Thanks for this question! Someone with horizontal canal BPPV will present with horizontal nystagmus during the supine roll test that is either geotropic or ageotropic. The direction of nystagmus is determined by the location of the debris in the canal; ageotropic nystagmus is caused by debris that are closer to the ampulla. This page by Timothy Hain describes horizontal/lateral canal BPPV well: https://dizziness-and-balance.com/disorders/bppv/lcanalbppv.htm. This excerpt may answer your question: “In idiopathic cases with geotropic nystagmus the “bad” ear is assigned to the side with the stronger nystagmus. With ageotropic nystagmus, the bad ear is assigned to the side with the weaker nystagmus. The rationale for this pattern is that excitation is stronger than inhibition (i.e. Ewald’s second law). This mechanism was not supported by a recent study of positional alcohol nystagmus on persons who had only one remaining labyrinth (Tomanovic and Bergenius, 2013).”
For example, say someone has right horizontal canalithiasis. If their nystagmus is:
– geotropic, they will be more symptomatic/have more nystagmus when their right ear is down.
– ageotropic they will be more symptomatic/have more nystagmus when their left ear is down. In other words, their least symptomatic side is affected.
Determining which side is affected is important, as this impacts the selection and direction of repositioning maneuvers.