Dix-Hallpike BPPV test: findings by canal

Benign paroxysmal positional vertigo is caused by displaced otoconia, tiny calcium carbonate crystals normally embedded in the utricle, that end up loose inside one of the semicircular canals instead. When the head moves into a position that lets gravity pull the debris through the canal, it drags endolymph with it and creates a brief, false sensation of rotation along with a matching pattern of nystagmus. Because the posterior canal sits in the most gravity-dependent position of the three canals, it accounts for the large majority of BPPV cases, with the horizontal canal a distant second and the anterior canal rare.

The Dix-Hallpike test is performed by turning the patient's head 45 degrees toward the side being tested, then quickly moving them from sitting to lying with the head hanging slightly below the level of the table. For posterior canal BPPV, a positive result shows up-beating nystagmus, meaning the fast phase moves toward the forehead, combined with a torsional component in which the upper pole of the eye rotates toward the affected, downmost ear. The nystagmus has a short latency, typically 5 to 20 seconds after reaching the test position, builds and fades in a crescendo-decrescendo pattern, resolves within about 60 seconds, reverses direction briefly when the patient sits back up, and weakens with repeated testing.

The same maneuver can reveal anterior canal BPPV, which is much less common. There, the nystagmus is down-beating instead of up-beating, still with a torsional component toward the affected ear, which is the key feature that separates it from the far more frequent posterior canal pattern.

Horizontal canal BPPV is not tested with Dix-Hallpike. Instead, the patient lies on their back with the head flexed slightly forward, and the head is turned first to one side, then the other, while the examiner watches for horizontal nystagmus in each position. Two patterns are possible, and telling them apart identifies both the type of debris and the affected ear.

In the more common geotropic pattern, the nystagmus beats toward the ground in both head-turn positions, and the affected ear is the one that produces the stronger response, which is the pattern typical of canalithiasis, free-floating debris in the canal. In the less common apogeotropic pattern, the nystagmus beats away from the ground, toward the uppermost ear, in both positions, and the affected ear is the one that produces the weaker response, consistent with cupulolithiasis, debris adherent to the cupula itself rather than floating freely. The rule for identifying the affected ear is opposite between the two patterns, which makes it the detail most often missed.

The time course of the nystagmus separates these two mechanisms directly. Canalithiasis, debris moving freely through the canal, produces nystagmus with a short latency, a crescendo-decrescendo shape that fades within about a minute, and a response that weakens each time the position is repeated. Cupulolithiasis, debris stuck to the cupula, produces nystagmus with little or no latency, sustained for as long as the provoking position is held rather than fading on its own, and a response that does not weaken with repetition. Canalithiasis generally responds well to standard repositioning maneuvers such as the Epley procedure, while cupulolithiasis is more resistant and often needs a different, liberatory technique.