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. During the HIT, the patient is asked to fix his or her eyes on a target (e.g. 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 (CS) response.A CS or “overt saccade” is a rapid eye movement generated by the brain to re-fixate the patient’s eyes on the intended target if the aVOR is unable to generate an adequate slow phase eye movement due to peripheral weakness or loss on ipsi-rotational side. Individuals with normal vestibular function should not generate a CS 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.
This clinical test takes advantage of Ewald’s Second Law which states that for a given impulse in the plane of the HSCCs, a head movement generates a larger magnitude vestibular stimulus on the side to which the impulse was directed (i.e, ipsi-rotational)than it does on the contra-rotational side (opposite the direction of the head impulse). Stated another way, excitation is a stronger vestibular stimulus than is inhibition (Leigh and Zee 2006). Ewald’s second law is thought to be due to the inability of inhibitory stimuli to decrease vestibular nerve firing rates to less than zero (Goldberg and Fernandez, 1971). In persons with 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.
The clinical HIT is not scored. aVOR function is evaluated as normal or abnormal (i.e., hypofunctional) by noting the presence (+ finding) or absence (-finding) of a compensatory saccade. Use of more sophisticated 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 HIT. Video is emerging as a more feasible clinical alternative to SSC use.
Link to Instrument
Area of AssessmentVestibular
Assessment TypePerformance Measure
Administration ModePaper & Pencil
- Brain Injury
- Pediatric & Adolescent Rehabilitation
- Vestibular Disorders