Principles
Mechanisms & biomechanics
Every repositioning manoeuvre is the same idea applied to a different canal: use gravity and the canal's orientation to roll debris back through the common crus into the utricle. Understanding the physics is what lets you adapt when the standard sequence fails.
Gravity and canal orientation
The balance canals are loops of fluid. When loose crystals sit in a loop, moving the head makes the fluid — and the crystals — shift, which the brain misreads as spinning. A manoeuvre tilts the head through a series of positions so gravity carries the crystals out of the loop and back into the chamber they came from.
Each position aligns the affected canal's lumen with the gravity vector so debris settles through the common crus down to the utricle. In the posterior canal the Epley and Semont use the canal's vertical orientation to move otoliths through the non-ampullated and ampullated limbs back to the vestibule.
Manoeuvre-specific success depends on a predictable direction of otoconial movement with a low risk of misdirection. For the horizontal canals, the Lempert and Gufoni work because their orientation relative to the horizontal plane gives a predictable path toward the utricle. In the steeply sloping anterior canal, deep head-hanging and Yacovino exploit gravity-assisted downbeat displacement into the common crus.3
Canalithiasis vs cupulolithiasis
The two mechanisms behave differently and demand different tactics. Canalithiasis — free-floating debris — produces a brief, latent, fatiguing nystagmus and responds to repositioning that lets gravity move the particles along the canal. Cupulolithiasis — debris stuck to the cupula — produces a persistent, less-fatiguing nystagmus and must first be liberated from the cupula (the rationale for the rapid Semont swing, and for the apogeotropic Gufoni/Casani manoeuvres) before it can be repositioned. The horizontal canal can convert from one state to the other, which is why the supine roll test can change between examinations.1
Dwell time and technique
Two technical variables decide whether a manoeuvre succeeds. The first is speed: liberatory manoeuvres rely on rapid angular acceleration to detach debris, so a hesitant swing fails. The second is dwell time: each position must be held long enough for the debris to transit and settle — 3D models and video-oculography both show that adequate hold times matter. Inadequate rotation or insufficient dwell time impedes debris migration and reduces effectiveness; this is why a careful, deliberate technique outperforms a hurried one for the slow repositioning manoeuvres.2
Canal conversion
The most common complication of repositioning is canal conversion — debris migrating from one canal into another, usually posterior-to-horizontal, mid-manoeuvre. It announces itself as a new pattern of positional nystagmus on re-testing and is treated as BPPV of the new canal. Recognising it prevents the error of declaring failure (or success) when the problem has simply moved.
Key points
- Manoeuvres align the canal with gravity to move debris through the common crus to the utricle.
- Canalithiasis is repositioned; cupulolithiasis must first be liberated from the cupula.
- Speed matters for liberatory manoeuvres; dwell time matters for all of them.
- Canal conversion is the key complication — a new positional nystagmus means a new canal.