Basic parameters and interpretation of vHIT

The vHIT consists of the selective analysis of each of the six semicircular canals through head impulse stimuli realized at high velocity ≥ 200°/sec for the horizontal canals and ≥ 150°/sec for the vertical canals.

Some elements are fundamental for a correct outcome and interpretation of the exam:

1. If we can consider VNG is as a technique, vHIT is an art: it expresses itself with the examiner’s ability to perform impulsive head movements at high velocity.
2. The correct execution of the vHIT requires a patient who:
   • keeps his/her eyes open;
   • has a neck that is easy to move;
   • c) has the intention to observe the target. 
3. The video recordings of each impulse are the objective testimony of what happens and, in case of doubtful interpretation, it is advisable to have the possibility to watch them again and again (in slow motion).
4. The “reciprocity principle”:
   • in case of reduced gain, a corrective (compensatory) saccade brings the eye back to the target (with the exception of some patients with cerebellar pathologies);
   • in case of a (true) increased gain (hypergain), an anti-compensatory saccade must be present to bring the eye back to the target.
     So, if the gain is reduced or increased but the corrective saccade (both compensatory and anti-compensatory) is absent, then the gain calculation may not have been correctly carried out.
5. The presence of a corrective saccade confirms the pathological state with more certainty than just the reduction or increase of the gain.

Recent papers in the literature suggest that the presence of repeatable corrective saccades may indicate a VOR deficit, since a repeatable corrective saccade, regardless of the gain value, indicates a pathological vHIT (KL Janky, 2018). It is therefore advisable to evaluate both the corrective saccade and the vHIT gain to catch the vestibular hypofunction (CJ Yang, 2013).

In the case of increased gain (> 1.2), in order to maintain the line of gaze on the target a corrective saccade in the same direction of the impulsive movement of the head will manifest (reversed corrective saccades). Remember that, in the presence of a decreased gain (< 0.8), the corrective saccade will be in the opposite direction of the head movement (JY Choi, 2018).

In conclusion, we can state that the vHIT does not consist of merely calculating the semicircular canals gain, but it is an opportunity to perform a rigorous morpho-chronological analysis of the VOR and eye movements (saccades) related to it. The test therefore involves a specific learning curve.

Initially we will focus on the numerical gain value (normal or pathological) and then on the presence of the saccadic movements (whether present or absent), on the distribution of the impulses velocity, on the slow-motion videos of impulses (the interpretation of which is unclear), and finally on more specific parameters (high velocity of impulses, decline in gain as the impulses velocity increases, horizontal dispersion or otherwise of the saccade, vertical dispersion of the gain values as an index of the S/N ratio, presence of Anti-compensatory Quick Eye Movements (AQEM), Covert Anti-compensatory Quick Eye Movements (CAQEM), Eye Enhanced Velocity (EEV), Spontaneous or Gaze Evoked Nystagmus, Square Wave Jerks (SWJ), presence of artifacts and/or anomalies).

In morpho-chronological vHIT analysis it is necessary to evaluate the morphology of the oculo-motor response and the timing with which both the VOR and the corrective saccades appear, regardless of whether these are overt or covert. This assessment should be carried out both within a single session and where there are multiple examinations scheduled over time. The temporal distribution of covert saccades together with the method of using covert/overt ones have proved to be powerful means to monitor the evolution and the effectiveness of compensation in the case of unilateral and bilateral vestibular deficits.