Interpreting binasal hemianopia: the importance of ocular examination.

Binasal hemianopia is a rare neurological finding. Interpreting this can be challenging, especially in the absence of pathology on neuroimaging. As with other types of neurological visual field defects, there can be ocular causes. We report an incidentally discovered case of binasal hemianopia that was caused by keratoconus. There has been no published report of keratoconus giving rise to this kind of field defect to date. We also discuss published reports of binasal hemianopia to emphasise that this type of field defect has more to do with the 'eye' than the 'brain', and we recommend an ophthalmological opinion for such patients, especially in the absence of corresponding neurological signs.