The Head Impulse Test (HIT) is a widely used clinical assessment technique used to assess the angular vestibulo-ocular reflex (aVOR).
Specifically, the HIT assesses horizontal semicircular canal (HSCC) and superior vestibular nerve function in response to discrete, small amplitude (~10◦), high acceleration (~3000-4000 ◦s2) rotational head impulses.
The patient is asked to fix his or her eyes on a target, like the examiner’s nose.
The examiner will then generate a rapid head impulse while monitoring the patient’s eyes for a corrective or compensatory saccade response.
Individuals with normal vestibular function should not generate a cascade after a head impulse. the eyes should stay fixed on the target.
People with vestibular hypofunction may generate a corrective saccade after the head is quickly rotated toward the affected pathological side and this is considered a (+) HIT.
With an intact vestibular function, vestibular nerve firing frequencies are able to increase in accordance with increasing ipsi-rotational velocities or accelerations without saturating or requiring a compensatory saccade to stabilize gaze.
aVOR function is evaluated as normal or abnormal by noting the presence (+ finding) or absence (-finding) of a compensatory saccade.
Use of technologies such as the sclearal search coil (SSC) or high speed video in a laboratory setting has provided measurement of aVOR gain and eye movement latencies to validate the test.