The VD and NVD groups each included 11 men and 9 women. The mean subjects’ age of the VD group participants was 56.45+ 16.64 years (mean + standard deviation) and did not significantly differ from that of the NVD group which was 50.75+ 14.58 years (unpaired t-test). The final diagnosis of the 20 patients included in the VD group was vestibular neuritis in 17, vestibular schwannoma in 2 and intra-tympanic gentamicin- induced vestibulopathy in 1 patient. The diagnosis of vestibular neuritis was reached only if all the following symptoms and signs were present: acute onset of prolonged severe rotatory vertigo, the presence of spontaneous nystagmus and postural imbalance, and documentation of unilateral reduced caloric response (caloric lateralization > 25%) on the caloric study of videonystagmography.

For the NVD group the average rightward hvHIT gain was 0.93+ 0.1 (95% CI 0.88-0.98) and the leftward gain 0.75+ 0.02 (95% CI 0.74-0.77). The difference was of statistical significance (p<0.0001; Wilcoxon matched-pairs signed-ranks test).

The rightward and leftward head velocity values did not significantly differ reaching 210+ 31.3 (95% CI 195.7-224.9) and 213.9+33.8 (95% CI 198.1-229.7) deg/sec respectively (Paired t-test).

No significant difference was found between the leftward head velocities of the VD and NVD groups registered as 207.3+ 33.2 (95% CI 191.8-222.8) and 213.9+ 33.8 (95% CI 198.1-229.7) respectively (Unpaired t-test).

The average left hvHIT gain of the VD group was significantly lower as compared to the NVD group (0.445+ 0.23 vs. 0.757+ 0.024; p<0.0001, unpaired t-test; Fig. 1) and the gain asymmetry average, where higher gains were recorded for the rightward stimuli, was significantly larger (39.2+ 24.11% vs. 18.12+ 9.05%; p<0.002, unpaired t-test; Fig. 2).

As detailed in section 2.3 the calculated normal range in our laboratory for the right hvHIT gain is 0.72-1.28 and 0.72-1.04 for the left hvHIT gain.

Gain values > 0.72 were found in all NVD patients and in 4 (20%) patients of the VD group (p<0.0001; Fisher’s exact test).

Significantly higher left-sided RS velocity and frequency were found in the VD group (221.35+ 89.18 vs. 131+ 42.93 degrees/sec; p<0.0002 and 83.2+ 33.2% vs. 24.15+ 29.8%; p<0.0001, Mann–Whitney test respectively; Fig. 3, Fig. 4). No differences between the groups were found in the RS latency (196.71+ 52.13 vs. 250.82+ 114.78 msec for the VD and NVD groups respectively. Not significant by unpaired t-test, Welch corrected for different standard deviations).

RS were detected in 19 (95%) of the VD patients and 12 (60%) of the NVD group (p<0.02; Fisher’s exact test).

Among the 19 VD patients in whom RS were detected 17 (89%) had RS frequency > 80%, while in all the 12 NVD patients having RS the frequency value was < 80% (p<0.0001 Fisher’s exact test).

RS velocity was higher than 150 deg/sec in 17 of the 19 VD patients (89%) and in only 3 of the 12 NVD patients (25%) having RS (p<0.0004 Fisher’s exact test).

17 of the VD group out of 19 (89%) and 1 of the NVD group out of 12 (8%) had RS with both frequency > 80% and velocity > 150 deg/sec (p<0.0001 Fisher’s exact test).

15 of the VD group participants (75%) had gain values < 0.72 and RS frequency > 80% while in none of the NVD group such association was recorded (p<0.0001 Fisher’s exact test).

15 of the VD group (75%) and 3 NVD group participants (15%) presented with gain values < 0.72 and saccade velocity > 150 deg/sec (p<0.0004 Fisher’s exact test).

The combination of gain values < 0.72, saccade frequency > 80% and saccade velocity > 150 deg/se was found in 15 (75%) of the VD group and none of the NVD group (p<0.0001 Fisher’s exact test).

The sensitivity, specificity, positive predictive value and negative predictive value of the following outcome measures towards the prediction of VD are detailed in Table 1.