Pathophysiology of lateral semicircular canal paroxysmal positional vertigo

Objective: To study the pathophysiology of lateral semicircular canal (LSC) paroxysmal positional vertigo (PPV). Study design: Retrospective study. Methods: Between June 2004 and June 2005 we observed 471 patients with PPV. In the apogeotropic forms of LSC-PPV, we tried to transform the nystagmus into the geotropic form, either by diagnostic or therapeutic menoeuvres. If we failed, we advised barbecue rotations toward the healthy side and sleeping on the affected side. Patients were evaluated once a week until resolution. Results: 91 patients suffered LSC involvement, in 61 cases in the geotropic form and in 30 cases in the apogeotropic form. Out of these, five transformed into the geotropic form during the first examination; in 5 patients we observed geotropic nystagmus at the follow-up visit; the last 20 never showed geotropic nystagmus before resolution. Conclusion: We hypothesize that in the geotropic form the debris is free floating in the posterior arm of the LSC (canalolithiasis). In the apogeotropic form the debris can be free floating in the anterior arm or attached to the cupula of the ampulla: if we observe transformation from the apogeotropic into the geotropic form this suggested a canalolithiasis, otherwise we have assumed a cupulolithiasis.