vHIT systems employing mono-ocular eye movements recording have an inherited right-left imbalance regarding the horizontal semicircular canal gain values depending on the location of the camera. A study including 212 healthy subjects from all ages showed a mean 9.1% gain value difference in favor of the right side in all age groups while using right eye recording system (6). Another recent study of commercially available vHIT systems reported on 5% rightward gain bias when the right eye movements only were captured (18). While using scleral search coils for accurate binocular eye movements recording it was found that the horizontal VOR gain for the adducting eye exceeded the gain of the abducting eye by an average of 15.3% (19). Several explanations have been offered for this right-left imbalance; a longer neural pathways (trisynaptic) for the adducting medial rectus, compared to the shorter (disynaptic) pathway of the abducting lateral rectus when the system camera captures the right eye movements alone (6,19). Alternatively, the higher gain might stem from the relative decrease in head velocity while a right-handed examiner uses his weaker left hand for the conduction of the rightward head thrust. Others have attributed the rightward higher gain to asymmetry in the maximal active force of the medial rectus when compared to the lateral rectus muscle which is about 26% greater for the adducting muscle (4,20).

Although we have found in the NVD group a significant gain difference in favour of the rightward head impulse which was conducted by a right-handed examiner, the head velocities to both sides were comparable. Thus, for our study the higher right-sided gain is not explained by possible examiner-related factors.

The reported bias in favour of lower left hvHIT gain when right eye movements alone are recorded, that is also supported by our results in the NVD group, might introduce false-positive results of left sided vestibulopathy especially when the VOR gain is used as the sole diagnostic criterion. In the current study we evaluated the added benefit of RS in reaching a final diagnosis of left horizontal semicircular canal hypofunction in using the right mono-ocular ICS impulse vHIT system.

The recommended published VOR gain cut-off value discriminating normal and pathological horizontal semicircular canal function is 0.8 (2,4-7,16). Based on our laboratory norms we have looked at a cut-off value of 0.72. Significantly lower average left-side VOR gain and higher gain asymmetry were found for the VD group, and all the NVD group participants had left hvHIT gain > 0.72. Still the sensitivity of gain <0.72 alone towards the diagnosis of VD was only 80%.

The existence of RS has been previously reported in healthy subjects, with increased frequency and velocity with higher age and lower gain values (6,21). In our study the mere presence of RS had 100% sensitivity but only 40% towards the diagnosis of VD. A recent research has suggested RS frequency of 80% as a parameter for their consistency (13). Adding the restriction parameter of RS frequency>80% increased the hvHIT specificity to 100% with 89% sensitivity.

Various RS velocity criteria have been proposed to distinguish normal vs pathological vestibular function. These include critical RS velocity of 50 (13), 100 (21), 133 (22) and 135 deg/sec (9).

Although RS average velocity was significantly higher in the VD group and significantly higher proportion of VD patients demonstrated RS with velocity > 150 degrees/sec, this RS velocity criterion alone or adding it to the RS frequency >80% parameter did not improve the vHIT prediction values.

Based on our results we recommend careful re-consideration of the hvHIT 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 hvHIT diagnostic accuracy (Fig. 5).

Our study limitations include its retrospective nature and the relatively small size of the cohort included. Although vestibular disease was ruled out in our NVD subjects, the ideal control group should have been comprised of healthy subjects with no complaint of dizziness, which we found difficult to recruit. Most (85%) of the VD group have suffered from vestibular neuritis. Still, the inclusion of other diagnoses might have introduced some disparity in our results.