What is Upbeating Nystagmus?
Nystagmus is a rhythmic, involuntary oscillation of the eyes. When the oscillation occurs without a visual trigger and appears in the primary position of gaze, it is called spontaneous nystagmus. Upbeating nystagmus is a form of vertical nystagmus in which the fast phase (or beat) moves the eyes upward.
Jerk versus Pendular Nystagmus
Eye movements in nystagmus can be classified by their waveform. A jerk nystagmus alternates a slow drift away from the intended gaze position with a quick corrective saccade. Upbeating nystagmus is usually a jerk nystagmus: the eyes slowly drift downward and then snap upward. A pendular nystagmus, by contrast, has a sinusoidal oscillation without clearly distinct fast and slow phases. While upbeating nystagmus is most commonly jerk, a pendular waveform can occasionally have a vertical component that appears similar to upbeating nystagmus and should be differentiated during the bedside examination.
Spontaneous Nystagmus and the Role of Fixation
Spontaneous nystagmus is present when the eyes are in the primary position with the head still. Visual fixation often suppresses peripheral vestibular forms of nystagmus, but may have less effect or a paradoxical effect on central forms. A spontaneous upbeating nystagmus that persists despite fixation suggests a central origin, especially when it is accompanied by other brainstem or cerebellar signs such as ataxia, dysarthria, or impaired saccades.
Periodic Alternating Nystagmus (PAN)
Periodic alternating nystagmus is a horizontal jerk nystagmus that spontaneously reverses its direction at regular intervals, typically every 90 to 120 seconds. Although PAN is horizontal rather than vertical, it is clinically relevant to any discussion of central nystagmus because it carries the same localizing value: PAN is almost always caused by a disorder of the cerebellar midline or the brainstem, and it often coexists with other central eye movement abnormalities. When evaluating a patient with upbeating nystagmus, the examiner should look for PAN as well, because the presence of both vertical and periodic horizontal nystagmus strongly supports a central lesion and may require a specific treatment, such as baclofen or management of the underlying disorder.
Central Causes of Upbeating Nystagmus
Upbeating nystagmus is considered a central nystagmus, meaning it arises from dysfunction of the brainstem, cerebellum, or their connections. A lesion of the ventral midline medulla or the dorsal vermis of the cerebellum is the classic localization. Spontaneous upbeating nystagmus that increases with upward gaze and decreases with convergence points toward the medulla, while a lesion of the midbrain may produce a different pattern of vertical eye movement abnormalities.
Common central causes include:
- Stroke in the medulla or cerebellum
- Multiple sclerosis
- Tumors of the posterior fossa, such as a meningioma at the foramen magnum
- Wernicke encephalopathy due to thiamine deficiency
- Cerebellar degeneration
- Medication-induced nystagmus, for example with lithium or certain anticonvulsants
Because upbeating nystagmus is frequently associated with lesions near the craniocervical junction, imaging should include the cervical medullary junction and not just the cerebral hemispheres.
Clinical Evaluation and Differential Diagnosis
The first step in evaluation is careful observation of the waveform. The examiner should note whether the nystagmus is jerk or pendular, whether it is spontaneous or gaze-evoked, and how it responds to fixation and convergence. Upbeating nystagmus that is present only on upward gaze is less specific than spontaneous upbeating nystagmus in the primary position. The presence of oscillopsia (the sensation that the world is moving) is common but not always present.
Key distinguishing features:
- Jerk upbeating nystagmus with a slow downward drift is typical of central lesions.
- Pendular vertical nystagmus may accompany disorders such as multiple sclerosis or brainstem infarction.
- Associating signs such as skew deviation, internuclear ophthalmoplegia, or limb ataxia strengthen the case for a central origin.
- A purely peripheral cause of vertical spontaneous nystagmus is rare; when vertical nystagmus is present, central disease should be assumed until proven otherwise.
In addition to PAN, the differential diagnosis includes downbeating nystagmus, seesaw nystagmus, and oculopalatal tremor. A careful neurologic examination and magnetic resonance imaging of the brain and upper cervical spine are usually sufficient to identify the cause. In selected patients, a lumbar puncture, blood tests, or a trial of medication may be needed.
Prognosis and Management
The prognosis of upbeating nystagmus depends entirely on the underlying disease. If the cause is a small stroke or a reversible condition such as Wernicke encephalopathy, the nystagmus may improve with treatment. In degenerative or demyelinating conditions, the nystagmus may persist and require symptomatic treatment. Medications commonly used for central nystagmus include gabapentin, memantine, baclofen, or clonazepam, but the response varies. Some patients benefit from convergence or pressure on the eye muscles, though these manoeuvres are not curative.
Summary
Upbeating nystagmus is a spontaneous vertical nystagmus with a fast phase beating upward. It is usually a jerk nystagmus, but a pendular component may be present in certain central disorders. The most important clinical lesson is that vertical spontaneous nystagmus, including upbeating nystagmus, is almost always central. The presence of PAN or other eye movement abnormalities further supports a brainstem or cerebellar origin. A stepwise neurologic work-up, including focused imaging, can identify treatable causes and guide management.