A Case of Right Apogeotropic Posterior Semicircular Canal BPPV that Initially Emulated as Left Anterior Semicircular Canal BPPV

In any patient with a history of rotational vertigo triggered by changes in the position of head relative to the gravity, whose oculomotor patterns elicit a positional downbeating nystagmus (p-DBN), the localization could be either central in the brainstem, midline cerebellum, or at the craniocerebral junction; or else peripheral due to one of the rare variants of benign paroxysmal positional vertigo of vertical semicircular canals. Most serious causes of central vertigo in patients with p-DBN can be diagnosed by magnetic resonance imaging of the posterior fossa and craniovertebral junction. However, the peripheral p-DBN could be either due to anterior semicircular canal benign paroxysmal positional vertigo (ASC-BPPV) or a recently described apogeotropic variant of posterior semicircular canal BPPV (apo-PSC-BPPV) and the two are almost impossible to differentiate initially. The usual clinical scenario in apo-PSC-BPPV is diagnosing it initially as ASC-BPPV. However, following diagnostic or therapeutic positioning maneuvers for the purported ASC-BPPV, the positional oculomotor pattern changes to an upbeating nystagmus with the reversal in the direction of the torsion as well, localizing it to the contralateral PSC with respect to the ASC initially diagnosed. The initial oculomotor pattern observed on the right Dix–Hallpike test in this patient, of a short latency downbeating left torsional (from the patient’s perspective) positional nystagmus suggested a diagnosis of left ASC-BPPV, which was accordingly treated with multiple sessions of reverse Epley maneuvers daily for a week. At the end of the week, a verifying right Dix–Hallpike test elicited an upbeating right torsional (from the patient’s perspective) positional nystagmus. It is extremely unlikely that this patient had resolution of her initial left ASC-BPPV with the daily sessions of reverse Epley maneuvers carried over a week and immediately suffered from commoner geotropic variant of the right PSC-BPPV (geo-PSC-BPPV). It is plausible to interpret that this patient suffered from the right apo-PSC-BPPV from the very outset, and the reverse Epley maneuver performed for the ostensive left ASC-BPPV led to an intracanal shift of otoconial debris from its nonampullary to the ampullary arm resulting in right geo-PSC-BPPV. The reasons why situations like this outwit the clinician resulting in inaccurate localization as well as lateralization is discussed. The patient was successfully treated with right Epley maneuver after transformation to geo-PSC-BPPV and was asymptomatic at follow-up for 4 weeks. A peripheral p-DBN with torsional component in any patient with a history of positionally triggered vertigo can be either ASC-BPPV or apo-PSC-BPPV. A very close follow-up at a short interval of time with meticulously executed positional tests is the only definitive way to differentiate the two conditions.