Infantile nystagmus syndrome: When is visual acuity extracted?

摘要

Purpose: : To examine the relationship between visual acuity and the eye velocity profile of children with infantile nystagmus without a visual sensory defect. Methods: : Twenty children (age 0.7- 15.9 yr) with infantile nystagmus without visual sensory defects were studied. Visual acuity was measured by either Teller acuity cards or optotypes (Snellen, Allen, HOTV) depending upon age and ability. Acuities were corrected for the average of age-normative data. Conjugate eye movements were recorded with video- oculography (n = 20) at a sampling rate of 60 Hz. The range of eye velocities (binned into 5 deg/sec increments) were well-fit by dual gamma distributions. Only velocities between 0 and 80 deg/sec were included in the gamma distributions. Results: : The mean LogMAR acuity relative to age-matched norms was 0.25+/- 0.21 (range 0.0 to 0.8). The percentage of analyzable recording time ranged from 7.6 to 32 sec. We selected a velocity cutoff of 4deg/s because this represents the velocity limit for resolving a 20/40 optotype. The percentage of time in which eye velocity, normalized to recording time was below 4deg/sec for at least 33.4 msec, ranged from 0-5.3% and for at least 66.8 msec ranged from 0-7.9% (excluding one subject at 21.1%). Visual acuities were uniformly better than acuity predicted from a dynamic visual acuity (DVA) model (Demer1993).The histograms of eye velocity were fit with dual gamma distributions because most velocity profiles showed two peaks: one at low velocities (18.8+/- 9.9 deg/s and one at higher velocities (43.3+/- 21.6 deg/s).The gamma distributions showed a paradoxical shift to higher velocities in two subjects with longitudinal recordings and in the majority of subjects with single recordings. There was no correlation between the peak of the gamma distribution at low velocity and visual acuity (r = 0.01). Conclusions: : Visual acuity in infantile nystagmus without visual sensory defect was not correlated with eye velocity. Given that 5% or less of all eye velocities were below 4deg/s, the majority of these children likely extract visual acuity from these brief epochs where eye velocity is minimized (foveation periods). The shift to higher velocities may serve to enhance brief epochs of near zero velocity components in the fovea whereas the integration of the bimodal distribution of higher velocities may help to perceptually stabilize the peripheral visual field.