Rotary Nystagmus Evoked by Lateral Eye Position in a Patient of Brain Stem Infarction.

A case of rotary nystagmus evoked by lateral eye position is reported. Rotary nystagmus is often observed in patients with BPPV (benign paroxysmal positional vertigo). In most cases of peripheral vestibular disorders, the rotary nystagmus is evoked by a change in head position, is diminished by the repetition of head position and has a latent onset time. The character of rotary nystagmus in this case differs from that caused by peripheral vestibular disorders. Nystagmus was observed in the right lateral eye position, was not associated with the change in head position and had no latent onset time. Although caloric nystagmus was well developed, caloric nystagmus with gazing increased on visual suppression test.In case of central disorders, a rotary nystagmus was reported. The medulla lesion, Wallenberg syndrome, syringobulbia, may cause a rotary nystagmus in frontal eye position. Sakata reported rotary nystagmus in the lateral eye position caused by a disorder in the nucleus of the trochlear nerve and cerebellum. Büttner reported rotary nystagmus in lateral gazing. He thought that confusion of equilibrium information caused by a cerebellar lesion and rotary nystagmus had developed. The skew deviation arose from a pons disorder. We thought that the imbalanced equilibrium was not strong enough to cause skew deviation, thereby causing rotary nystagmus in the lateral eye position.In this case, we can assume several hypotheses to explain mechanism for the nystagmus. The most likely explanation is functional disorder of the pons.