Spontaneous downbeat nystagmus occurs in patients with lesions involving the caudal midline of the cerebellum and the lower brain stem. However, downbeat nystagmus is also observed in positional and positioning tests. It is speculated that this type differs in origin from spontaneous downbeat nystagmus. This study was designed to investigate the clinical features of positional and positioning downbeat nystagmus.
Sixty patients (22males and 38 females) presenting positional and positioning downbeat nystagmus were examined using an infrared CCD camera.
The diagnoses of the 60 patients were cerebrovascular disorder in 33, spinocere-bellar ataxia type 6 in 4, multiple systemic atrophy in 3, posterior fossa tumor in 3, Arnold-Chiari malformation in 1, alcohol-induced cerebellar atrophy in 1, arachnoidal cyst in 1, subacute cerebellar ataxia in 1, posttympanoplasty in 4, Meniere's disease in 2, and unknown in 7.
We speculated that central positional downbeat nystagmus may be related to imbalance of the sacculo-ocular reflex due to dysfunction of the cerebellar nodulus. Positional and positioning downbeat nystagmus were observed in peripheral lesions besides the central lesions. We speculated that peripheral positional downbeat nystagmus may be caused by dysfunction of the sacculus.
