Light cupula: the pathophysiological basis of persistent geotropic positional nystagmus

Area of Science:

• Neurology
• Ophthalmology
• Vestibular System

Background:

• Benign paroxysmal positional vertigo (BPPV) is a common cause of vertigo.
• Horizontal canal BPPV (H-BPPV) presents with geotropic or apogeotropic positional nystagmus.
• The pathophysiology of persistent geotropic positional nystagmus (PGN) requires further elucidation.

Purpose of the Study:

• To define the time constant (TC) of nystagmus in patients with H-BPPV.
• To investigate the relationship between slow-phase eye velocity (SPV) and head rotation angle in supine position.
• To clarify the pathophysiological basis of PGN in H-BPPV.

Main Methods:

• Positional nystagmus (geotropic and apogeotropic) was recorded using video-oculography.
• Nystagmus data were analyzed in three dimensions.
• The time constant (TC) and slow-phase eye velocity (SPV) were calculated.

Main Results:

• Geotropic positional nystagmus in H-BPPV was classified into transient (TC ≤35s) and persistent (TC >35s).
• Persistent apogeotropic positional nystagmus (AN) also had a TC >35s.
• SPV direction in PGN was opposite to AN, with symmetrical relationships to head rotation angle.

Conclusions:

• PGN, with a TC >35s, is suggested to be caused by cupula deviation due to gravity.
• Cupula deviation in PGN is opposite to that in AN, with PGN potentially caused by a light cupula.
• This contrasts with the heavy cupula theory for AN.