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

Introduction

Benign paroxysmal positional vertigo (BPPV) is a mechanical disorder of the membranous labyrinth in which attacks of true rotational vertigo occur when the head changes its position relative to the gravity. The typical situations are laying on the bed, taking lateral recumbent positions, getting up from supine to sitting position, stooping forward (e.g., tying shoelaces), and extending neck (e.g., placing objects on a high shelf). The pathophysiology of BPPV involves inappropriate detachment of the otoconial debris, mainly composed of calcium carbonate derived from the degenerative utricular macula and its sequestration into one of the three semicircular canals (most commonly posterior, less frequently horizontal, and rarely anterior semicircular canal [ASC]). The motion of head in a direction normally moves the endolymph within the semicircular canal aligned in the direction of head movement, bending the cupula to generate an appropriate nerve impulse, which apprises the brain via vestibulo-ocular reflex (VOR), the plane, and the angle the head has moved. The brain reflexely (via VOR) makes corrective eye movements equal in angle, but the opposite direction and the point of fixation thereupon always falls on the fovea centralis.

►Table 1 shows that 56 to 93% of all patients diagnosed with BPPV at any specialty clinic have posterior semicircular canal BPPV (PSC-BPPV).^1-11 Owing to its dependent location, the PSC is most frequently involved in BPPV, and the sequestered otoconial debris in canalolithiasis usually gets entrapped in its ampullary arm. The otoconial debris rarely enters the ASC due to its relatively higher location in the membranous labyrinth, and any debris that enters it is liable to get automatically cleared for the same reason.

The diagnosis of BPPV is largely dependent on the elicitation of specific oculomotor patterns (positional nystagmus) by the positional tests, that localize as well as lateralize the affected semicircular canal as per Ewald's laws.¹² The treatment of BPPV due to canalolithiasis is performed by canalith repositioning maneuvers, innovated for each semicircular canal by the vestibulologists across the globe by conceptualizing the three-dimensional model of the membranous labyrinth. The precise location of otoconia in the semicircular canal is deciphered by observing the oculomotor patterns elicited on the diagnostic positional tests. However, despite the unerring diagnostic precision of the oculomotor patterns obtained by the diagnostic positional tests, some situations outwit the clinician resulting in inaccurate localization as well as lateralization.

In 1995, Agus et al¹³ first observed on positional tests for PSC-BPPV, a reversed oculomotor pattern of positional nystagmus in respect of its vectorial components. It consisted of a positional downbeating nystagmus (p-DBN), which was clockwise torsional in the right head-hanging position and counterclockwise torsional in the left head-hanging position (from the examiner's perspective). He attributed this finding to the presence of free-floating otoconial debris in the distal portion of the nonampullary arm of PSC that would result in its ampullopetal shift under the effect of gravitational force at the end of the positioning maneuvers. He also reported that on repeating the diagnostic positional maneuvers (Semont's diagnostic maneuver or Dix–Hallpike maneuver), the oculomotor pattern reversed (upbeating plus counterclockwise torsional in the right head-hanging position and clockwise torsional in the left head-hanging position, from the examiner's perspective). It was hypothesized that at the end of the next “sitting repositioning” maneuver, under the influence of gravitational force, the ampullopetal movement of the otoconial debris shifts it to the juxta-cupular position in the proximal portion of the PSC. In 2012, Vannucchi et al¹⁴ reported in six patients of positional vertigo: an identical pattern of temporal course and first named it apo-PSC-BPPV. All six patients had an initial oculomotor pattern of torsional DBN on positional tests, that localized the disorder to the ASC, and accordingly, all six were treated with some form of physical therapy for it. However, during regular follow-up over the next few days, all six patients developed a different oculomotor pattern of upbeating positional nystagmus with a reversal in the direction of torsion as well, which localized the disorder to the contralateral PSC. It is extremely unlikely that these six patients had a resolution of their initial ASC- BPPV of one side but within a few days developed geo-PSC- BPPV (the commoner geotropic variant) of the contralateral side. Ever since the variant apo-PSC-BPPV from Italy has been reported in 23 cases by Califano et al,¹⁵ 22 of whom were successfully treated with a quick liberatory rotation maneuver; Vannucchi et al¹⁶ in 16 patients; and Asprella- Libonati and Pecci¹⁷ in 28 patients described two innovative maneuvers, namely Demi-Semont and 45 degrees forced prolonged positioning, with successful outcomes in 11/16 and 23/28 patients, respectively. Outside Italy there are only two case reports of three patients with apo-PSC-BPPV, two from Illinois, United States,¹⁸ and one from Cleveland, Ohio, United States.¹⁹ The apo-PSC-BPPV due to nonampullary arm posterior semicircular canalolithiasis appears to be an important contributor to the expanding realm of the etiology of peripheral p-DBN. However, in a study of 77 patients of geo- PSC-BPPV, 22 out of those 77, who were successfully treated with Epley maneuver, also exhibited a transient p-DBN on a verifying Dix–Hallpike test 1 hour later. The presence of residual debris in the distal nonampullary portion of the PSC was attributed to this finding, and 5 out of the 22 patients (23%) developed a geo-PSC-BPPV again at a 1-week follow-up.²⁰

We are reporting the case of a 48-year-old female patient with 2 days history of vertigo on lying supine, getting up from supine, and on taking either of the lateral recumbent positions along with a prominent sense of continuous dizziness. She was initially diagnosed as left ASC-BPPV, based on the right Dix–Hallpike maneuver eliciting after a short latency, a downbeating counterclockwise (from the patient's perspective) torsional positional nystagmus. Accordingly, she was treated with sessions of reverse Epley maneuvers for 1 week. At the end of 1 week, the verifying right Dix–Hallpike test elicited an upbeating clockwise (from the patient's perspective) torsional nystagmus suggesting a diagnosis of the typical right geo-PSC-BPPV. Such a temporal course suggests that the patient was suffering from the recently described rare atypical variant of the right apo-PSC-BPPV from the outset, which transformed into the typical geo-PSC-BPPV as a result of the intracanal shift of the otoconial debris from the nonampullary arm of the PSC to the juxta-cupular region in its ampullary arm. To the best of our knowledge, such a rare variant of apo-PSC-BPPV has not been reported from India hitherto.

Case Description

Discussion

Subjecting a patient with a history of rotational vertigo triggered by changes in the position of head relative to the gravity, to the Dix–Hallpike test, opens a Pandora's box of a diagnostic dilemma if a p-DBN is elicited. If such a patient does not have any other central nervous system symptoms or signs, the responsible lesion could be; either serious localizing to the intracranial posterior fossa and craniovertebral junction; or benign due to conditions like ASC-BPPV. A recently described variant of PSC-BPPV due to nonampullary canalolithiasis, called apo-PSC-BPPV, is another diagnostic consideration in such patients. The potentially serious causes of p-DBN can be ruled out by obtaining magnetic resonance imaging of the brain, with dedicated views of the posterior fossa structures and the screening of the craniovertebral junction. However, normal magnetic resonance imaging in such patients does not ease out the situation, for the treating otoneurologist. An analysis of the genesis of oculomotor patterns during the right Dix–Hallpike positioning in this patient of right apo-PSC-BPPV, with involved pathophysiologic mechanisms that resulted in positional nystagmus identical in its vectorial components to the left ASC-BPPV is discussed here. In the right Dix–Hallpike test, the otoconial debris located in the ampullary arm of the left ASC shifts ampullofugal, under the effect of gravity triggering excitatory discharges in the left anterior ampullary nerve. However, if otoconial debris is in the nonampullary arm of the right PSC, its ampullopetal movement in the right Dix–Hallpike positioning will trigger inhibitory discharges in the right posterior ampullary nerve. In either of the situations, this will cause upward/clockwise (from patient's perspective) slow phase VOR, due to the combined action of the right (inferior oblique and superior rectus [SR]) and the left (superior oblique and SR). Therefore, the fast-phase VOR which is a refixation saccade (clinically appreciated as positional nystagmus), will be downbeating in both eyes with incyclotorsional component in the right eye and excyclotorsional component in the left eye (downwards and counterclockwise from patient's perspective) (►Fig. 1). It is extremely unlikely that this patient had resolution of her initial left ASC-BPPV with the daily sessions of reverse Epley maneuver carried over a week to be immediately followed by a paroxysm of the commoner geo-PSC-BPPV. It is plausible to interpret that this patient suffered from the right apo-PSC-BPPV from the very outset. Reverse Epley maneuver sessions performed to treat suppositious left ASC-BPPV led to an ampullopetal shift of the otoconial debris jammed in the nonampullary arm of the right PSC to the juxta-cupu- lar location. Thus, this may be looked upon as an intracanal type of canal-switch phenomenon that occurred in the right PSC leading to the changing oculomotor patterns on the right Dix–Hallpike positioning observed on day 1 (January 23, 2020) and day 8 (January 30, 2020).

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Fig. 1

In the right Dix–Hallpike position, there is an ampullofugal shift of otoconial debris in the ampullary arm of the left anterior semicircular canal, which generates excitatory impulses in the left anterior ampullary nerve. This causes the co-contraction of the right (inferior oblique [IO] and superior rectus [SR]) and the left (superior oblique [SO] and SR). Otoconial debris in the nonampullary arm of the right posterior semicircular canal during the right Dix–Hallpike positioning will result in its ampullopetal shift, which generates inhibitory impulses in the right posterior ampullary nerve, which also causes the co-contraction of the same pair of extraocular muscles. In either of the situations, the generated slow phase VOR is directed upwards (straight arrow) and is clockwise torsional (curved arrow) from the patient’s perspective. Therefore, the fast phase VOR (which is appreciated as positional nystagmus) is downbeating and counterclockwise torsional from the patient’s perspective. IR, inferior rectus; LA, left anterior semicircular canal; LH, left horizontal semicircular canal; LP, left posterior semicircular canal; LR, lateral rectus; MR, medial rectus; RA, right anterior semicircular canal; RH, right horizontal semicircular canal; RP, right posterior semicircular canal; VOR, vestibulo-ocular reflex.

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Conclusion

In any patient presenting with a history of positionally triggered vertigo, a change in oculomotor patterns elicited on positional tests, suggesting an initial ASC-BPPV (positional downbeating torsional nystagmus) to contralateral geo- PSC-BPPV (positional upbeating nystagmus with a reversed vectorial torsional component) over a short period is highly indicative of apo-PSC-BPPV from the very outset. The positional tests or therapeutic maneuvers, which are performed to diagnose/treat the putative condition, lead to an ampullo- petal shift of the otoconial debris present in the nonamp- ullary arm of the PSC to the juxta-cupular location. It can be regarded as a form of intracanal switch canalolithiasis within the PSC due to the shift of the otoconial debris from its nonampullary arm to the ampullary arm. The transformation of a nonampullary to the ampullary arm posterior semicircular canalolithiasis makes this disorder treatable by the better-established treatment options.