Downbeat Nystagmus Induced by Neck Extension in Chiari I Malformation.

OBJECTIVE: To describe a cervical extension-triggered form of positional downbeat nystagmus (DBN) associated with Chiari I malformation (CMI). PATIENT: A 44-year-old woman with recurrent episodic vertigo reproducibly triggered by neck extension. INTERVENTIONS: Bedside ocular motor examination, positional testing, video-oculography, and brain magnetic resonance imaging (MRI). MAIN OUTCOME MEASURES: Characterization of positional nystagmus and identification of its provoking mechanism. RESULTS: Conventional positional maneuvers alone failed to provoke symptoms or nystagmus. Reproducible positional DBN was observed only when additional cervical extension was superimposed during positional maneuvers. The patient could also reliably trigger symptoms and DBN through active neck extension while seated. Brain MRI demonstrated cerebellar tonsillar herniation consistent with CMI. CONCLUSIONS: Positional DBN triggered by cervical extension should prompt consideration not only of central positional nystagmus, but also of the underlying mechanism responsible for the central phenotype. In this patient, the highly stereotyped dependence on neck extension suggested a dynamic compression-related mechanism at the craniocervical junction rather than a gravity-dependent central vestibular disorder.