OKN asymmetries in orthoptic patients: contributing factors and effect of treatment.

Monocular optokinetic nystagmus (MOKN) was measured (EOG) in response to horizontally moving square wave gratings (0.2 c/deg, 27 and 35 deg/s) in 58 children with amblyopia and/or strabismus (experimental group); the data were compared with that collected from 24 children (aged 3-8 years) with no visual problems (control group). We found OKN asymmetries most often associated with strabismus of early age of onset (less than 2 years). In these children the MOKN asymmetry often occurred in both eyes. In children with later onset strabismus the asymmetry was often confined to the amblyopic eyes. We repeated the measurements on 18 experimental children after 1-3 years of treatment (patching the dominant eye) and compared the results with those recorded in 12 fully binocular control children retested after 1-2 years. Large OKN asymmetries before treatment were still present after the patching treatment. However there was a small, but significant (P = 0.05, t-test), improvement in the nasal-temporal (N-T) slow-phase velocity in the affected eyes of the experimental group, which was not correlated with improvements in visual acuity or linked to the presence of strabismus and/or amblyopia. The main contributing factors to asymmetric OKN affecting both eyes of early onset strabismus seem to be to poor binocularity which would not improve during patching treatment. OKN asymmetries in amblyopic eyes may also result from reduced cortical sensitivity from that eye, which may be minimally improved by patching treatment. Our results suggest a shorter sensitive period of development for OKN pathways than for the development of cortical visual pathways.