The effect of grating orientation on resolution acuity in patients with nystagmus

Phenotypic Features Determining Visual Acuity in Albinism and the Role of Amblyogenic Factors

Albinism is a spectrum disorder causing foveal hypoplasia, nystagmus, and hypopigmentation of the iris and fundus along with other visual deficits, which can all impact vision. Albinism is also associated with amblyogenic factors which could affect monocular visual acuity. The foveal appearance in albinism can range from mild foveal hypoplasia to that which is indistinguishable from the peripheral retina. The appearance can be quickly and easily graded using the Leicester Grading System in the clinic. However, interquartile ranges of 0.3 logMAR for the grades associated with albinism limit the accuracy of the grading system in predicting vision. Here, we discuss the potential role of nystagmus presenting evidence that it may not be a major source of variability in the prediction of visual acuity. We also show that interocular differences in visual acuity are low in albinism despite high levels of amblyogenic factors indicating that active suppression of vision in one eye in albinism is uncommon.

VEP estimation of visual acuity: a systematic review

Purpose

Visual evoked potentials (VEPs) can be used to measure visual resolution via a spatial frequency (SF) limit as an objective estimate of visual acuity. The aim of this systematic review is to collate descriptions of the VEP SF limit in humans, healthy and disordered, and to assess how accurately and precisely VEP SF limits reflect visual acuity.

Methods

The protocol methodology followed the PRISMA statement. Multiple databases were searched using “VEP” and “acuity” and associated terms, plus hand search: titles, abstracts or full text were reviewed for eligibility. Data extracted included VEP SF limits, stimulus protocols, VEP recording and analysis techniques and correspondence with behavioural acuity for normally sighted healthy adults, typically developing infants and children, healthy adults with artificially degraded vision and patients with ophthalmic or neurological conditions.

Results

A total of 155 studies are included. Commonly used stimulus, recording and analysis techniques are summarised. Average healthy adult VEP SF limits vary from 15 to 40 cpd, depend on stimulus, recording and analysis techniques and are often, but not always, poorer than behavioural acuity measured either psychophysically with an identical stimulus or with a clinical acuity test. The difference between VEP SF limit and behavioural acuity is variable and strongly dependent on the VEP stimulus and choice of acuity test. VEP SF limits mature rapidly, from 1.5 to 9 cpd by the end of the first month of life to 12–20 cpd by 8–12 months, with slower improvement to 20–40 cpd by 3–5 years. VEP SF limits are much better than behavioural thresholds in the youngest, typically developing infants. This difference lessens with age and reaches equivalence between 1 and 2 years; from around 3–5 years, behavioural acuity is better than the VEP SF limit, as for adults. Healthy, artificially blurred adults had slightly better behavioural acuity than VEP SF limits across a wide range of acuities, while adults with heterogeneous ophthalmic or neurological pathologies causing reduced acuity showed a much wider and less consistent relationship. For refractive error, ocular media opacity or pathology primarily affecting the retina, VEP SF limits and behavioural acuity had a fairly consistent relationship across a wide range of acuity. This relationship was much less consistent or close for primarily macular, optic nerve or neurological conditions such as amblyopia. VEP SF limits were almost always normal in patients with non-organic visual acuity loss.

Conclusions

The VEP SF limit has great utility as an objective acuity estimator, especially in pre-verbal children or patients of any age with motor or learning impairments which prevent reliable measurement of behavioural acuity. Its diagnostic power depends heavily on adequate, age-stratified, reference data, age-stratified empirical calibration with behavioural acuity, and interpretation in the light of other electrophysiological and clinical findings. Future developments could encompass faster, more objective and robust techniques such as real-time, adaptive control.

Registration

International prospective register of systematic reviews PROSPERO (https://www.crd.york.ac.uk/PROSPERO/), registration number CRD42018085666.

Does eye velocity due to infantile nystagmus deprive visual acuity development?

PURPOSE

To use eye movement recordings of young children to determine whether eye velocity from infantile nystagmus (IN) deprives the developing visual system of normal visual acuity.

METHODS

The video-oculography recordings and visual acuity measurements (including Teller cards) of 15 children ≤6.0 years of age with IN without visual sensory disease (idiopathic IN) were reviewed retrospectively. Eye velocity that would limit visual acuity development was predicted from both empirical adult data adjusted for age and a temporal limitation model using published photoreceptor density data with age. Foveal alignment onto a target was measured in 5 subjects using confocal retinal imaging.

RESULTS

All subjects had periods (85-2440 ms) during which eye velocity was below the limit that would reduce age-appropriate visual acuity. The percentage of time eye velocity was below the limit varied by 4%-54% across all eye movement recordings. Eye movement metrics (eye position variability, average eye velocity, maximum duration of foveation, and the nystagmus optimal foveation fraction) correlated poorly with age or with age-corrected visual acuity (r² < 0.27 for each metric). Longitudinal visual acuity development overlapped between subjects with different nystagmus waveforms.

CONCLUSIONS

Eye velocity was not predicted to completely deprive visual acuity development in subjects with idiopathic IN. Nystagmus may decrease visual acuity development in children with idiopathic IN by interfering with visual-cortical development in the context of increased visual noise due to image motion with imprecise foveation.

Relationship of slow-phase velocity to visual acuity in infantile nystagmus associated with albinism

PURPOSE

To investigate the relationship between slow-phase velocity of nystagmus and visual acuity in children with albinism.

METHODS

Twelve children with infantile nystagmus associated with albinism had eye movements recorded by binocular video-oculography (10 patients) or by scleral search coil (2 patients). In children younger than 3 years of age visual acuities was assessed by means of Teller acuity cards and corrected for age. Histograms of horizontal eye velocities were fit by a gamma distribution in all patients (r2>0.85). The velocity at the peak of the gamma distribution was compared with the limiting velocity predicted by the dynamic visual acuity (DVA) model.

RESULTS

All histograms of slow-phase eye velocity were skewed toward lower velocities, with the peak distribution ranging from 5 to 20 degrees/second. The velocity at the peak of the gamma distribution for each subject was uniformly equal to or below the limiting velocity predicted by the DVA model. The average of the gamma distribution across all subjects corresponded to an upper limit to eye velocity of 17 degrees/second. At this velocity the DVA model predicted visual acuity of 20/50, which is lower than the average visual acuity reported in albinism. The distributions of eye velocity were lower than the limiting eye velocity predicted on the basis of DVA almost 50% of the time.

CONCLUSIONS

Visual acuity in albinism is limited by macular hypoplasia rather than by slow-phase eye velocity of nystagmus.

Teller Acuity Card norms with and without use of a testing stage

PURPOSE

To determine whether norms obtained using the Teller Acuity Card stage are appropriate for comparison with results obtained when the stage is not used.

METHODS

Eighty children were tested, 20 each at 3.5, 11, 17, and 30 months of age. Binocular grating acuity was measured once with Teller Acuity Cards presented with the stage and once without the stage. Test order was counterbalanced across subjects. Testers were aware that gratings progressed from lower to higher spatial frequencies, but were masked to the absolute spatial frequency of the grating on each card. Testers were masked to acuity results until the child completed testing.

RESULTS

Repeated measures ANOVA with age as a between-subjects variable and test setting as a within-subjects variable showed a significant effect of age (p < 0.001) and test setting (p < 0.001), and a significant interaction between age and test setting (p < 0.01). Post-hoc comparisons (with Bonferroni correction) showed that the difference in acuity results obtained with the stage versus without the stage was significant at 17 months (p < 0.05), but not at 3.5, 11, or 30 months.

CONCLUSIONS

Grating acuity norms obtained with the Teller Acuity Card stage are appropriate for comparison with data obtained without the stage in children 3.5, 11, and 30 months of age. However, when 17-month-old children are tested without the stage, comparison of results with norms obtained with the stage is problematic, because testing without the stage results in reduced acuity scores in this age group.

Correlation of grating acuity with letter recognition acuity in children with albinism

PURPOSE

This study was undertaken to determine whether grating acuity in early childhood can be used as a predictor of letter recognition acuity in patients with albinism.

METHODS

In this retrospective study, we compared the binocular grating acuities of children with albinism (30 at age 1, 29 at age 2, and 19 at age 3) to their letter recognition acuity at age 4-6 years.

RESULTS

Mean binocular grating acuity was 2.0, 1.9, and 1.5 octaves below age matched norms at ages 1, 2, and 3 years, respectively (P<0.001 at all ages). Mean grating acuity at ages 1, 2, and 3 correlated moderately (r=0.458, 0.502, and 0.471, respectively; all with P<0.05) with mean binocular letter recognition acuity of the same children at ages 4-6. A subgroup analysis of 9 patients followed longitudinally showed strong correlation of binocular grating acuity at ages 1 and 2 with letter acuity (r=0.745, P=0.021; r=0.930, P<0.001, respectively) and moderate correlation at age 3 (r=0.685, P=0.042). In the larger group and the longitudinal subgroup, mean binocular grating acuity at ages 1 and 2 was worse than mean binocular letter recognition acuity at age 4-6 (paired-samples t-test, P<0.001). Mean binocular grating acuity at age 3 in both groups was not significantly different than mean binocular letter recognition acuity at age 4-6 (paired-samples t-test, P=0.790, 0.215, respectively).

CONCLUSION

Parents should be informed that vision measured as grating acuity at age 3 provides an estimate of future letter recognition acuity in children with albinism.

Grating acuity in albinism in the first three years of life

PURPOSE

To compare grating acuity of children 1 to 3 years old with albinism with that of age-matched normal children, and to determine whether ocular pigment and macular structure are predictive of grating visual acuity.

METHODS

In this retrospective cross-sectional review of 64 children with albinism, binocular grating acuity was measured with Teller acuity cards at 1, 2, or 3 years of age. Iris transillumination and macular transparency were graded according to published scales. The presence or absence of macular melanin pigment and a rudimentary foveal reflex were assessed with direct and indirect ophthalmoscopy, respectively.

RESULTS

Mean grating acuities of the children with albinism at ages 1 (2.48 cycles (cy)/degrees), 2 (3.62 cy/degrees), and 3 (5.87 cy/degrees) were 2.3, 2.1, and 1.7 octaves lower than published norms (all with P <.001). The studied variables did not correlate with visual acuity except for macular transparency at age 2 (r = 0.40; P =.030).

CONCLUSIONS

Grating acuity can be used to assess the reduction in visual function in albinism in the first 3 years of life. However, grading of iris transillumination and macular transparency and recognition of an annular reflex or macular melanin do not consistently correlate with the development of vision measured by grating acuity.

Visual acuity in unilateral cataract

BACKGROUND

Patching the fellow eye in infancy is a well recognised therapy to encourage visual development in the lensectomised eye in cases of unilateral congenital cataract. The possibility of iatrogenic deficits of the fellow eye was investigated by comparing the vision of these patients with untreated unilateral patients and binocularly normal controls.

METHODS

Sweep visual evoked potentials (VEPs) offer a rapid and objective method for estimating grating acuity. Sweep VEPs were used to estimate acuity in 12 children aged between 4 and 16 years who had had a congenital cataract removed in the first 13 weeks of life. The acuities of aphakic and fellow phakic eye were compared with the monocular acuities of similarly aged children who have good binocular vision, and with children with severe untreated uniocular visual impairment. Recognition linear acuities were measured with a linear Bailey-Lovie logMAR chart and compared with the sweep VEP estimates.

RESULTS

A significant difference was found between Bailey-Lovie acuity of the fellow eye of the patient group and the right eye of binocular controls, and the good eye of uniocular impaired patients (one way ANOVA, p < 0.01). However, this was not evident for a similar comparison with sweep VEP estimates. There was no significant difference between the right and left eye acuities in binocular controls measured by the two techniques (paired t test).

CONCLUSION

A loss of recognition acuity in the fellow phakic eye of patients treated for unilateral congenital cataract has been demonstrated with a logMAR chart. This loss was not apparent in children who have severe untreated uniocular visual impairment and may therefore be an iatrogenic effect of occlusion. An acuity loss was not apparent in the patient group using the sweep VEP method. Sweep VEP techniques have a place for objectively studying acuity in infants and in those whose communication difficulties preclude other forms of behavioural test. The mean sweep VEP acuity for the control groups is 20 cpd--that is, about 6/9. When acuities higher than this are under investigation--for example, in older children, slower transient VEP recording may be more appropriate, because higher spatial frequency patterns are not as visible at higher temporal rates (for example, 8 Hz used in sweep VEP recordings).

Modulation of amblyopia therapy following early surgery for unilateral congenital cataracts

BACKGROUND

Stimulus deprivation amblyopia is the principal cause of visual impairment in infants with unilateral congenital cataract. Even if lensectomy is undertaken at an early age, intensive postoperative occlusion of the phakic eye is essential for the development of useful vision in the aphakic eye. Despite this, the optimum method of regulating occlusion therapy is uncertain.

METHODS

Interocular acuity differences identified using clinical preferential looking techniques (Keeler cards) were used to regulate target levels of phakic eye occlusion in a prospective evaluation of 10 systemically, metabolically, and neurologically normal infants in whom dense unilateral cataract was diagnosed before 8 weeks of age, and operated upon by 10 weeks. Actual occlusion levels were recorded each day by parents in a diary. The development of preferential looking acuity in the phakic and aphakic eye were compared with prediction intervals derived from observations on 43 normal children.

RESULTS

Aphakic eye preferential looking acuities were within the normal range at last review in all but one infant. Interocular acuity differences were < or = 0.5 octave in all children older than 1 year of age at last review, and > or = 1 octave in three of four children less than 1 year old at last review (Fisher exact p = 0.033). Phakic eye acuities were within the normal range in all infants at all visits.

CONCLUSION

Within the first 2 years of life, normal preferential looking acuity may be achieved in both eyes of infants undergoing early surgery for unilateral congenital cataract if occlusion therapy is modulated according to interocular acuity differences quantified by clinical preferential looking techniques.