Is traditionally considered uncommon. Unless a person had a head injury, I would typically consider bilateral BPPV highly unlikely. I have noticed that sometimes left sided posterior canal canalithiasis can mimic a less intense form of right sided posterior canal canalithiasis by causing right torsional upbeating nystagmus in the right Dix-Hallpike position. Lately, however, I have had a few clients (within one week) who had intense torsional, upbeating nystagmus in the same direction of the Dix-Hallpike maneuver on both sides. This nystagmus was typical BPPV type nystagmus (crescendo/fatigue/symptomatic). These findings made me think of the following question: Is bilateral BPPV really uncommon?
If there is a vascular or hormonal cause of BPPV, one could hypothesize that if the inner ear is not getting the nutrients it needs to be healthy on one side, why couldn't it experience a similar state on the opposite side.