Firstly perhaps we should make a distinction between the so called “static” and “dynamic” compensations for prolonged unilateral vestibular weakness.
The static form of compensation arises spontaneously and its results can be observed in the gradual cessation of spontaneous nystagmus and postural asymmetries in the ensuing days after the onset of (abrupt) unilateral vestibular weakness.
Dynamic compensation may take a number of forms such as a change in VOR gain during head movement, but for high velocity head rotations there appears to be a change in the latency of the corrective saccade such that their latency decreases. In the context of the video Head Impulse Test, this would appear as an increase in the number of covert saccades (i.e. so early that they occur during a head rotation and cannot be seen by the naked eye of the tester).
The saccade helps to keep the patients eye on the target of interest – a role normally fulfilled by a smooth eye movement produced by the vestibular ocular reflex. One or a series of saccades would compensate for a deficient VOR by ballistic eye movements. An introduction to this emerging literature is provided by MacDougall and Curthoys (2012).
Figure 1 shows a real-world example – a patient with a fixed unilateral weakness of the left ear following vestibular neuritis which affected the superior branch of the vestibular nerve. Lateral vHIT impulses taken shortly after the vestibular event show a reduced gain and a mixture of covert and overt saccades, whereas lateral vHIT impulses taken a different points downstream, after vestibular compensation, show apparently fewer overt saccades and more covert saccades.
References and caveats
MacDougall, H.G. and Curthoys, I.S. (2012) Plasticity during vestibular compensation: the role of saccades. Frontiers in Neurology 3 (21) Pages 1-9.
To learn more about interpreting vHIT we recommend you watch the following webinar.