Nystagmus

Nystagmus may be attributed to 2 broad categories of disorders: peripheral disorders and central nervous system (CNS) disorders. Peripheral causes of nystagmus are related to the vestibular system and the oculomotor nuclei. The presence and nature of nystagmus helps identify vestibular disorders as well as distinguish central from peripheral vertigo.

Vestibular (peripheral) nystagmus has a slow component caused by the vestibular input and a quick, corrective component that causes movement in the opposite direction. The direction of the nystagmus is defined by the direction of the quick component because it is easier to see. Vestibular nystagmus may be rotary, vertical, or horizontal and may occur spontaneously or when gazing or moving the head. Vestibular nystagmus is unidirectional and can be slowed or suppressed with visual fixation.

Central nystagmus caused by CNS disorders can be vertical (strongly associated with a central cause), horizontal, torsional, or mixed, and can change direction. The effects of visual fixation can vary as the nystagmus can increase, decrease, or remain the same. Central nystagmus can be associated with other neurological deficits.

Initial inspection for nystagmus is performed with the patient sitting with unfocused gaze (+30 diopter or Frenzel lenses can be used to prevent gaze fixation). The patient is then asked to gaze to the left and right. The direction and duration of nystagmus are noted.

If nystagmus is not detected, the Dix-Hallpike (Barany) maneuver is performed.

In the Dix-Hallpike maneuver, the following occur:

• The patient sits erect on an examination table so that when lying back, the head extends beyond the end of the examination table.

• With support, the patient is rapidly lowered to a horizontal position, and the head is extended back 45° below horizontal and rotated 45° to the left.

• The patient is told to fixate the eyes on a single location; visual fixation can shorten or even abolish nystagmus, so the maneuver is ideally performed with the person wearing Frenzel lenses to make visual fixation on anything impossible.

• The patient is returned to an upright position, and the maneuver is repeated with rotation to the right.

• Vertigo and nystagmus can take about 5 to 10 seconds (sometimes up to 30 seconds) to appear (latency). Symptoms last 10 to 30 seconds, then decrease and disappear (ie, fatigue).

The direction and duration of nystagmus and development of vertigo are noted. Nystagmus occurs when the head is turned to the affected ear in benign paroxysmal positional vertigo (BPPV). Any position or maneuver that causes nystagmus should be repeated to see whether nystagmus fatigues.

Nystagmus due to BPPV has a latency period of 3 to 30 seconds and is fatigable and torsional, beating toward the affected ear. In contrast, nystagmus secondary to a central nervous system disorder has no latency period and does not fatigue. During induced nystagmus, the patient is instructed to focus on an object. Nystagmus caused by peripheral disorders is inhibited by visual fixation. Because Frenzel lenses prevent visual fixation, they must be removed to assess visual fixation.

Caloric stimulation of the ear canal induces nystagmus in a person with an intact vestibular system. This procedure is performed with the patient supine and the head elevated 30°; each ear is then irrigated sequentially with cold water or air (30° C). Alternately, warm water or air (40 to 44° C) can be used, taking care not to burn the patient with overly hot water. Cold water causes nystagmus toward the side opposite to the affected ear; warm water irrigated into the affected ear causes nystagmus toward the same side as the affected ear. A mnemonic device is COWS (Cold to the Opposite and Warm to the Same). Quantification of caloric response is best performed with formal (computerized) electronystagmography or videonystagmography. Failure to induce nystagmus or a > 20 to 25% difference in velocity for the slow wave of nystagmus between sides suggests a lesion on the side of the decreased response (1).