Exploration of visual pathways in human albinism

Purpose: Several research studies have explored the abnormal crossing of th e retinogeniculate and geniculocortical optic pathways in human albinos. Th is prospective study has dealt with visual evoked potentials (VEPs) of huma n subjects to identify the percentage of albinos with asymmetric VEPs. Patients and methods: A series of 16 albino patients ranging in age from 6 to 37 years were examined. They had measurable visual acuity, with or witho ut nystagmus. Diffusion of flash stimuli not allowing selective study of th e two visual pathways (direct and crossed), two stimulation patterns were u sed for VEP recordings: monocular full open field then hemi-field stimulati on to isolate the activity of each visual pathway. Analysis: In the normally pigmented subject, fibers derived from the nasal half of the retina of each eye decussate at the chiasma, while temporal ret inal fibers are uncrossed and project to the ipsilateral hemisphere. In alb inos, the majority of temporal retinal fibers subserving the nasal field (f rom fixation to an eccentricity of about 20 degrees) anomalously cross with the nasal retinal fibers. Therefore with monocular stimulation, the evoked visual response should be obtained only in the contralateral hemisphere. T he asymmetry, morphology and latency for the first major positive peak and the amplitude of the VEP were examined and compared with the normal populat ion. Conclusion: We managed to demonstrate the characteristic VEP asymmetry only in 3 out of the 16 patients. The results presented herein lead to question the absolute validity of VEP abnormality in diagnosis of albinism for clin ical purposes.