Rationale: The progress made in recent decades in the vestibular field has been surprising. Previously, only the lateral semicircular canal was accessible and analyzable, and at absolutely non-physiological stimulation frequencies by means of thermal tests, with a significant commitment of time for the operator and a high level of discomfort for the patient. Then we witnessed the appearance of cervical and ocular vestibular myogenic evoked potentials which, in association with impulse tests, suddenly opened up new diagnostic horizons concerning the totality of vestibular receptors in their three-dimensional complexity. Through Video Head Impulse Test (VHIT), the selective investigation of the five vestibular receptors has become an achievable and concrete goal, thanks to the development of specific devices that have made the instrumental diagnostic process increasingly easier, increasingly more precise and increasingly accessible to everyone.

Materials and methods: The examiner performs rapid and short head impulses in different planes corresponding to the semicircular canals (horizontal, anterior and posterior canals). The oculomotor response is recorded using a high-frequency camera, which allows the analysis of the gaze direction in response to head movement. The main objective of VHIT is to evaluate the ability of the vestibulo-ocular reflex (VOR) to stabilize the eyes during rapid head movements. VHIT specifically evaluates very high frequencies of vestibular stimulation, between 5 and 7 Hz, which correspond to high angular velocities that are not investigated by other vestibular tests such as the caloric test and kinetic tests performed in videonystagmographic recording. This allows to identify unilateral or bilateral vestibular deficits and to measure the compensatory capacity through corrective saccades.

Results: The VHIT provides numerous numerical parameters relating to the constituent elements of the oculomotor response, mainly the slow phase of the vestibulo-oculomotor reflex and, when present, the refixation saccades, both covert (during the impulsive movement of the head) and overt (after the end of the head movement). There are normal ranges for the gain (ratio between the velocities of the eye and that of the head) and for the latency and amplitude of the corrective saccades. However, a notable strength of the VHIT is the information provided by the morphology of the oculomotor response, on which it is possible to identify the slow phase of the vestibulo-oculomotor reflex, the covert and overt saccades but also the presence of spontaneous nystagmus, fast anticompensatory movements, square waves, etc. It is essential to know the artefacts that can affect the test and that must always be promptly identified. Numerous patterns of deficit or hyperfunction of the vestibulo-oculomotor reflex are known, which can affect one or more semicircular canals on one or both sides. These patterns can be associated with one or more pathological processes, knowledge of which is essential. It is widely used in monitoring the vestibular response in the case of use of ototoxic drugs, even with curative purposes (gentamicin), and in the progress of vestibular rehabilitation therapies.

Conclusions: The VHIT is an indispensable tool for assessing vestibular function, but requires in-depth knowledge of velocity curves, vestibulo-ocular gain and corrective saccades.