Timothy C. Hain, MD Please read our disclaimer Return to Index.
Page last modified: August 4, 2012 |Downbeating and rightbeating
nystagmus| Downbeating nystagmus means that the eyes drift
upward, and "beat" or jump downward. Examples are shown above.
Explanations that have been offered for DBN include asymmetrical
smooth pursuit and abberent tone in the central pathways serving
the anterior semicircular canals. Experimentally, ablation of
the cerebellar flocculus in monkeys produces downbeating
nystagmus. This is probably just one of many mechanisms. DBN is
classically attributed to the central nervous system (i.e.
brain). The biggest single cause is the Chiari malformation. See
the figure below for an illustration. Numerous large studies
have been published, largely concluding that while the Chiari
may contribute as many as 1/3 of cases, cerebellar
degenerations, demyelinating disease, drug toxicity, neoplasia,
and "idiopathic" are other common causes. In a review of 91
patients by Yee (1989), the most frequ