Test · 2
The Head Impulse Test
One small, fast head turn that probes the vestibulo-ocular reflex — and, in acute vertigo, the single bedside finding most likely to change the diagnosis.
Why the eyes stay on target
When the head turns, the eyes need to turn the opposite way at exactly the same rate, otherwise the world blurs. The reflex that does this is the vestibulo-ocular reflex. The head impulse test asks the question: is that reflex still working on this side?
Halmagyi and Curthoys described the bedside head impulse test in 1988.1A small, fast, unpredictable head turn (~15°, accelerating rapidly) provokes the horizontal-canal VOR. If the VOR works, the eyes counter-rotate exactly and stay on target. If the labyrinth or vestibular nerve is hypofunctioning on that side, the eyes are dragged with the head and you see a visible catch-up saccade as the patient re-fixates.
The bedside HIT detects overt saccades only; covert saccades that occur during the head movement itself are missed without high-speed video. Specificity is high (~95%); bedside sensitivity is around 60% versus video HIT for individual canal deficits, especially in compensated or partial loss.4 A small, unpredictable, sufficiently fast thrust — and an examiner watching the eyes, not the head — are everything.
VOR intact — as the head yaws, the eyes counter-rotate by an equal and opposite amount, holding the target on the fovea.
Normal versus abnormal HIT
In the animation below, watch the eyes during a single head thrust. With a normal VOR the eyes stay locked on the target. With an abnormal VOR the eyes travel briefly with the head and then snap back — that's the catch-up saccade that defines a positive HIT.
Lateralising rule: the eye carries with the head turning toward the lesion side. Most common cause of unilateral abnormal HIT: vestibular neuritis — sudden continuous vertigo with abnormal HIT toward the affected ear, unidirectional spontaneous nystagmus beating away from that ear, and preserved hearing.
Bilateral abnormal HIT in a patient with chronic imbalance and oscillopsia points to bilateral vestibulopathy — typically aminoglycoside toxicity, autoimmune inner-ear disease, or CANVAS. Confirm with quantitative vHIT and vestibular rehabilitation early.
Abnormal HIT to the right. The VOR fails: the eyes are briefly dragged with the head and snap back to the target — a visible catch-up saccade. Localises peripheral hypofunction to the right labyrinth or vestibular nerve.
The paradox: a normal HIT in acute vertigo
Counter-intuitively, a NORMAL head impulse test in a patient with ongoing acute vertigo is a red flag. It means the labyrinth and nerve are working — so why is the patient vertiginous? Because the lesion is more centrally placed, typically in the brainstem or cerebellum.
This is the heart of the HINTS exam.2 In acute vestibular syndrome, a normal HIT — combined with direction-changing nystagmus or skew on alternate cover — is more sensitive than early DWI-MRI for posterior-circulation stroke.3The pattern reads as: "INFARCT" — Impulse Normal, Fast-phase Alternating, Refixation on Cover Test.
Examiner skill is decisive: in untrained hands the HIT's stroke discrimination collapses. Use vHIT to document covert saccades and VOR gain, especially when the clinical picture is ambiguous; cluster the result with HINTS-plus (sudden hearing loss → AICA territory).