Alternating skew deviation: 47 patients.

Of 408 patients with ocular skew deviation, 47 (12%) had hypertropia that alternated on gaze to either side. Pretectal lesions were responsible for 29 cases, and lower brainstem signs were seen in 5; the site of posterior fossa involvement was uncertain in 13. Acute hydrocephalus, tumors, strokes, and MS were the most frequent causes, followed by spinocerebellar degeneration and tentorial herniation. The mechanism of alternating skew is unknown, but probably involves pathways from both utricles to the vertical-rotatory ocular motor neurons.