OBJECTIVETo discuss the objective characteristics and mechanism of nystagmus direction, intensity and time in horizontal semicircular canal benign paroxysmal positional vertigo (HSC-BPPV) .

METHODSA total of 233 patients with HSC-BPPV, whereas 179 horizontal semicircular canalithasis (HSC-Can) and 54 horizontal semicircular cupulolithiasis (HSC-Cup) were involved respectively. The induced nystagmus in roll tests recorded by video-nystagmograph(VNG) , whose direction, intensity and time characteristics were compared in various BPPV.

RESULTSHorizontal nystagmus was both induced by turning left or right in HSC-BPPV roll tests. The direction of the induced nystagmus was the same with turning in HSC-Can. The latency, duration time and intensity ([AKx(-)D] ± s) turning to lesion and normal side were (1.922 ± 1.501)s and (1.447 ± 0.855)s, (25.620 ± 10.409)s, and (22.110 ± 10.931)s, (56.441 ± 33.168)°/s and (24.239 ± 13.892) °/s in HSC-Can. The latency, duration time and intensity turning to lesion side were larger than normal side (t = 3.715, 15.219 and 4.070, P < 0.01) , the difference was statistically significant, and the intensity rate was about 2: 1. The direction of the induced nystagmus was opposite to turning in HSC-Cup. The intensity turning to normal side was larger than lesion side obviously. The intensity rate was about 2: 1, the difference was statistically significant (t = -7.634, P < 0.01) . While the latency and intensity of turning to lesion side in HSC-Can were larger than turn to normal side in HSC-Cup, and the difference detected no statistically significant difference (t = 1.554 and 0.305, P > 0.05).

CONCLUSIONSThe induced nystagmus intensity of head to two sides in roll tests for HSC-BPPV both follow Ewald's law, and the ratio between stronger and weaker are both 2: 1. These nystagmus parameters of VNG in roll tests are an objective guideline for BPPV diagnosis.