I had written a blog entry back in 2017 about a case study of mine that I had felt was similar to a Horizontal Semicircular Canal Jam: “Stuck BPPV: A Canalith Jam Case Study”. Recently, a new article by Michael Schubert, Janet Helminski, David Zee and a few other researchers was published detailing two specific case studies of horizontal semicircular canal jams. They provide a clear picture of what should be considered diagnostic criteria for this phenomenon. They also touch on a theory about the “light cupula” phenomenon. Overall, it’s a fantastic article/case study and you should read it!
First let’s talk about the prevalence of Horizontal Canal BPPV. “Horizontal semicircular canal (hSCC) benign paroxysmal positional vertigo (BPPV) accounts for 22% of all BPPV, with paroxysmal nystagmus during positional testing being geotropic in 70%, apogeotropic in 28%, and direction-fixed in 1.8%”2 So overall, the chances of seeing direction fixed nystagmus during positional testing, where the velocity of the nystagmus seems dependent on the patient’s head position, is very small. However, it’s important to have an idea of what you’re looking at if you happen to evaluate a patient with this presentation! This type of patient could easily be mistaken for having a central issue rather than BPPV.
Can you picture the different types?
What is a Canal Jam?
A canal jam occurs when there is a partial or complete obstruction within the lumen of the canal. The study suggests that this can occur when debris is moving from a larger section of the canal and gets bottle-necked into a more narrow portion of the canal. This can either happen spontaneously, or even during canalith repositioning maneuvers.
When an obstruction occurs, it is thought that the jam may create a prolonged inhibitory or excitatory stimulus due to the prolonged deflection of the cupula. This would cause persistent direction fixed horizontal nystagmus regardless of head position.
What happens when the jam breaks up? The answer is geotropization. This is the liberation of the jam which causes an immediate change of the nystagmus from direction-fixed to geotropic nystagmus. Once converted to geotropic nystagmus, the BPPV can be treated using the barbecue roll (log roll) or Gufoni's liberatory maneuver.2
Article Case Studies
Case 1: Overview
Conclusion: The patient presented with a Right Horizontal Semicircular Canal Jam in which the cupula was deflected to create a prolonged inhibitory response.
Case 2: Overview
Conclusion:The patient presented with a Right Horizontal Semicircular Canal Jam in which the cupula was deflected to create a prolonged excitatory response.
“An interesting phenomenon that I have occasionally observed while undertaking the canalith repositioning procedure (CRP), is a sudden conversion of transient nystagmus to a rapid form that persists irrespective of head position. Simultaneously, the patient usually complains of intense vertigo. I believe the mechanism to be a jamming of the canaliths…”.1 John Epley (1995)
How to Identify a Canal Jam
This article provides great criteria to use for identifying whether your patient has a canal jam:
Determining the side of involvement could be difficult without the conversion of the unidirectional positional nystagmus to geotropic nystagmus. Take both cases in this article for example. Both patients had a Right Horizontal Semicircular Canal Jam, but each presented differently due to the direction of prolonged cupula deflection. Once the jam is cleared, and the patient presents with a more typical presentation of canalithiasis or cupulolithiasis, the side of involvement can be identified.
Based on the presentation of these two cases, there remain few differential diagnoses that would cause the sudden reversal of nystagmus during positional testing, and thus this finding is critical to be confident in diagnosing canalith jam. Vestibular migraine can cause a persistent nystagmus that is position dependent, though the velocities of the nystagmus tend to be less than reported in the case studies.2
In one last final note, the authors suggest that “Semicircular canal membrane plugs (jams), even perhaps more than one in a single canal, may be more common than appreciated and might be responsible for other unusual patterns of nystagmus such as persistent geotropic nystagmus with vertigo that currently is attributed to the “light cupula” phenomenon.”2
After reading through this study, it’s safe to say that my patient in 2017 was not experiencing a true canal jam, but more likely experiencing a form of Horizontal Canal BPPV, Canalithiasis-type where the bolus of debris was moving freely within the canal but was too large to pass through the canal opening back into the utricle.
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