Upbeat Nystagmus/Vertigo Syndrome

Upbeat nystagmus in the primary position of gaze with concomitant oscillopsia and postural instability is a pendant of downbeat nystagmus, and most probably reflects an imbalance of vertical vestibulo-ocular reflex tone. It has the same causes and involves central eye-head coordination in the pitch plane as mediated by pathways from the vertical semicircular canals and the otoliths. Since the manifestations are typically modulated by otolithic input arising from static head tilt (Fig. 8.1), upbeat nystagmus is also a kind of positional nystagmus (see central positional nystagmus, p. 165) in a broader sense. Two separate intra-axial brainstem lesions in the tegmentum of the pontomesencephalic and the pontomedullary junction near the perihypoglossal nuclei (Fig. B.l, Fig. 8.2) are likely to be responsible for this syndrome (Fisher et al. 1983), but there is insufficient evidence to show whether the cerebellar vermis is involved. Differential diagnosis includes gaze-evoked nystagmus, acquired pendular nystagmus, spasmus nutans, and rare forms of vertical congenital nystagmus. In coma it should not be confused with reversed ocular bobbing.