured usingvisual-evoked potentials (VEP), with behavioural tests, in children withDS. The goal was to determine whether reduced VA could be detectedin VEP recordings from the rst stages of the response pathway.Methods: The subject group contained 34 children with DS and 35
controls, aged 3 months to 14 years. VA was measured using steady-state, swept VEP (Norcia and Tyler, 1985). VA was also measuredusing behavioural techniques.Results: Visual acuity was analysed using a subject group (DS vs
controls) test type (behavioural vs VEP) ANOVAANOVA with age as acovariate. A signicant effect of subject group was observed(F1,59 8.632, p < 0.001) and a signicant interaction between subjectgroup and test type (F1,59 5.169, p 0.027). The DS group showedreduced VA compared with the controls in both VEP and behaviouraltests, but the decit was more pronounced with behavioural testing.Reduced VA in the DS group was still seen when analysis was restrictedto children who accommodate accurately (F1,26 8.047, p 0.009).Conclusions: Reduced VEP acuity in the DS group supports the idea
of an underlying sensory decit in the DS visual system. Increasedreduction with behavioural techniques implies additional losses at laterstages of processing. Accommodative inaccuracy does not fully explainreduced VA in DS. Children with DS who do not have signicant levelsof defocus still exhibit reduced VA when compared with controls.
Norcia, A. M. and Tyler, C. W. (1985) Spatial-frequency Sweep VEP visual acuity during the 1st year of life. Vision Res. 25, 13991408.
Does accommodation inaccuracy matter in childrenwith Down syndrome?
R. E. Stewart,a J. M. Woodhouse,a M. Cregg,a
V. Pakeman,a H. Gunter,a L. D. Trojanowska,a
M. Parkera and W. I. Fraserb
aDepartment of Optometry and Vision Sciences, CardiffUniversity, Cardiff CF10 3NB, UKbWelsh Centre for Learning Disabilities, University ofWales College of Medicine, Cardiff CF14 3BL, UK
Purpose: Almost 57% of children with Down syndrome have signi-cant refractive error (hypermetropia +3.00 D, myopia )1.00 D)
(Woodhouse et al., 1997); they are 17 times more likely to havesignicant long or short sight than their typically developing peers.Also, for the age range 4.5 months to 11.1 years, 34% of children withDown syndrome have strabismus, compared with 47.5% of controlchildren (British College of Optometrists, 199194). In addition, 82%of our cohort of children with Down syndrome (n 100) havereduced accommodation at near. In order to investigate theinuence of accommodation accuracy on refractive error and strabis-mus, this study compared the above parameters in children withDown syndrome who accommodated accurately with those whodid not.Methods: Accommodation was measured using dynamic retinosco-
py technique at testing distances of 25, 16.6 and 10 cm. An accom-modative lag of plano to 0.75 D is normal for typically developingchildren (Rouse et al., 1984). Accurate accommodation for our cohortwas therefore dened as 0.75 D lag of accommodation (at aminimum of two of three testing distances). Children with Downsyndrome who accommodated accurately were age matched with achild with inaccurate accommodation, based on their most recent visitfor which a full data set was available.Results: Chi-square analysis of the data for the two groups revealed
that there was a greater number of children with signicant hyper-metropia (p 0.003) and strabismus (p 0.003, Fishers exact test) inthe under-accommodating group.Conclusion: Children with Down syndrome who under-accommo-
date are more likely to be strabismic and have signicant hyper-metropia.
Rouse, M. W., Hutter, R. F. and Shiftlett, R. (1984) A normativestudy of the accommodative lag in elementary school children. Am.J. Optom. Physiol. Opt. 61, 693697
British College of Optometrists (199194). Childrens Visual Problems.Radcliffe Medical Press, Oxford, UK.
Woodhouse, J. M., Pakeman, V. H., Cregg, M., Saunders, K. J.,Paker, M., Fraser, W. I., Sastry, P. and Lobo, S. (1977) Refractiveerrors in young children with Down Syndrome. Optometry VisionScience. 74, 844851.
Vision status of children with oculocutaneous albinism
A. O. Oduntan,a M. Raliavhegwaa and P. M. Lundb
aDepartment of Optometry, University of the North,Private Bag X1106 Sovenga 0727, South AfricabCell and Molecular Biosciences, School of Scienceand Environment, Coventry University, Priory Street,Coventry CV1 5FB, UK
Purpose: Black South African children with oculocutaneous(OCA) albinism were examined optometrically to establish the levelof vision improvement that could be achieved following opticalcorrection.Methods: The children (N 153) (males 50.3%; females 49.7%),
aged 717 years (mean 10.76 2.25 years) were examined with alogMAR VA chart, cover tests, retinoscopy (dry), subjective refraction,
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Vistech contrast sensitivity test, ophthalmoscopy, Randot stereotest,Ishihara and Farnsworth Panel D-15 tests.Results: Many (34.6%) of the children had strabismus, 225 prism
dioptres. Uncorrected distance VAs were: OD: Finger counting (FC)to 6/7.5 ) 2, OS: 6/7.5 + 2 to 6/7.5 ) 3, OU: 6/60 ) 1 to 6/7.5. Most(67.6%) of the children had myopia. Others (30.8%) had hyperopia oremmetropia (1.7%). Astigmatism was present in 92.25% of thechildren. The nearest equivalent spherical powers were: OD: )12.00to +2.75 D (mean )1.48 2.28 D) and OS: )8.00 to +5. 75 D(mean 1.36 2.18 D).Following optical correction, VA improvement ranged from one
to three lines in 71.2% of the children. The corrected VAs were, ODFC to 6/6 ) 1, OS 6/7.5 + 2 to 6/6 ) 1, and OU 6/60 to 6/6. Many(84.3%) of the children had corrected VA worse than 6/18 in thebetter eye and were therefore classied as partially sighted or blind(VA worse than 3/60). Stereoacuity was poor (50070 sec arc) inmost (89.5%) of the children, whilst others could not perceive theminimum 500 sec arc on the test. Contrast sensitivity was poor,maximum spatial frequency being 18 cpd with peak sensitivity at3 cpd. Many (83%) of the children, however, had normal colourvision.Conclusion: These children with OCA had poor VA, which could
be improved signicantly with optical correction. Their poorcontrast sensitivity and depth perception, however, were notimproved. Most of the children with OCA had normal colourvision.
Relative letter legibility of Punjabi optotypes
A. Sailoganathan, J. Siderov and E. Osuobeni
Department of Optometry and Ophthalmic Dispensing,Anglia Polytechnic University, East Road, CambridgeCB1 1PT, UK
Purpose: To investigate relative letter legibility of Punjabi optotypes.Methods: In experiment 1, the inuence of font type on letter
legibility was studied on nine subjects optically defocused using a+1.00 DS lens in front of one eye. Two Punjabi font types used inwidely read Punjabi language newspapers and magazines wereselected. Legibility was determined according to the method ofStrong and Woo (1985). In experiment 2, a single Punjabi font(selected from experiment 1) was used to investigate relative letterlegibility in 10 subjects who were optically defocused with a+1.00 DS, +2.00 DC, axis 180 and 90. The Punjabi letters weremodied to t into a 5 5-grid using Coral draw and presented ona standard PC monitor at high contrast using Powerpoint. Themethod of Strong and Woo (1985) was used to determine relativeletter legibility.Results: The results of experiment 1 showed no effect of font type
on letter legibility (ANOVAANOVA, p > 0.05). The results of experiment 2were analysed in two ways. Firstly, ANOVAANOVA revealed an interactionbetween blur condition (sphere, cyl axis 90, cyl axis 180) and letterlegibility (p < 0.05). Post-hoc analysis (analysis of main effects andTukey HSD test) showed a signicant effect of letter legibility onlyunder the +2.00 DC axis 90 blur condition (p < 0.05). Secondly,the percentage difference in relative legibility under cylinder blur(axes 180 and 90) was plotted against the legibility under sphericalblur. We selected 11 letters of equal legibility on the basis of adifference in relative legibility for each cyl condition of no morethan 10% of the value of the spherical blur condition.Conclusion: Relative legibility of Punjabi letters was determined
resulting in the selection of 11 letters. These letters will form the basisof a new, standardised vision chart in Punjabi.
Strong, G. and Woo, G. C. (1985) A distance visual acuity chartincorporating some new design features. Arch. Ophthalmol. 103,4446.
Accuracy of wavefront measurement and the potentialeect of erroneous custom corneal correction on themodulation transfer function
L. Diaz-Santana,a N. Daviesb and D. Lara-Saucedoc
aApplied Vision Research Centre, Department ofOptometry and Visual Science, City University,Northampton Square, London EC1V 0HB, UKbDepartment of Ophthalmology, The MiddlesexHospital, Mortimer Street, London W1T 3AA, UKcPhotonics Group, Physics Department, ImperialCollege of Science, Technology and Medicine, LondonSW7 2BW, UK
Purpose: To assess the accuracy of repeated measurements of ocularaberrations using wavefront sensing in a small group of observers andto assess the potential effect of measurement error on custom cornealcorrection.Methods: A ShackHartmann wavefront sensor was used to meas-
ure the ocular wavefront in nine eyes. Head position was stabilisedusing a dental bite bar and the pupil centred using a CRT monitor andcircular grating. Twenty ShackHartmann images were collected foreach measurement. Each observer had three sets of measurementstaken; the rst and the second after careful alignment and the nalafter regrasping the bite bar in the same position as for the secondmeasurement, but without pupil realignment. The modulation transferfunctions for each set were calculated and the effect of ideal customtreatments on the modulation transfer function (MTF) was estimated.Results: There were highly statistically signicant differences in a
large number of Zernike modes between the three sets of measurements.TheMTFs calculated for the residual wavefronts following ideal customtreatment were below the diffraction limit. The RMS wavefront errorswere consistently better for the residual wavefronts obtained using therealigned data than using data taken without pupil realignment.Conclusions: Sequential measurement of ocular aberrations shows
statistically signicant differences in a large number of Zernike modes.If aberrations determined by a single measurement are to be used in acustom correction the resulting modulation transfer function is likelyto remain below the diffraction limit. Pupil realignment is critical inreduction of the residual root mean square wavefront values to aminimum.
On-eye spherical aberration of soft contact lensesand eective lens power
H. H. Dietze and M. C. Cox
Department of Optometry, University of Bradford,Bradford BD7 1DP, UK
Purpose: Soft contact lenses (SCL) produce a signicant level ofspherical aberration (SA). A simple model assuming that a thin SCLaligns to the cornea predicts that these effects are similar on-eye andoff-eye. We investigate the effect SA has on the power of an SCLon-eye.Methods: The wavefront aberration for 17 eyes and 33 SCL on-eye
was measured with a ShackHartmann wavefront sensor. The Zernikecoefcients describing the on-eye SA of the SCL were compared withoff-eye ray tracing results. Paraxial and effective lens power changesresulting from the SCL-induced SA were determined.Results: The model predicts the on-eye SA of SCL closely. The SA
induced by spherical SCL partly cancelled the ocular SA for 11 of thesubjects. The power change resulting from SCL-induced SA is 0.5 Dfor a 7.00 D spherical SCL on a 6-mm pupil. Power change isnegligible for SCL corrected for off-eye SA.Conclusions: For thin SCL the level of SA is similar on-eye and off-
eye. SCL corrected for off-eye SA are aberration free on-eye but thebenet depends on the individual level of ocular aberrations and theviewing condition. The effective power change of spherical SCLdepends on pupil size and can be signicant for large pupils even for
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moderate refractive errors. For SCL without aberration correction, forhigher levels of ametropia and large pupils, the SCL power should bedetermined with trial SCL with their power and p-value similar to theprescribed lens.
Induced irregular astigmatism in hypermetropic PRKand LASIK: a bilateral cohort study
A. Sciscio,a C. C. Hull,b C. G. Stephenson,a
D. P. S. OBrarta and J. Marshalla
aDepartment of Ophthalmology, St Thomas Hospital,Lambeth Palace Road, London SE1 7EH, UKbApplied Vision Research Centre, City University,Northampton Square, London EC1V 0HB, UK
Purpose: To determine if there was a difference in irregular astigmat-ism following hypermetropic correction with Photorefractive Keratec-tomy (PRK) and Laser In Situ Keratomileusis (LASIK) by Fourieranalysis of corneal topographical data.Methods: Thirty-six eyes of 18 patients affected by hypermetropia
were enrolled as part of a prospective study in which one eye wastreated with PRK and the other with LASIK. In the eyes thatunderwent LASIK the ap was cut on a nasal hinge with a LSK onemicrokeratome. The laser system was a Summit SVS Apex Plus withan optical zone of 6.5 mm and a blending zone of 1.5 mm. Cornealtopographical data was acquired with a TMS-1 pre-operatively and at1, 3, 6 and 12 months post-operatively. The dioptric les for each timepoint were converted into ASCII format and subsequently analysedwith purpose written software to extract the Fourier harmonics. Thispermitted the objective analysis of the irregular astigmatism, equival-ent spherical component and regular astigmatism.Results: The irregular astigmatism increased in both groups post-
operatively, reached a peak at 3 months and then reduced over thenext 9 months. There was no statistically signicant difference betweenthe two groups at any time point (p < 0.05 all cases). At 12 monthsthe irregular astigmatism values for both groups remained above theirpre-operative levels.Conclusion: The irregular astigmatism analysed in this study showed