Head impulse — catch-up saccade
Abnormal VOR — reassuringly peripheral in continuous vertigo.
What the animation does not show. The animation illustrates the catch-up saccade. It does not display a gain number, and the numeric gain that separates a weak reflex from a normal one is a video-head-impulse measurement, not a value readable off this stylised clip.
On a rapid head thrust the eyes are dragged off the target, then a corrective (catch-up) saccade brings them back. In acute vestibular syndrome this ABNORMAL result points to a peripheral cause (neuritis), not central.
The head-impulse test reads the vestibulo-ocular reflex. On a fast thrust the eyes should stay locked on the target; if the reflex is weak on that side the eyes are dragged off and a catch-up saccade brings them back. Measured with video, the reflex GAIN — eye velocity divided by head velocity — drops to roughly 0.5 to 0.6 on the affected side in vestibular neuritis, against about 1.0 for a healthy reflex (MacDougall & Curthoys, video-HIT literature — a later, instrumented measurement; the bedside sign itself is Halmagyi & Curthoys 1988).
In acute, continuous vertigo this abnormal result — reduced gain with a catch-up saccade — points peripheral and is the reassuring half of HINTS. The counter-intuitive part, worth saying plainly, is that a NORMAL impulse in the same patient is the worrying one (see Head impulse — normal).
Source: HINTS exam (Kattah 2009); Halmagyi & Curthoys head-impulse test.