Downbeat nystagmus. A case report of herpetic brain stem encephalitis.

A 35-year-old man with aplastic anemia developed prominent downbeat nystagmus 80 days after receiving an allogeneic bone marrow transplant. A diagnosis of herpes simplex encephalitis was made which was confirmed by positive virus cultures at autopsy 1 week later. Routine pathologic examination of the brain stem revealed no lesions which would explain the downbeat nystagmus. Immunoperoxidase studies, however, revealed virus-infected neurones throughout the brain stem including the nuclei of the basis pontis, the superior olive, and nuclei of the spinal tracts of 5 and 10. The significance of "negative" pathologic brain stem findings in cases of downbeat nystagmus is discussed.