What does lateral nystagmus (abnormal involuntary eye movement) indicate?

From the Guidelines

Lateral nystagmus is a significant indicator of a disorder affecting the vestibular system, cerebellum, or brainstem, and its presence warrants a thorough evaluation to determine the underlying cause, which can include vestibular disorders, multiple sclerosis, stroke, brain tumors, or medication toxicity, as suggested by the most recent study 1.

Key Considerations

• Lateral nystagmus appears as involuntary, rhythmic eye movements in the horizontal direction and can result from various conditions.
• The direction of the fast phase of nystagmus can help localize the lesion—nystagmus that beats away from the side of a peripheral vestibular lesion or toward the side of a central lesion.
• Alcohol and certain medications like anticonvulsants (particularly phenytoin) can induce lateral nystagmus when reaching toxic levels.
• During assessment, it's essential to note the direction, amplitude, and triggering factors of the nystagmus, as these characteristics help determine the underlying cause and guide appropriate treatment.

Diagnostic Approach

• The supine roll test is the preferred maneuver to diagnose lateral canal BPPV, as recommended by 1.
• The HINTS examination can help distinguish between peripheral and central causes of vertigo, with a high sensitivity for detecting posterior circulation infarct, as noted in 1.
• Imaging may be required to rule out stroke or other central causes of vertigo, especially if the HINTS examination is not available or is inconclusive, as suggested by 1.

Underlying Causes

• Vestibular disorders, such as benign paroxysmal positional vertigo (BPPV), are common causes of lateral nystagmus, as discussed in 1 and 1.
• Central causes, including stroke, brain tumors, and multiple sclerosis, can also present with lateral nystagmus, as mentioned in 1 and 1.
• Medication toxicity, particularly with anticonvulsants like phenytoin, can induce lateral nystagmus, as noted in the example answer.

From the Research

Lateral Nystagmus Indications

Lateral nystagmus, an abnormal involuntary eye movement, can indicate various conditions. The following points highlight its possible implications:

• Lateral nystagmus can be a sign of acute vestibular syndrome, which may be caused by central or peripheral vestibular disorders 2.
• In cases of multiple sclerosis, lateral nystagmus can be an indication of a demyelinating plaque in the brainstem, particularly in the vestibular nucleus 3, 4.
• The presence of lateral nystagmus, along with other symptoms such as vertigo, nausea, and vomiting, can suggest a central cause of acute vestibular syndrome, such as a stroke or multiple sclerosis 2, 3.
• Lateral nystagmus can also be seen in patients with peripheral vestibular disorders, such as vestibular neuritis, but the presence of other central signs, such as skew deviation or abnormal head impulse test, can help differentiate between central and peripheral causes 2, 5.
• The direction and characteristics of nystagmus can provide clues about the underlying cause, with some studies suggesting that unidirectional, horizontal nystagmus that follows Alexander's law and is suppressed with fixation may indicate a central cause 3.

Diagnostic Considerations

When evaluating patients with lateral nystagmus, the following diagnostic considerations are important:

• A comprehensive neurologic examination, including the HINTS Plus examination, can help identify central signs and differentiate between central and peripheral causes of acute vestibular syndrome 2.
• Imaging studies, such as MRI, may be necessary to confirm the diagnosis and identify the underlying cause of lateral nystagmus 3, 5.
• Bedside oculomotor tests, including the head impulse test and video head impulse test, can provide valuable information about vestibular function and help diagnose vestibular disorders 5, 4.