Typical features of bppv as described by hall & ruby:
1. Canalithiasis mechanism - This explains the latency of the nystagmus as a result of the time needed for motion of the material within the posterior canal to be initiated by the gravity.
2. Duration of the nystagmus - is correlated with the length of time required for the dense material to reach the lowest part of the posterior canal.
3. The vertical (upbeating) and torsional (superior poles of the eye beating towards the lowermost ear). The nystagmus is more vertical when the patient looks away from the lowermost ear, and more torsional when looking towards the lowermost ear.
4. The reversal of nystagmus when the patient returns to the sitting position is due to retrograde movement of material in the lumen of the posterior canal back towards the ampula, resulting in ampulo petal deflection of the cupula.
5. The fatiguability of the nystagmus evoked by repeated dix hallpike positional testing is explained by dispersion of material within the canal.
