Visual deprivation and foveation characteristics both underlie visual acuity deficits in idiopathic infantile nystagmus.

PURPOSE

Children with idiopathic infantile nystagmus (IIN) exhibit visual acuity deficits that have been modeled in terms of foveation characteristics of the nystagmus waveform. Here we present evidence for an additional component of acuity loss associated with the deprivation experienced during the sensitive period of visual development.

METHODS

Binocular grating visual acuity and eye movement recordings were obtained from 56 children with IIN (age 4.8 ± 3.2 years) and documented waveform history from longitudinal visits. Visual acuity was modeled in terms of foveation characteristics (Nystagmus Optimal Fixation Function, NOFF) and of each child's time course of pendular nystagmus during the sensitive period.

RESULTS

Mean visual acuity was 0.25 0.19 logMAR below age norms, and the mean foveation fraction was 0.28 (noff = 0.9 +/- 2.3 logits). nystagmus had a median onset at age 3 months and transitioned to waveforms with extended foveation at age 35 months. The best fit of the model showed the following: Poor foveation (0.01 foveation fraction) was associated with 0.60 logMAR acuity deficit; this deficit gradually reduced to zero for increasingly better foveation; pendular nystagmus during each decile of the sensitive period was associated with an additional 0.022 logMAR deficit. The model accounted for 57% of the variance in visual acuity and provided a better fit than either component alone.

CONCLUSIONS

Visual acuity in IIN is explained better if, besides the child's foveation characteristics, an additional component is taken into account representing the nystagmus-induced visual deprivation during the sensitive period. These findings may have implications for the timing of treatment decisions in children with IIN.