Odontoid aplasia

The Dix–Hallpike test — or Nylén–Bárány test — is a diagnostic maneuver used to identify benign paroxysmal positional vertigo (BPPV). Procedure When performing the Dix–Hallpike test, patients are lowered quickly to a supine position (lying horizontally with the face and torso facing up) with the neck extended 30 degrees below horizontal by the clinician performing the maneuver.The Dix–Hallpike and the side-lying testing position have yielded similar results. [Source: Wikipedia ]