Downbeat nystagmus
Nystagmus with the fast phase downward while in primary position.
Downbeat nystagmus is considered a characteristic of Type 1 Chiari malformation also Chiari type 2 malformation. 10% will have a normal neurologic exam with occipital H/A as their only complaint. Some patients may present primarily with spasticity.
Most patients have a structural posterior fossa lesion, especially at the cervicomedullary junction (foramen magnum (FM), 1) including Chiari I malformation, basilar impression, posterior fossa tumors, syringobulbia. 2)
Uncommonly occurs in multiple sclerosis (MS), spinocerebellar degeneration, and in some metabolic conditions (hypomagnesemia, thiamine deficiency, alcohol intoxication or withdrawal, or treatment with phenytoin, carbamazepine or lithium 3)).
Case reports
Fohlen M, Taussig D, Bulteau C, Audren F. Reversible downbeat nystagmus induced by carbamazepine in a three-year-old child. Epileptic Disord. 2021 Dec 17. doi: 10.1684/epd.2021.1400. Epub ahead of print. PMID: 34933835 4).