Mechanism of posterior semicircular canal stimulation in patients with benign paroxysmal positional vertigo

A quantitative study of the stimuli and vestibulo-ocular response associate d with benign paroxysmal positional vertigo (BPPV) was made to test and fur ther develop the canalithiasis theory of BPPV. The angular velocities of th e head in the planes of the semicircular canals during the Dix-Hallpike tes t were measured in four healthy subjects using electromagnetic sensors to r ecord the position of the head in a sir degrees of freedom paradigm. Next, the nystagmus reactions in seven patients diagnosed with idiopathic BPPV we re recorded with video-oculography. The characteristics of the vestibulo-oc ular reflex (VOR) response were analyzed using, three-dimensional vector te chniques. The angular velocity of the head was primarily, but not exclusive ly, in the plane of the posterior semicircular canal (PSC) in question. Bot h the anterior and horizontal canals were also stimulated by a lesser degre e. The duration of the motion stimulus in the PSC was < 1.3 s with peak ang ular velocities of 150 deg/s. The eye response in BPPV patients began 4 s a fter the test and had a duration of 15-20 s. Peak slow-component eye veloci ties of about 42 deg/s were reached 3-5 s after onset of nystagmus, The mot ion of the eye, as predicted by the cupulolithiasis theory, is disconjugate d and has torsional, vertical, and horizontal components. In the eye ipsila teral to the tested ear it is primarily torsional (0.80, 0.54, 0.16) and in the contralateral eye it is mainly vertical (0.57, 0.73, 0.08). These resu lts suggest that particles, initially resting on the floor of the cupula do me in the PSC, are perturbed by the Dix-Hallpike test and disperse freely i nto the endolymph where they are propelled by gravity into the canal lumen. This creates abnormal pressure on the cupula and the specific VOR activati on of the ipsilateral superior oblique and the contralateral inferior rectu s muscles, whose force vectors are indistinguishable from the measured eye motion vectors. The estimated pressure exerted on the crista is approximate ly 10(-2) dyn/cm(2).