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
- First Law: The direction of the nystagmus is directly correlated to the canal that is being stimulated… not the position of the head.
- Second Law: Fluid flow (or otoconia/cupula) moving towards the utricle in the HORIZONTAL canal is an EXCITATORY stimulus.
- Third Law: Fluid flow (or otoconia/cupula) moving away from the utricle in the VERTICAL (posterior and superior/anterior) canals is an EXCITATORY stimulus.
- Note: Nystagmus always beats to the more neurally active (or excited) ear
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.
- The patient bows the head to 90 degrees in the sitting position
- The patient leans the head backward to 45 degrees in the sitting position
- The patient’s head is turned to the right to 90 degrees in supine
- The patient’s head is turned to the left to 90 degrees in supine
- The patien’ts head is turned to the right or left about 15-20 degrees in supine to find the null position.4
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.
- Of the 19 patients studied, the male-to-female ratio was 5:14 indicating that there is a higher incidence among females which is consistently observed in other studies.
- Of 18 patients who’s involved side could be identified, 13 were right side involved, and 5 left side
- The authors noted that over the span of their study, 134 patients exhibited geotropic nystagmus. 19 of which were diagnosed with a light cupula making them 14.2% of patients with geotropic nystagmus. This indicates that a light cupula may not be a rare disease entity and may often be misdiagnosed as a canalithiasis-type of horizontal canal BPPV.4
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:
- A reduction in blood flow to the inner ear may disturb the endolymphatic homeostasis, leading to possible changes of the specific gravity during the production of endolymph, thus making it heavier in comparison to the cupula.4
- Inflammation or injury that may occur in the inner ear may cause plasma with leukocytes in the endolymph to increase, creating denser endolymph.4
- A hemorrhagic event may lead to blood plasma proteins to leak into the inner ear fluids due to breakage of the blood-labyrinthine barrier which may increase the specific gravity of the endolymph3
- There is the idea floating around that “light debris” could be attached to the cupula making the cupula “light”. Although “light debris” hasn’t been identified yet, several candidates have been proposed3:
- free floating cells within the endolymph degenerate and swell up to become lighter (when I read this… all I could think of were those little chia seeds. Hard and dense when you buy them, turning into little jelly-like balls when you let them sit in your overnight oats)
- attached otolith particles expand and become lighter
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!
- Bergenius, Johan & Tomanovic, Tatjana. (2006). Persistent geotropic nystagmus - A different kind of cupular pathology and its localizing signs. Acta oto-laryngologica. 126. 698-704. 10.1080/00016480500475609.
- Imai T, Matsuda K, Takeda N, et al Light cupula: the pathophysiological basis of persistent geotropic positional nystagmus BMJ Open 2015;5:e006607. doi: 10.1136/bmjopen-2014-006607
- Kim M-B, Hong SM, Choi H, et al. The Light Cupula: An Emerging New Concept for Positional Vertigo. Journal of Audiology & Otology. 2018;22(1):1-5. doi:10.7874/jao.2017.00234.
- Kim, C. , Kim, M. and Ban, J. H. (2014), Persistent geotropic direction‐changing positional nystagmus with a null plane: The light cupula. The Laryngoscope, 124: E15-E19. doi:10.1002/lary.24048