Semicircular Canals & BPPV
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The semicircular canals & benign paroxysmal positional vertigo (BPPV)
In benign paroxysmal positional vertigo, calcium carbonate crystals from the utricle fall into one of the semicircular canals— most often the posterior canal — and stimulate positional vertigo.
- We draw an oval-shaped cranium and label its anterior and posterior aspects, and then its right and left sides.
- The left anterior semicircular canal faces anterolaterally.
- The left posterior semicircular canal lies perpendicular to it (the posterior canal lies along the axis of the petrous ridge).
- The horizontal canal faces laterally.
- We draw the right-side semicircular canals as mirror images of the left.
In normal, upright head position, the horizontal canal is tilted upward about 30 degrees to the horizontal plane and the anterior and posterior canals are roughly within the vertical plane. When sitting upright, if the head is tilted down 30 degrees, the horizontal canals are brought into the earth-horizontal plane.
AMPULLA & CUPULA
- At the utricular end of each canal is a membranous enlargement, called the ampulla, which contains the sensory cell system of the semicircular canal.
- Attached to the ampulla is the cupula, which is a gelatinous mass that deflects during angular acceleration. When the cupula bows, the hair cells are activated and, as a result, the semicircular canals generate eye movements in their plane of orientation.
- The left anterior canal and the right posterior canal drive the eyes along the same diagonal.
- The right anterior and left posterior canals drive the eyes along the same diagonal.
- The horizontal canals drive the eyes laterally.
We will divide the directionality of the eye movements generated by the anterior and posterior semicircular canals (the vertical canals) based on whether the canal is anterior or posterior, right or left. Eye movements are commonly described based on the direction of the fast-phase of the nystagmus they produce. Here, we draw the slow phase of the nystagmus.
eye movement directionality from the observer's (or examiner's) perspective in coronal view.
- The anterior canals produce upward movements.
- The posterior canals produce downward movements.
- The right-side horizontal semicircular canals stimulate movements in a clockwise direction.
- The left-side horizontal semicircular canals stimulate movements in a counterclockwise direction.
- If the right posterior canal is activated, then from the observer's perspective, the slow phase of the nystagmus is downward with clockwise rotation.
- The fast phase, which is how the nystagmus is actually named, is in the opposite direction: upward and counterclockwise.