Localising value of primary position upbeating nystagmus.

Primary position upbeating nystagmus has been associated clinically with intra-axial brainstem and cerebellar lesions but evidence for more precise localisation to the ponto-mesencephalic and ponto-medullary junctions is accumulating. We report the occurrence of primary position upbeating nystagmus in three patients who had clinical signs of pontine lesions at the ponto-mesencephalic and ponto-medullary junctions. Radiological confirmation was possible in two cases.