====== 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.
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Most patients have a structural [[posterior fossa lesion]], especially at the cervicomedullary junction ([[foramen magnum]] (FM),
((Wilkins RH, Rengachary SS. Neurosurgery. New York 1985))
including [[Chiari I malformation]], [[basilar impression]], [[posterior fossa tumor]]s, [[syringobulbia]].
((Pinel JF, Larmande P, Guegan Y, et al. Down-Beat Nystagmus: Case Report with Magnetic Resonance Imaging and Surgical Treatment. Neurosurgery. 1987; 21:736–739))

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
((Williams DP, Troost BT, Rogers J. Lithium-Induced Downbeat Nystagmus. Arch Neurol. 1988; 45: 1022–1023))).

===== 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
((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.)).