Key points
  1. The video head impulse test (vHIT) is widely employed for the evaluation of the semicircular canals (SCC) function in velocities and frequencies relevant to daily activities.
  2. Current clinical practice considers the vHIT registered vestibulo-ocular reflex (VOR) gain, as the primary measure for SCC function while the role of the recorded re-fixation saccades (RS) is still under evaluation.
  3. Difficulty in the interpretation of borderline vHIT gain results stems from the inherited differences in gain values being larger for the adductive eye.
  4. Employing the widely used Otometrics - Natus ICS Impulse device (Taastrup, Denmark), in which the camera captures the right eye movements alone, we found that when borderline gain values of 0.72-0.8 are recorded for the left-word impulses raising the possibility of left canal dysfunction, the presence of RS with frequency >80% largely improve the diagnostic accuracy of vestibular pathology.
  5. Based on our results we recommend careful re-consideration of the vHIT test results implications when left-sided VOR gain values < 0.8 are recorded. Gain < 0.72 is highly specific for the diagnosis of vestibular dysfunction. However, for gain values in the range of 0.72-0.79 the presence of RS with frequency > 80% would largely improve the diagnostic accuracy
Introduction
The bed-side head impulse test was formulated in 1988 as a measure of the lateral semicircular canal function (1). For this test pathological response is based on the detection of re-fixation saccades (RS) that compensate for the low gain of the failing vestibulo-ocular reflex (VOR). In 2009 the video head impulse test (vHIT) was introduced (2). This technology, which is based on the capturing of eye-movements at a frequency of around 250 Hz, enables the quantification of the VOR gain and the recording of RS both during the head movement and following it (”covert” and ”overt” RS respectively). The RS parameters of latency, frequency, and velocity are provided by the commercially available vHIT systems (3). However, current clinical practice considers the VOR gain, for which norms and pathological values have been published, as the primary measure for semicircular canal function while the role of the registered RS is still under evaluation (4-7).
It was found that the RS velocity increases with decreasing gain (8,9). Also, in a group of dizzy patients with normal VOR gains the frequency of the RS was reported to increase with age (8). Several previous studies have examined the possible role of RS parameters in addition to the VOR gain towards the diagnosis of canal dysfunction. High velocity RS were demonstrated together with improved gain values among patients recovering from vestibular neuritis supporting the possible diagnostic value of RS in addition to the gain criterion (10). Overt but not covert RS were frequently recorded in asymptomatic older patients (9,11) differentiated from those found in unilateral semicircular canal dysfunction by their lower frequency and slower peak velocity. The combination of gain values <0.78 and RS frequency > 82% was recently suggested to improve diagnostic accuracy over the low gain parameter alone (9). Others even assert that the interpretation of vHIT results should first rely on the occurrence of RS and only second on the gain values (12). Another study proposed that the presence of RS albeit normal gain values indicate the existence of peripheral vestibulopathy and localizes the side of the lesion (13). For the recently introduced protocol of suppressive head impulse (SHIMP), the amplitude of anti-compensatory saccades was suggested as an indicator of residual semicircular canal function (14).
When mono-ocular eye movements registration is employed, the horizontal vHIT gain values were found to vary according to the side of the eye against which the recording camera is placed. Higher gains and longer saccade latencies were found for rightward impulses when the right eye movements only were captured (4,6,9,11,15).