In this video we see a strong and clear nystagmus consistent with a right ear (PC-BPPV Nystagmus: It’s not so rotary after all) PC-BPPV. The nystagmus, while described as “rotary-torsional”, will actually have a more visible upward and oblique movement than may be anticipated by new practitioners.
Often times, practitioners new to BPPV are perplexed when they begin see their first PC-BPPV patients after learning in their lectures about “rotary-torsional” nystagmus. It is important to remember Ewald’s 1st Law. Simply put, vertical canal BPPV produces vertical nystagmus. Posterior canal is upbeat, anterior canal (rare) downbeat, both with an oblique “twist” toward the ground. Also worth considering is the varying viscoelastic properties of the extra-ocular muscles. The rectus muscles are larger and stronger and the obliques, smaller and weaker. This often causes an earlier fatigue of the “rotary” aspect of the nystagmus but leaving the vertical component, which is rectus, dominated.
For a comprehensive discussion of eye movement, corresponding muscles and canal involvement see Roberts and Gans, Chapter 9, In Jacobson and Shephard (Eds.), Balance Function Assessment and Management, Plural, 2008.