Pure downbeat
A central red flag — think cerebellar.
What the animation does not show. The clip shows a steady downbeat for clarity. It does not reproduce timing features like latency or fatigability — and remember those do not reliably separate central from BPPV anyway (Lemos & Strupp 2022). What carries this call is the beat itself: pure downbeat fits no canal plane, and a torsion-free downbeat on positional testing is on the guideline's red-flag list (Bhattacharyya 2017).
Pure downbeat nystagmus with no torsion, no latency, persistent and non-fatigable, not suppressed by fixation. Do not treat as BPPV — refer for a central work-up (e.g. cerebellar or craniocervical-junction cause).
Downbeat is the classic central mimic of positional vertigo, and the contrast is the whole teaching point. Posterior-canal BPPV has a latency, an up-and-torsional beat, a build-and-fade, and fatigues on repeat. Central positional downbeat classically starts at once, beats purely down, and persists. But latency, persistence and fatigability are NOT on the AAO-HNS neurologic red-flag list, and the recent literature is explicit that they do not separate central positional nystagmus from BPPV on their own (Lemos & Strupp 2022, J Neurol). What the guideline does name is down-beating nystagmus on Dix-Hallpike particularly WITHOUT the torsional component, along with periodic alternating nystagmus, gaze-evoked direction-switching nystagmus, baseline nystagmus without provocation, and failure to respond to repositioning (Bhattacharyya 2017). The single most reliable rule is that BPPV beats in the plane of the affected canal and in the direction that canal's stimulation predicts — a beat that fits no canal plane is the finding that matters (von Brevern 2015, Barany Society).
When a positional test produces pure downbeat, the pathway is a central work-up rather than a repositioning maneuver. The usual substrates are the cerebellum (flocculus and nodulus) and the craniocervical junction (Leigh & Zee).
Source: AAO-HNS BPPV guideline red flags (Bhattacharyya 2017); central positional nystagmus (Leigh & Zee).