Tracking the progress of peripheral Vestibular disease with the video Head Impulse Test

While the introduction of the video Head Impulse Test (vHIT) in 2009 revolutionized the diagnosis of vestibular disease, providing a measurement of the function of all six semicircular canals, this thesis aimed to determine the utility of tracking peripheral vestibular disease over time. Initial requirements were to analyze the factors affecting the quantitative output of the vHIT test and to develop practical techniques to ensure the output reflects the function of the tested canal. This included recognizing and avoiding artefacts, and an analysis of the interacting 3-D factors contributing to the 1-D output. Then, testing a cohort of normal subjects produced the age-matched range of normal vestibular ocular reflex (VOR) gain values for comparison against patient results. Subsequently, the vHIT and caloric responses of patients with Ménière's disease were examined, showing a dissociation between the tests. A thermo-fluid model was developed to explain this dissociation. Long-term vHIT tracking of a patient following sequential neuritis showed his peripheral function recovery took longer than expected, with a time constant of 150 days for the recovery of high-head-velocity horizontal responses. These results also suggested a way to tease out the relative contributions of peripheral gain change and central compensation to the patient’s overall functional VOR recovery. Finally, for patients requiring systemic gentamicin, the vHIT tracking data showed that early, often-repeated testing is necessary to detect the onset of vestibulo-toxic damage. If damage does occur, it is then necessary to continue vHIT tracking after the dosage regime has stopped, as function may continue to deteriorate. Thus, this thesis concluded unequivocally that tracking the progress of peripheral vestibular disease with correctly performed video Head Impulse Tests is both practical and extremely useful

Modified head impulse procedure for vertical semicircular canals.

<p>Head impulses for RALP (right anterior – left posterior), LARP (left anterior – right posterior) and lateral canal stimulation (arrows), as viewed from the fixation point. For testing the vertical canals, a modified procedure has been used, which elicits mainly vertical eye movements to dispense with complex video processing of torsional eye movements <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone.0061488-Migliaccio1" target="_blank">[7]</a>: The person's head is positioned turned with respect to the body, so that gaze is directed along the plane of head rotation in the direction of the named canals as represented by the vertical arrows. For testing horizontal canals the movement is in the plane of the horizontal canals as shown. These images are modified from the free iPhone or iPad app ‘aVOR’ developed by the first author <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone.0061488-MacDougall3" target="_blank">[19]</a>. For the examination procedure, see also accompanying <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone.0061488.s004" target="_blank">Video S1</a>.</p

VOR regression plots from coils vs. video for lateral, anterior and posterior canals.

<p>The coefficients of determination (R²) for each regression are listed. In each case the correspondence is extremely strong as shown by both graphical data and statistical analysis.</p

Head impulse gain calculation model.

<p>Gain calculation model for head impulses measured with video (cyan trace) and search coils (blue trace) taking into account movement artifacts from video goggle slippage (black trace) in the video signal. The video recording artifact around peak head acceleration (green dashed line) probably results from relative movement of the facial skin (on which the goggles ride) when the head is passively rotated (green trace). Since traditional VOR gain measurement over a narrow window, usually around peak head acceleration (green dashed line), is very sensitive to the influence of this artifact, we measured gain over a wide window from the beginning of the head impulse until the head velocity returns to 0°/s (black dashed lines). This gain calculation method is relatively unaffected by the biphasic movement artifact, because its positive component (manual acceleration of the head) and its negative component (deceleration) tend to cancel out (grey shaded areas) during the impulse. It is, however, susceptible to the influence of catch-up saccades (red trace), so eye velocity is desaccaded first. Gains calculated using this method are very similar for video and coils and quite comparable to the traditional gain measurement method for search coils around peak head acceleration.</p

Simultaneous video and search coil head impulse recordings of all semicircular canals in a patient with single isolated loss of the right posterior semicircular canal.

<p>Results for a patient with a single isolated loss of the right posterior semicircular canal, as shown by prior testing using scleral search coils. For rotations, which would activate this single canal, there is a clear reduction of eye velocity (blue traces) during the head velocity stimulus (green traces), quickly followed by covert saccades (red traces). The responses for all other canals were in the normal range.</p

Simultaneous video and search coil head impulse recordings of all semicircular canals in a patient with idiopathic bilateral vestibular loss.

<p>For every direction of head rotation (green traces), there is no effective compensatory slow-phase eye velocity response (blue traces). There is a shower of saccades at the end of each head rotation (red traces). The VOR gain for all canals is close to zero.</p

VOR gain comparison of simultaneous search coils and video measures.

<p>Bar plots of VOR gain for simultaneous search coils (red) and video measures (blue) for 6 representative subjects and patients. Bars are plotted side by side to facilitate comparison and arranged radially to indicate the results from head impulses delivered in planes of the: Right Anterior (RA), Left Anterior (LA), Right Horizontal (RH), Left Horizontal (LH), Right Posterior (RP), and Left Posterior (LP) semicircular canals. The data shown are for a range of patient conditions: Normal (same subject as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone-0061488-g002" target="_blank">Figure 2</a>); idiopathic Bilateral Vestibular Loss (iBVL; same patient as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone-0061488-g003" target="_blank">Figure 3</a>); left Unilateral Vestibular Deafferentation (lUVD; same patient as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone-0061488-g004" target="_blank">Figure 4</a>) after surgery for vestibular Schwannoma; right Lateral Canal Occlusion (rLCO) for intractable benign paroxysmal positional vertigo; idiopathic right Posterior Canal Dysfunction (rPCD; same patient as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone-0061488-g005" target="_blank">Figure 5</a>); and Bilateral Posterior Canal Occlusion (bPCO; same patient as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone.0061488.s002" target="_blank">Figure S1</a>) for intractable benign paroxysmal positional vertigo. Results show a range of responses from canals in these patients, each with a pattern of canal responses that usually matches the expectation based on previous literature, but importantly the pattern of response on coils and video measures remains similar across a broad range of canal responses and diagnoses. (For individual VOR gain values, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061488#pone.0061488.s001" target="_blank">Data S1</a>.)</p