Peripheral Downbeat Positional Nystagmus: Apogeotropic Posterior Canal or Anterior Canal BPPV.

Background And Purpose: Downbeat nystagmus (DBN) during the Dix-Hallpike test (DHT) suggests excitation of the anterior canal (AC) or inhibition of the posterior canal (PC) underlying benign paroxysmal positional vertigo (BPPV). This case series describes 2 individuals presenting with DBN in positional testing suggestive of a PC BPPV variant termed apogeotropic PC-BPPV and due to inhibition of the PC.

Case Descriptions: Case 1 illustrates a DBN during positional testing (PC inhibition) that changes to an upbeating nystagmus (PC excitation) representing the otoconial material changing location and direction of movement within the PC. Case 2 describes a canal jam in the nonampullary segment of the PC.

Differential Diagnosis: Apogeotropic PC-BPPV can cause DBN due to inhibition of the vestibular afferent. Apogeotropic PC-BPPV may be due to a canal jam of debris within the nonampullary segment or cupulolithiasis with debris attached to the inferior-most aspect of the cupula within the PC. It can be difficult to differentiate AC-BPPV from the apogeotropic PC-BPPV variant. In both forms, the affected canal may be provoked in 1 or both positions of the DHT and straight head hanging position. However, in AC-BPPV there may only be a slight or absent torsional component toward the involved ear. In apogeotropic PC-BPPV, a strong torsion away from the involved ear is typically observed. The straight head hanging position may resolve AC-BPPV or convert apogeotropic PC-BPPV to typical PC-BPPV.

Summary: These 2 cases illustrate atypical variants of BPPV that clinicians must consider in their interpretation of DBN during positional testing, particularly in the absence of other neurological signs.