First, some simple anatomy
There is a big difference between being “weightless” and being “light”. Just ask the cupula.
The cupula is a gelatinous billow-like structure that sits at the end of each semicircular canal of the vestibular organ (inner ear). It doesn’t move unless you produce head movement like shaking your head “no”, nodding your head “yes”, or tilting your head side to side. Think of it as jello sitting on a plate. It shouldn’t move unless you move the plate.
The movement of fluid known as endolymph pushes or deflects the cupula to either “excite” or “inhibit” each canal. Normally, the density of the cupula is equal to the fluid that surrounds it. That’s how it stays stationary when the head is stationary. It neither floats or sinks. Gravity has no effect on it, or at least it shouldn’t. If a person’s cupula floated or sunk depending on what position they were in, they may experience vertigo for however long they stay in that position.
Benign Paroxysmal Positional Vertigo (BPPV) is the most common cause of vertigo associated with changes in head position. This condition is typically caused by the displacement of otoconia or crystals in the semicircular canals of the vestibular organ. When they move through the canal as your head position changes (like stones sinking in water), they inappropriately deflect the cupula, causing the canal to either be excited or inhibited when it shouldn’t be. Free floating otoconia can be skillfully guided out of the canals with maneuvers like the Epley maneuver for the posterior canal, and the log roll or Gufoni maneuver for the horizontal canal. There is also the chance that the crystals could be stuck to the surface of the gelatinous cupula. This is a problem because now the cupula becomes heavier than the fluid around it.
Let's Get Technical
Remember Ewald's Laws
BPPV in the Horizontal Semicircular Canal is generally diagnosed with the roll test. During this test, the patient is supine on the table with their head elevated with approximately 30 degrees of cervical flexion. The patient’s head is then turned to each side anywhere between 45-90 degrees. (Remember: raising the head 30 degrees is important in order to position the canals perpendicular to the floor/parallel to gravity for optimal otoconia or cupula displacement)
Generally speaking we're taught that when the patient’s head is turned during the test, nystagmus that beats to the ground (geotropic) and is only transient/temporary in duration is BPPV Canalithiasis-type. In this case, the crystals are free floating. If the nystagmus beats away from the ground and is persistent (doesn’t stop/fatigue for longer than 1 minute), the patient has BPPV Cupulolithiasis-type. The crystals are likely adhered to the cupula creating a “heavy” cupula. So, what does it mean if you come across a patient with persistent geotropic nystagmus on both sides of the roll test?
To picture what needs to happen in order to achieve persistent geotropic nystagmus, think in terms of Ewald’s Second Law: Fluid flow (or otoconia/cupula) moving towards the cupula/utricle in the HORIZONTAL canal is an EXCITATORY stimulus. Picture the cupula in the right horizontal semicircular canal. In order to have nystagmus beating towards the floor in a right roll test, the right horizontal canal would have to be excited (more neurally active). The cupula would have to deflect toward the utricle. In order to have nystagmus beating toward the floor in a left roll test, the right horizontal canal would have to be inhibited, making the left horizontal canal technically more neurally active in comparison. The right cupula would have to deflect away from the utricle.
Imai et al2 compared 107 patients who had either persistent ageotropic nystagmus or persistent geotropic nystagmus. Their findings showed that persistent geotropic nystagmus was consistent with the time constant and relationship between slow phase velocity and the angle of head rotation as observed in persistent ageotropic nystagmus. The difference being the direction of the nystagmus. “Therefore, it is suggested that the response of the lesioned horizontal semicircular canal cupula to the head position in patients with persistent geotropic nystagmus was opposite to that in those with ageotropic nystagmus.” Instead of the cupula being weighted down by adhered otoconia, the cupula is actually lighter than the fluid around it.
Diagnostic Criteria for Light Cupula
Kim et al4 studied 19 patients who presented with geotropic direction changing nystagmus. Their methods included a sequence of positions which can be used to test for a light cupula.
Essentially, this boils down to 3 tests: The Bow and Lean, The Roll Test, and The Null Point Test.
The study by Kim et al4 yielded some additional interesting findings.
The Null Point
When the head is slightly turned to the affected side in the supine position until the horizontal semicircular canal’s cupula is aligned with the plane of gravitational vector, the nystagmus stops.3 This is referred to as the null point. At this point, the cupula neither floats or sinks and doesn’t stimulate or inhibit the canal.
What causes a Light Cupula?
In reality, there is a lot of controversy and there are different theories reported about the pathophysiological mechanism for a light cupula phenomenon. Here’s rough breakdown of what was reported in the studies used for this blog:
What to do with a Light Cupula
According to the research, “we don’t know” just about sums it up. The reason why the cupula has become lighter is unclear, but there is the understanding that canalith repositioning maneuvers do not have any effect on the condition.
“Symptoms of vertigo and positional nystagmus resolve spontaneously within 2 weeks in most patients with light cupula. However, in some cases in whom persistent geotropic nystagmus is accompanied by sudden sensorineural hearing loss, positional vertigo may persist for several months or even longer.”3
If patients are extremely symptomatic, an over the counter vestibular suppressant may help to keep the patient comfortable. Perhaps we can also educate the patient to position themselves in the null point when trying to go to sleep at night. Habituation exercises like the Brandt-Daroff exercises may also be considered.
If you’re still reading this at this point, is there anything you’ve found that has helped your patient? Leave a comment below!
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