Why does the Dix-Hallpike maneuver provoke vertigo and nystagmus in posterior canal BPPV?

• A. Because gravity mobilizes displaced otoconia (canaliths) within the posterior canal, altering endolymph flow and stimulating the hair cells.
• B. Because gravity changes in the middle ear.
• C. Because it compresses the optic nerve.
• D. Because otoliths dissolve in canal.

Answer: (C)

Dix-Hallpike provokes vertigo and nystagmus in posterior canal BPPV because gravity moves displaced otoconia (canaliths) within the posterior semicircular canal. As these crystals shift with the head position, they cause endolymph flow that deflects the cupula in the canal’s crista ampullaris. This deflection changes the firing rate of vestibular hair cells, sending a false sense of motion to the brain and producing the vertigo along with the characteristic nystagmus. Other ideas, like changes in middle ear pressure, optic nerve compression, or dissolution of otoliths, don’t produce the same canal-specific endolymph movement and hair cell stimulation, so they don’t explain the observed symptoms.