Lying Down Nystagmus in Lateral Canal Paroxysmal Positional Vertigo

<b>Introduction</b>: The aim of this study was to determine the position of otoconial debris in lateral ampullar or non-ampullar canalolithiasis, based on two parameters: (1) the direction of the nystagmus appearing when the patient lies down, if present, and (2) the positional nystagmus evoked by the supine roll test. <b>Methods</b>: Theoretical results were compared with a population of 170 patients observed over the past ten years for horizontal canal benign paroxysmal positional vertigo (HC-BPPV). The series included 141 geotropic and 29 apogeotropic cases. <b>Results</b>: Among the geotropic forms, 80 showed no supine nystagmus (Geotropic Nystagmus with no supine nystagmus, GT0) (56.7%), 51 had supine nystagmus directed toward the healthy side (Geotropic Nystagmus with supine nystagmus congruent, direct toward the healthy side, GT+) (36.2%), and 10 toward the affected side (Geotropic Nystagmus with supine nystagmus incongruent direct to the affected side, GT-) (7.1%). In the apogeotropic group, 10 showed no supine nystagmus (Apogeotropic nystagmus with no supine nystagmus, AGT0) (34.6%), 16 had nystagmus toward the affected side (Apogeotropic Nystagmus with supine nystagmus congruent, direct toward the affected side, AGT+) (55.2%), and 1 toward the healthy side (Apogeotropic Nystagmus with supine nystagmus Incongruent, direct toward the healthy side, AGT-) (3.4%). Two cases presented monopositional apogeotropic nystagmus (mAGT), consistent with a "sieve-type canal jam" (6.8%). Overall, 90 out of 170 patients (52.9%) showed no nystagmus in the supine position, with a statistically significant difference between variants (<i>p</i> = 0.0474, Yates correction). <b>Conclusions</b>: The comparison between lying-down nystagmus and positional nystagmus, assessed through the Supine Roll Test as the leading diagnostic maneuver for horizontal canal involvement, may help identify the initial location of debris within the lateral semicircular canal and guide the appropriate liberatory maneuver, while the effectiveness and side of the maneuver allow the distinction between canal-side and utricular-side jams.