Nystagmus is a rhythmic, biphasic, oscillatory eye movement consistingof a slow phase in one direction followed by either a fast phase (jerk nystagmus) or another slow phase (pendular nystagmus) in the opposite direction. Jerk nystagmus is named for the direction of its fast phase. For example, upbeat nystagmus refers to a pattern of slow downward drift followed by a quick upbeat saccade. Not all abnormal oscillatory eye movements are nystagmus. Some consist only of fast eye movements (e.g., opsoclonus), and others are less rhythmic than nystagmus (e.g., ocular bobbing); these are collectively known as nystagmoid eye movements.

The Heimann-Bielschowsky phenomenon refers to pendular vertical oscillation of one eye associated with severe vision loss in the same eye. It usually affects children or young adults with severe unilateral optic nerve disease or amblyopia but generally does not occur in older adults with acquired vision loss.

Dissociated abducting nystagmus is present in one eye (the abducting eye) but not the other adducting eye when the patient is looking to one side. It indicates a contralateral internuclear ophthalmoplegia (INO). For example, if the left eye has abducting nystagmus in left gaze, it is due to a right INO, and one should look for incomplete adduction of the right eye.

Oculopalatal tremor/myoclonus is caused by brainstem lesions, specifically within the area between the red nucleus, dentate nucleus of the cerebellum, and inferior olive (i.e., Mollaret triangle).

Congenital nystagmus consists of large amplitude pendular horizontal nystagmus in primary gaze and jerk nystagmus in lateral gaze. It retains a horizontal vector in upgaze, unlike most acquired forms of nystagmus (e.g., gaze-evoked nystagmus), which either disappear or acquire an upbeat waveform in upgaze. It usually presents between 2–4 months of age and can occur with or without vision loss.

Oculomasticatory myorrhythmia consists of slow (1–2 Hz) pendular convergent-divergent eye movements with synchronous jaw movement. It is pathognomonic for Whipple disease.

Seesaw nystagmus consists of repetitive cycles in which one eye elevates and intorts and the other eye depresses and extorts. Both phases may be of equal velocity, or one may be faster than the other. It localizes to the midbrain or the parasellar region.

Opsoclonus (“dancing eyes”) refers to multidirectional fast phase (saccades) conjugate eye movements without an intersaccadic interval (a pause between individual eye movements). It is seen with the opsoclonus-myoclonus-ataxia syndrome, which is associated with viral encephalitis (e.g., West Nile virus), as an autoimmune process, or as a paraneoplastic syndrome associated with neuroblastoma in childhood and various malignancies (e.g., breast cancer, associated with anti-Ri antibodies) in adulthood.

Ocular bobbing is a nonrhythmic movement with a quick, large amplitude, downward ocular deviation followed by a pause and then a slow upward drift. The reverse, known as ocular dipping, can also occur. These are seen in diffuse bihemispheric dysfunction (e.g., severe toxic-metabolic encephalopathy) or large pontine lesions.

Convergence-retraction “nystagmus” (technically not true nystagmus, as it lacks a slow phase) occurs with dorsal midbrain lesions (i.e., Parinaud syndrome) and is typically accompanied by other pretectal signs such as pupillary light-near dissociation, vertical gaze paresis, and eyelid retraction. Co-contraction of the medial recti with other extraocular muscles results in retraction of the globes into the orbits.

Square-wave jerks are small horizontal saccadic intrusions that occur in primary gaze and are separated by a brief pause between each conjugate eye movement (an intersaccadic interval). More than 10–15 per minute are associated with neurodegenerative diseases such as progressive supranuclear palsy. When similar movements are variable in amplitude and occur following a visually guided saccade, consider macrosaccadic oscillations—essentially, an extreme form of the hypermetric saccades seen in cerebellar disease. Ocular flutter is distinguished by the lack of an intersaccadic interval, and has clinical implications similar to opsoclonus.

Nystagmus that has a rotary component without a corresponding unidirectional horizontal component (e.g., purely rotary, mixed vertical-rotary) is broadly known as central vestibular nystagmus and indicates dysfunction of the brainstem vestibular system.

Downbeat nystagmus, best seen in lateral gaze, typically indicates a lesion at the cervicomedullary junction. Chiari malformations are a common cause. It can also be seen in cerebellar degeneration. Upbeat nystagmus can be seen in brainstem, particularly midbrain, and cerebellar lesions.

A mixed horizontal-rotary jerk nystagmus typically indicates peripheral vestibular dysfunction (semicircular canals, vestibulocochlear nerve), though can be seen on occasion with lesions of the vestibular nuclei. It is unidirectional with the fast phase beating away from the affected side, retains a horizontal waveform in up and downgaze, increases when looking in the direction of the fast phase, and decreases when looking in the direction of the slow phase.

Periodic alternating nystagmus changes direction in cycles of 45–90 seconds. It indicates a lesion of the cerebellum and its projections to the dorsal medulla.

The most common type of nystagmus encountered in clinical practice is physiologic nystagmus, which is a normal finding. It consists of small amplitude horizontal nystagmus present only in extreme gaze that beats left in left gaze and right in right gaze, and extinguishes after several beats. Larger amplitude nystagmus present in both maximal and submaximal gaze that does not extinguish is gaze-evoked (sometimes called “direction-changing”) nystagmus and indicates a lesion in the brainstem or cerebellum. It can also be seen with some medications (e.g., phenytoin).