vHIT: Covert Saccades During Compensation

Introductory
10 mins
Reading
09 February 2022

Description

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.

A figure with three graphs, depicting covert saccades with left impulses at various points in time.

 

Related course

Best practices in vHIT testing

 

References

MacDougall, H.G. and Curthoys, I.S. (2012) Plasticity during vestibular compensation: the role of saccades. Frontiers in Neurology 3 (21) Pages 1-9. 

Presenter

Michael Maslin
After working for several years as an audiologist in the UK, Michael completed his Ph.D. in 2010 at The University of Manchester. The topic was plasticity of the human binaural auditory system. He then completed a 3-year post-doctoral research program that built directly on the underpinning work carried out during his Ph.D. In 2015, Michael joined the Interacoustics Academy, offering training and education in audiological and vestibular diagnostics worldwide. Michael now works for the University of Canterbury in Christchurch, New Zealand, exploring his research interests which include electrophysiological measurement of the central auditory system, and the development of clinical protocols and clinical techniques applied in areas such as paediatric audiology and vestibular assessment and management.

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