Positional nystagmus is a very fast eye movement over which a patient has no control and is brought on or influenced by the position of the body and/or head. These quick eye movements usually go along with feelings of dizziness. There are many types of positional nystagmus differentiated by both the position which causes the nystagmus to occur and the direction of the eye movements. The most common cause of positional nystagmus is benign paroxysmal positional vertigo (BPPV). One specific type of nystagmus, positional alcohol nystagmus, is unrelated to any illness or dysfunction but relates to the absorption of alcohol into the blood stream.
Examples of positional nystagmus classifications include upbeating, downbeating, lateral, and direction changing. Upbeating nystagmus is seen when the patient is lying down with the head tilted and indicates BPPV in the posterior canal of the inner ear. Downbeating nystagmus is similar but will occur without tilting of the head and indicates BPPV in the anterior canal. Lateral positional nystagmus is seen as a side-to-side movement when the patient is lying down, but the cause of this type is less certain. Direction changing positional nystagmus causes movements that change direction depending on whether the patient is lying down or sitting. Another variant, cervical nystagmus, is linked to compression of a cervical disk on the vertebral artery.
BPPV is explained as an accumulation of tiny pieces of debris sometimes called “ear rocks.” These tiny crystals are formed from calcium carbonate, similar in theory to kidney stones, and are officially known as otoconia. They slowly redissolve in the solution of the inner ear so any symptoms they cause can stop for a period of time before recurring. Most cases of BPPV are caused by head trauma or simply by aging. Certain viruses and the effects of surgery on the ear are also known causes, but in many cases a clear cause is never determined.
Eye movements caused by positional alcohol nystagmus (PAN) are sometimes confused as signs of horizontal gaze nystagmus. Police officers use horizontal gaze nystagmus (HGN) as one test for alcohol intoxication or illegal narcotics. To accurately view HGN, the subject lies on his or her side with the head in alignment with the spine. If the head is tilted to one side, any nystagmus that is observed is caused by PAN which shows the presence of alcohol but does not indicate a level of intoxication. A false positive for HGN may then be recorded.