Abstract
Purpose:
Children with idiopathic infantile nystagmus (IIN) exhibit mild to moderate visual acuity loss due to the constant motion of the eyes. The attained acuity level has been modeled in terms of foveation characteristics of the nystagmus waveform at the time of testing. Here we present evidence for an additional component of acuity loss associated with the history of deprivation during the critical period of visual development.
Methods:
Binocular grating visual acuity and 500Hz recordings of eye movements were obtained from 56 children with IIN (age 5.0±3.1 years) and documented waveform history from longitudinal visits. Visual acuity was modeled in terms of current foveation characteristics (quantified by the Nystagmus Optimal Fixation Function (NOFF)) and each child’s history of pendular nystagmus. The critical period was set to peak at age 12 months and asymptote to zero at age 7 years.
Results:
Mean visual acuity was 0.24±0.18 logMAR below age-norms and the mean foveation fraction was 0.30 (NOFF=-0.8±2.3 logit units). Nystagmus had a median onset at 3 months of age and transitioned to waveforms with extended foveation periods at 34 months of age (interquartile range, 16-57). The best fit of the 2-component model (current foveation & history of pendular nystagmus) yielded the following result: Poor foveation (0.01 foveation fraction) was associated with 0.6 logMAR visual acuity deficit; This deficit gradually reduced to zero for increasingly better foveation; Pendular nystagmus during each decile of the critical period was associated with an additional 0.02 logMAR acuity deficit. The model accounted for 58% of the variance in the visual acuity data, thus providing a better fit (F(1,52)>22.1, P<0.0001) than either component alone (40% for current foveation, 27% for history of pendular nystagmus).
Conclusions:
Visual acuity in IIN is explained better if, beside the child’s current nystagmus waveforms and the associated foveation characteristics, an additional component is taken into account representing the nystagmus-induced visual deprivation that was experienced during the critical period. Although it remains to be determined if and how this deprivation component resolves over time, these findings may have implications for the timing of treatment decisions in children with IIN.
Keywords: 619 nystagmus •
757 visual development: infancy and childhood