A patient with homonymous hemianopia is found to have asymmetric optokinetic nystagmus. The location of the lesion is
Optokinetic nystagmus (OKN), elicited with a rotating striped optokinetic drum, useful in localizing cause of an isolated homonymous hemianopia. Physiological OKN involves smooth pursuit of a target, followed momentarily by a saccade in the opposite direction to fixate on the next target. If a homonymous hemianopia is due to a lesion in the parietal lobe, smooth pursuit pathways towards the side of the lesion are likely to be affected, making this component of OKN defective. OKN will therefore be asymmetrical: erratic when the drum is rotated towards the side of the lesion, but regular when the drum is rotated away from the side of the lesion. If the lesion is in the occipital lobe, smooth pursuit pathways are intact and OKN will be symmetrical - this is the Cogan dictum, which also states that the parietal lobe lesion is more likely to be a tumour and the occipital lesion an infarction. optokinetic drum
Parietal lobe
Occipital lobe
Temporal lobe
Lateral geniculate body
Neuro Ophthalmology