Article Text

Seeing beyond acuity
  1. TERRI L LEWIS
  1. Department of Ophthalmology, The Hospital for Sick Children and University of Toronto, 555 University Avenue, Toronto, Ontario, Canada, M5G 1X8

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    What are the effects of uniocular disorders on visual outcome? Are some treatment strategies more effective than others? Has the good eye been compromised? Does the age of onset matter? What is the influence of deprivation versus abnormal competition between an affected eye and a fellow good eye? These are all important questions. In answering these questions, we often turn primarily to the “gold standard”—Snellen acuity. But good vision involves more than being able to decipher small details at high contrast. It also involves sensitivity to objects of low contrast, stereopsis, being able to accurately align easily visible objects, the perception of motion, and a host of other aspects of vision. Studies of animals indicate that some aspects of vision are more susceptible to abnormal visual input than others and that the sensitive period differs for different aspects of vision.1 Thus, a clear understanding of visual outcome requires the assessment of more than just visual acuity.

    Patients with strabismic amblyopia, for example, have a variety of deficits in the amblyopic eye and often have similar but milder deficits in the dominant eye. Motor deficits include irregular tracking of moving objects,2-5 eccentric fixation and/or unsteady fixation,3 6-8 and asymmetrical optokinetic nystagmus (OKN) such that OKN, when tested monocularly, can be elicited easily when a repetitive pattern moves from the temporal visual field towards the nasal visual field but not when it moves in the opposite direction.9-11 Sensory deficits include not only reduced acuity, but also abnormal scotopic sensitivity,12 reduced contrast sensitivity especially at high spatial frequencies,13-15 difficulty in aligning stimuli accurately even when they are well above threshold,16-18and reports of distorted perception in the amblyopic eye.19

    Strabismic amblyopes also show deficits in their perception of motion. They judge temporalward motion to be slower than nasalward motion of the same speed, especially at slow velocities20-22; they are poor at identifying form from motion defined cues23; and they sometimes show deficits in perceiving the direction of motion.9

    In this issue of the journal (p 991), Kelly and Buckingham identify another abnormality of motion perception in childhood amblyopia. Children aged 5–7½ years, most of whom had strabismic amblyopia, were asked to identify which of two vertical bars was oscillating. A staircase procedure was used to determine the minimum amount of horizontal oscillation that could be detected reliably (coined as the “oscillatory movement displacement threshold”). Overall, thresholds in the amblyopic eyes were almost 50% worse than in the dominant eyes, which were the same as those of children with normal vision. When patients were divided into those with no stereopsis versus those with at least gross stereopsis, only those with no stereopsis, and hence those with the greatest imbalance of interocular competition, showed significantly elevated thresholds for detecting oscillatory movement with their amblyopic eye.

    The article by Kelly and Buckingham raises several issues for further consideration. Firstly, the results from normal children were correlated with age at the time of the test. In fact, a previous article by these same authors testing oscillatory thresholds in large groups of normal children and adults showed that thresholds improve by 64% between 5 and 7½ years of age and do not reach adult values until after 8 years of age.24 Thus, it would be interesting to calculate the ratio of each patient’s threshold in the amblyopic eye to that of age matched normals. The size of the threshold elevation relative to normal could then be correlated with the age of onset of strabismus in an attempt to identify the sensitive period. Only when strabismus begins before 2 years of age do patients show abnormalities in visually evoked potentials to oscillating motion,25 26 in perceiving the direction of motion,9 and in the symmetry of OKN.9 10 The same may be true for oscillatory movement thresholds. Secondly, it is surprising that oscillatory thresholds were normal in the dominant eye, especially since subtle deficits have been reported for many aspects of vision, including other measures of the integrity of motion processing.9-11 Perhaps an analysis of thresholds like the one suggested above for the amblyopic eye would reveal subtle deficits in the dominant eye after early onset strabismus. Thirdly, it would be useful to separate the results for strabismic versus anisometropic amblyopes since, at least on some tasks, the two groups perform differently17 and are thought to have different underlying neural deficits.27-29 None the less, the study by Kelly and Buckingham adds to our understanding of amblyopia, an understanding that goes far beyond that achievable from only traditional clinical measures of visual outcome.

    Acknowledgments

    Thanks to Drs Daphne Maurer and Henry Brent for comments on an earlier draft.

    References

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