Overview of nystagmus

INTRODUCTION — Nystagmus is a rhythmic regular oscillation of the eyes. It may consist of alternating phases of a slow drift in one direction with a corrective quick "jerk" in the opposite direction, or of slow, sinusoidal, "pendular" oscillations to and fro. Jerk nystagmus is more common than pendular nystagmus. (See "Jerk nystagmus" and "Pendular nystagmus".)

Nystagmus can be continuous or paroxysmal, or evoked by certain maneuvers such as specific gaze or head positions. Nystagmus associated with some pathologies may only be seen transiently with hyperventilation or coughing and sneezing.

An overview of nystagmus, its treatment, and the vestibular physiology relevant to nystagmus and vertigo is presented here. The approach to vertigo is discussed separately. (See "Evaluation of the patient with vertigo".)

TYPES OF NYSTAGMUS — The two major types of nystagmus are jerk nystagmus and pendular nystagmus.

Jerk nystagmus — Jerk nystagmus is subdivided by trajectory and the conditions under which it occurs (table 1). Some forms are always present, even when the eyes are in the primary position (ie, looking straight ahead). Acquired jerk nystagmus in the primary position is classified according to trajectory:

●Downbeat

●Upbeat

●Horizontal

●Torsional

●Mixed

The direction named is the direction of the fast phase.

Other forms emerge only under specific conditions such as peripheral gaze (gaze-evoked) and certain head positions (positional). (See "Jerk nystagmus".)

Most acquired jerk nystagmus are the result of an asymmetry in vestibular semicircular canal inputs, in either the central or peripheral nervous system. Understanding the diagnostic value of the trajectory of jerk nystagmus requires some knowledge of the physiology of the semicircular canals. (See 'The vestibular sensory organs' below.)

Congenital nystagmus often has a jerk trajectory, and is almost always horizontal, even with the eye in up or down gaze. It can be complex, varying between jerk, pendular, and pseudopendular waveforms, with these variations occurring over time or in different gaze positions even in the same patient. It is usually not a diagnostic challenge because most patients have known about this since childhood and do not experience oscillopsia, with rare exceptions [1].

Pendular nystagmus — Pendular nystagmus has a sinusoidal oscillation without fast phases. The waveform of pendular nystagmus may occur in any direction; it can be torsional, horizontal, vertical, or a combination of these, resulting in circular, oblique, or elliptical trajectories. It can be different in the two eyes, sometimes even monocular. A video demonstrating pendular nystagmus is available online.

The diagnosis of pendular nystagmus is an exercise in pattern recognition, since pendular nystagmus is subdivided into a number of types recognized mainly by very specific trajectories. These are described in detail separately (see "Pendular nystagmus"):

●Acquired pendular nystagmus

●Congenital nystagmus

●Pendular nystagmus with visual loss

●Specific variants include:

•Spasmus nutans

•Oculopalatal myoclonus

•See-saw nystagmus

•Oculomasticatory myorhythmia (Whipple's)

BASIC CLINICAL VESTIBULAR PHYSIOLOGY

The vestibular sensory organs — There are two components to the peripheral vestibular system:

●The otolithic organs – The otolithic organs are the saccule and the utricle, each of which has a plane of hair cells embedded in a heavy slab of material containing calcium crystals, called the otoconia. These slabs have a significant amount of inertia. Any linear acceleration of the head (eg, gravity) causes the head to move relative to the otoconia, resulting in a bending of the hair cells and a change in neural activity.

●The semicircular canals – The semicircular canals detect angular accelerations (turns or rotations) of the head. Each canal contains fluid and a gelatinous structure called the ampulla, in which hair cells are embedded. Rotation of the head causes the canal to move relative to the fluid within it, thereby generating a force against the ampulla that bends the hair cells. An increase or decrease from the baseline firing rate occurs, depending upon the direction of rotation.

There are six canals arranged along three cardinal planes on each side. The two horizontal canals lie in one plane, not quite horizontal but tilted downward 30 degrees. The vertical (anterior and posterior) canals are all at approximately right angles to the horizontal plane but positioned in two planes 45 degrees from the sagittal plane. Thus, each plane has a pair of canals: the two horizontal canals paired with each other, and the anterior canal of one side paired with the posterior canal of the other side.

The vestibular system uses these pairs of canals in a "push-pull" reciprocal fashion. An angular movement of the head along one of these planes will activate one canal and inhibit its counterpart. Thus, the normal resting firing rate of the nerve from one canal increases while that of its counterpart decreases. The net change in both directions is summed algebraically by the brain to reveal the change in head position. This information is transmitted to the ocular motor nuclei, causing the eyes to move smoothly an equal and opposite amount to the head turn (the vestibuloocular reflex), enabling the eyes to remain stationary in space despite the head movement.

The directions that excite each canal are given by a simple rule: "A canal is excited by head motion towards the canal, in the appropriate plane." Thus, the right horizontal canal is excited by right head turns, the right posterior canal is excited by right head tilts or "posterior" head tilts (neck extension), and the left anterior canal is excited by left head tilts or "anterior" head tilts (neck flexion).

Vestibuloocular responses — The vestibuloocular reflex keeps the line of sight stable in space while the head is moving. Thus, it uses information about head rotation to drive the eyes an equal amount in the opposite direction. To accomplish this, each semicircular canal has excitatory projections to a pair of extraocular muscles, one in each eye, and inhibitory projections to an antagonistic pair. The muscles maximally activated by a canal are approximately aligned with the plane of that canal.

To understand the effects of such activation requires knowledge of the direction of pull of each extraocular muscle. The horizontal recti are simple: the lateral recti abduct and the medial recti adduct the eye. The vertical recti and obliques are more complex: they all have primary, secondary, and tertiary actions upon the globe (table 2). It is useful to think of the line of pull of these muscles as approximately aligned with the planes of the vertical semicircular canals. While the line of pull remains roughly constant, the effect of the pull upon the eye in part depends upon whether the eye is abducted or adducted at the time.

●Contraction of the superior rectus elevates the eye, especially when it is in the abducted position. When the eye is adducted, however, this strong primary action of the muscle is less and there is more of an intorsional motion.

●Contraction of the inferior rectus leads to the opposite pattern: depression of the eye in abduction with some extorsion in adduction.

●Contraction of the superior oblique causes mainly intorsion of the eye, as it pulls the top of the eye towards the pulley at the anteromedial corner of the roof of the orbit. This is maximal in abduction. When the eye is adducted, however, the pull of the top of the eye toward the pulley now causes depression.

●Contraction of the inferior oblique, whose origin is at the anteromedial corner of the floor of the orbit, leads to the opposite pattern, extorsion of the eye in abduction, with elevation in adduction.

With these data it is easy to see which pair of muscles is activated (or inhibited) by each semicircular canal to keep the line of sight steady with head turns (table 3).

Generation of nystagmus — Understanding the planes of action and ocular activations of the semicircular canals and their push-pull relationship with opposing canals on the other side is essential to understanding how jerk nystagmus is generated. With a lesion of the right horizontal canal, for example, there is a loss of tonic leftward vestibular input at rest, since there is always a baseline level of activity. The unopposed rightward "push" from the intact left horizontal canal creates a slow rightward drift of the eyes, the "slow phase," which is eventually counteracted by a leftward fast phase. Thus, loss of the right horizontal canal creates left-beating nystagmus.

Similarly, loss of the right anterior canal creates a nystagmus that is a mix of upbeat and counterclockwise torsional fast phases, and loss of the right posterior canal creates a downbeat/counterclockwise torsional nystagmus. If all three canals on the right side are lost, the upbeat and downbeat components cancel each other out, but the counterclockwise torsional effects add together and summate with the left-beating horizontal nystagmus to create a mixed horizontal-torsional nystagmus. This is the typical finding with "peripheral nystagmus," that is, nystagmus from acute unilateral damage to the labyrinth or vestibular nerve.

This also explains why pure vertical nystagmus is rarely if ever generated by peripheral vestibular disease and indicates a brainstem problem. To obtain pure downbeat nystagmus, a lesion would have to affect the two posterior canals, summating the resulting loss of upward slow drifts, canceling the opposing torsional drifts, and sparing all other canals. The odds are against such a selective bilateral lesion of the labyrinth. The same holds for pure upbeat nystagmus. On the other hand, the central vestibular pathways in the brainstem are organized so that such deficits are possible.

Pure torsional nystagmus conceivably could result from a lesion that affected both the anterior and posterior canal on one side, sparing the horizontal canal. However, the regional blood supply and nerve subdivisions of the labyrinth segregate the anterior canal and horizontal canal from the posterior canal. Clinical studies suggest that vestibular neuronitis, for example, is a partial defect whose nystagmus suggests either a combination of horizontal and anterior canal dysfunction, or almost pure horizontal canal loss. Thus, partial lesions of the periphery are unlikely to cause pure torsional nystagmus. Pure horizontal nystagmus, however, could be either central or peripheral.

Some nystagmus is the result of hyperfunction of a canal rather than hypofunction of one labyrinth. The classic example is benign paroxysmal positional vertigo. Movement of debris in the posterior canal leads to abnormal activation, generating slow downward and torsional drifts, and therefore a mixed upbeat/torsional nystagmus. (See "Benign paroxysmal positional vertigo".)

SYMPTOMS

●Vertigo – Vertigo is the primary symptom associated with nystagmus and is most prominent with acute unilateral peripheral vestibular disease.

●Oscillopsia – Oscillopsia is a to-and-fro illusion of environmental motion. Depending upon the nystagmus, this may be continuous, intermittent, or gaze-evoked. This contrasts with the oscillopsia from bilateral vestibular hypofunction, which only occurs when the head is moving, as when walking or riding in a car. Oscillopsia is not specific to vestibular disease; it can also occur with saccadic intrusions, such as opsoclonus and ocular flutter.

●Blurred vision – Blurred vision occurs because the retinal image is smeared by stimulus motion.

●Abnormal head positions – Patients who find that their oscillopsia or blurred vision is least troublesome in certain gaze positions that minimize nystagmus may assume abnormal head positions.

●Imbalance – Vestibular information is used not just to stabilize gaze, but also for stability and control of body position through vestibulospinal reflexes. Thus, imbalance often accompanies nystagmus, even if it is not a direct consequence of nystagmus. Imbalance can also occur from associated damage to the otoliths.

Some patients with nystagmus are asymptomatic.

DIFFERENTIAL DIAGNOSIS — The rhythmic nature and slow speeds of nystagmus distinguish it from a number of other abnormal involuntary eye movements.

●Saccadic intrusions are irregular bursts of rapid eye movements, almost always indicating a problem of the midline cerebellum. (See "Overview of cerebellar ataxia in adults", section on 'Clinical syndromes'.)

These movements include opsoclonus, ocular flutter, square wave jerks, and macrosaccadic oscillations. Voluntary nystagmus is a misnomer; it too is a type of saccadic intrusion, which most resembles ocular flutter.

●Ocular bobbing and its variants have slow drifts and fast phases but are much more irregular than nystagmus. They occur in comatose and locked-in individuals. (See "Stupor and coma in adults", section on 'Eye movements' and "Locked-in syndrome", section on 'Clinical features'.)

●Oculogyric crises are irregular prolonged deviations of the eyes, usually up and lateral, that lack rhythmicity and slow phases. They are most frequently encountered with phenothiazine intoxication. (See "Etiology, clinical features, and diagnostic evaluation of dystonia", section on 'Clinical features'.)

SYMPTOMATIC THERAPY — The general approach to nystagmus therapy involves three basic considerations [2,3]:

●What is the type of nystagmus?

●Are there underlying causes that are treatable? Consider intoxications, metabolic derangements, infections, and operable structural lesions before resorting to symptomatic treatment.

●What is the potential benefit of symptomatic treatment?

The goals of improvement in the symptoms listed above (vertigo, oscillopsia, motion-degraded acuity, or head turn) should be clear prior to treatment. Patients without these symptoms do not require symptomatic treatment.

There are four main types of symptomatic nystagmus therapy:

●Medication

●Botulinum toxin injections

●Prism lenses and optical solutions

●Surgery

Medication — Appropriate medication use depends upon the type of nystagmus. This is discussed separately in individual topic reviews. (See "Pendular nystagmus" and "Jerk nystagmus".)

Botulinum injections — Botulinum injections into muscle or the intraconal space have been used to weaken the extraocular muscles and diminish the amplitude of nystagmus. These improve vision, oscillopsia, and nystagmus somewhat [4-6], but they also weaken normal eye movements. In some reports the most telling detail is that patients have declined to repeat the experience [5,7].

Prism lenses and optical solutions — Prism lenses and optical solutions may be useful in the following circumstances:

●If patients have a position of gaze in which nystagmus is minimal, prisms can be used so that the eyes are in this "null position" when the patient is trying to look directly ahead. Oscillopsia is thereby minimized for this most frequent gaze position, but gaze to the side will still bring on symptoms.

●If convergence dampens nystagmus, the patient can wear prisms that make the eyes more convergent while fixating in the distance [8,9].

●High hyperopic glasses can be combined with high myopic contact lenses to create a situation in which the retinal image does not shift with eye movement [10,11]. This is useful for reading, but the field of view narrows to a tunnel, and the desirable effects of normal eye movements are impaired in that they do not shift or steady gaze normally. Thus, the system cannot be worn while walking around. Most patients find the device cumbersome.

●Contact lenses can dampen congenital nystagmus for unknown reasons [12].

Surgery — Most surgery tries to shift the attachment of the muscles so that the muscle tensions appropriate to a gaze position where nystagmus was minimal or absent now place the eyes in center gaze. In essence, this is similar to the strategy of prism therapy and, as with prisms, oscillopsia will still be present in lateral gaze. Surgery has been used mainly with congenital nystagmus [13-15], where it has been reported to improve visual acuity in some cases [13]. Four-muscle tenotomy and reattachment is not aimed at shifting the null point, but at reducing proprioceptive feedback to an ocular motor control loop; it has been reported to broaden the null point and improve visual acuity in congenital nystagmus [16].

SUMMARY

●Definition and pathogenesis – Nystagmus is a rhythmic regular oscillation of the eyes. The rhythmic nature and slow speeds of nystagmus distinguish it from a number of other abnormal involuntary eye movements.

Jerk nystagmus occurs most often when there is an imbalance in the activation of the semicircular canals because of either peripheral vestibular disease or disruption of central vestibular pathways in the brainstem. (See 'Generation of nystagmus' above.)

●Associated symptoms – Nystagmus may be associated with the clinical symptoms of vertigo, oscillopsia, abnormal head position, blurred vision, and/or imbalance. (See 'Symptoms' above.)

●Examination features

•Jerk nystagmus consists of alternating phases of a slow drift in one direction with a corrective quick "jerk" in the opposite direction. It is further classified according to trajectory and the conditions under which it occurs (table 1). (See "Jerk nystagmus".)

•Pendular nystagmus consists of slow, sinusoidal, "pendular" oscillations to and fro subdivided into a number of types recognized mainly by very specific trajectories (eg, acquired pendular nystagmus, congenital nystagmus, etc). (See "Pendular nystagmus".)

●Symptomatic treatment – Appropriate medication use depends upon the type of nystagmus.

Prism lenses and other optical solutions and ocular surgery may be helpful in specific circumstances. (See 'Symptomatic therapy' above.)