Nystagmus

Nystagmus is a severe form of oculomotor disorders, manifested in spontaneous oscillatory movements of the eyes and accompanied by a significant decrease in visual acuity - low vision.

Nystagmus is a repetitive involuntary pendulum-like oscillation of the eyes, which can be physiological and pathological. Thus, nystagmus that appears in response to the rotation of an optokinetic drum or body in space is normal and serves to maintain good vision. Eye movements that fixate on an object are called foveating, and those that move the fovea away from the object are called defoveating. In pathological nystagmus, each cycle of movement usually begins with an involuntary deviation of the eye from the object, followed by a reverse refixation jerky movement. In direction, nystagmus can be horizontal, vertical, torsional or non-specific. In amplitude, nystagmus can be small-caliber or large-caliber (the amplitude of nystagmus is determined by the degree of deviation of the eyes), and the frequency of nystagmus can be high, medium and low (determined by the frequency of eye oscillations).

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What causes nystagmus?

The development of nystagmus can be caused by the influence of central or local factors.

Nystagmus usually occurs with congenital or early acquired vision loss due to various eye diseases (optical opacities, optic nerve atrophy, albinism, retinal dystrophy, etc.), as a result of which the visual fixation mechanism is disrupted.

Physiological nystagmus

1. Adjustment nystagmus is a small jerky nystagmus of low frequency at extreme gaze abduction. The fast phase is in the direction of gaze.
2. Optokinetic nystagmus is a jerky nystagmus caused by repeated movements of an object in the visual field. The slow phase is the pursuit movement of the eyes after the object; the fast phase is the saccadic movement in the opposite direction, so that the eyes fixate on the next object. If the optokinetic tape or drum moved from right to left, the left parieto-occipital region controls the slow (pursuit) phase to the left, and the left frontal lobe controls the fast (saccadic) phase to the right. Optokinetic nystagmus is used to detect malingerers who feign blindness and to determine visual acuity in young children. It may also be useful in determining the cause of isolated homonymous hemianopsia (see below).
3. Vestibular nystagmus is a jerky nystagmus caused by altered input from the vestibular nuclei to the horizontal eye movement centers. The slow phase is initiated by the vestibular nuclei, and the fast phase is initiated by the brainstem and frontomesencephalic pathway. Rotatory nystagmus is usually associated with vestibular pathology. Vestibular nystagmus can be induced by caloric stimulation:
   • When cold water is poured into the right ear, a left-sided jerky nystagmus appears (i.e., a fast phase to the left).
   • When warm water is poured into the right ear, a right-sided jerky nystagmus appears (i.e. a fast phase to the right). The mnemonic "COWS" (cold - opposite, warm - same) helps in remembering the direction of nystagmus.
   • When cold water is poured into both ears simultaneously, a jerky nystagmus with a fast upward phase appears; warm water in both ears causes a fast downward phase nystagmus.

Motor imbalance nystagmus

Motor imbalance nystagmus results from primary defects in the efferent mechanisms.

Congenital nystagmus

Inheritance can be X-linked recessive or autosomal dominant.

Congenital nystagmus appears 2-3 months after birth and persists throughout life.

Symptoms of congenital nystagmus

• Horizontal nystagmus, usually of the jerky type.
• May be weakened by convergence and is not observed during sleep.
• There is usually a bullet point - the direction of gaze at which nystagmus is minimal.
• When the eyes are set at the zero point, an abnormal position of the head may be observed.

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Nodding spasm

This is a rare condition between 3 and 18 months.

Symptoms

• • Unilateral or bilateral small-amplitude high-frequency horizontal nystagmus with head nodding.
  • Nystagmus is often asymmetrical, with an increase in amplitude during abduction.
  • Vertical and torsional components may be present.

Reasons

• Idiopathic nodding spasm resolves spontaneously by age 3 years.
• Anterior optic glioma, empty sella syndrome and porencephalic cyst.

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Latent nystagmus

Associated with infantile esotropia and not associated with vertical deviation. Characterized by the following:

• When both eyes are open, there is no nystagmus.
• Horizontal nystagmus occurs when one eye is covered or the amount of light entering the eye is reduced.
• Fast phase in the direction of the unclosed fixating eye.
• Sometimes a latent element is superimposed on the manifest nystagmus, so if one eye is covered, the amplitude of the nystagmus increases (latent-manifest nystagmus).

Periodic alternating nystagmus

Symptoms

• Concomitant horizontal jerky nystagmus, periodically changing direction to the opposite direction.
• Each cycle can be divided into an active phase and a stationary phase.
• During the active phase, the amplitude, frequency and velocity of the slow phase of nystagmus first increase progressively, then decrease.
• This is followed by a short, calm interlude lasting 4-20 seconds, during which the eyes make low-amplitude, often pendulum-like movements.
• This is followed by a similar sequence of movements in the opposite direction, the full cycle lasting 1-3 minutes.

Causes: Cerebellar diseases, demyelination, ataxia-telangiectasia (Louis-Bar syndrome), drugs such as phenytoin.

Convergence-retraction nystagmus

Caused by simultaneous contraction of the extraocular muscles, especially the medial rectus.

Symptoms

• Jerky nystagmus caused by downward movement of the OCN monitoring tape.
• The superior disfixation saccade brings the eyes toward each other in a convergent movement.
• Combined with retraction of the eye into the orbit.

Causes: Pretectal lesions such as pinealomas and vascular accidents.

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Downward-beating nystagmus

Symptoms: vertical nystagmus with a fast phase, "beating" downwards, i.e. which is more easily evoked by looking downwards.

Reasons

• Craniocervical junction pathologies at the level of the foramen magnum, such as Aniold-Cliiari malformation and sningobulbia.
• Medicines (lithium compounds, phenytoin, carbamazepine and barbiturates).
• Wernicke encephalopathy, demyelination and hydrocephalus.

Nystagmus, "beating" upwards

Symptoms: vertical nystagmus with a fast phase "beating" upward.

Causes: posterior fossa pathology, medications and Wernicke encephalopathy.

Maddox reciprocating nystagmus

Symptoms: Pendular nystagmus, in which one eye rises and turns inward, while the other eye at the same time falls and turns outward; thus, the eyes turn in opposite directions.

Causes: parasellar tumors, often causing bitemporal hemianopsia, syringobulbia and brainstem stroke.

Ataxic nystagmus

Ataxic nystagmus is a horizontal nystagmus that occurs in the abducted eye of a patient with internuclear ophthalmoplegia (see below).

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Sensory deprivation nystagmus

Sensory deprivation nystagmus (ocular) is a consequence of visual impairment. The severity of the condition is determined by the degree of vision loss. Horizontal and pendulum nystagmus may decrease with convergence. To reduce the amplitude of nystagmus, the patient may adopt a forced head position. The cause of sensory deprivation nystagmus is severe central vision impairment at an early age (e.g., congenital cataract, macular hypoplasia). As a rule, nystagmus develops in children under 2 years of age with bilateral vision loss.

Symptoms of Nystagmus

With some types of nystagmus, a fairly high visual acuity is maintained; in such cases, the reason for its development lies in disturbances in the regulation of the oculomotor apparatus.

Depending on the direction of the oscillatory movements, there are horizontal (most frequently observed), vertical, diagonal and rotational nystagmus; by the nature of the movements, there are pendulum-shaped (with equal amplitude of oscillatory movements), jerk-like (with different amplitudes of oscillations: the slow phase - in one direction and the fast - in the other) and mixed (either pendulum-shaped or jerk-like movements are manifested). Jerk-like nystagmus is called left- or right-sided depending on the direction of its fast phase. With jerk-like nystagmus, there is a forced turn of the head towards the fast phase. With this turn, the patient compensates for the weakness of the oculomotor muscles, and the amplitude of the nystagmus decreases; therefore, if the head is turned to the right, the "right" muscles are considered weak: the outer rectus of the right eye and the inner rectus of the left eye. Such nystagmus is called right-sided.

Nystagmus can be large-caliber (with an amplitude of oscillatory eye movements of more than 15°), medium-caliber (with an amplitude of 15-5°), or small-caliber (with an amplitude of less than 5°).

To determine the amplitude, frequency and nature of oscillatory nystagmoid movements, an objective research method is used - nystagmography. In the absence of a nystagmograph, the nature of the nystagmus amplitude can be determined by the degree of displacement of the light reflex from the ophthalmoscope on the cornea. If the light reflex during oscillatory eye movements moves from the center of the cornea to the middle of the distance between the center and the edge of the pupil, they say about small-caliber, small-swinging nystagmus, if it goes beyond these limits - large-caliber. If the movements of both eyes are not the same, such nystagmus is called dissociated. It is observed extremely rarely.

Types of nystagmus

1. Jerky nystagmus with a slow defoveating "drifting" movement and a fast corrective refoveating jerky movement. The direction of the nystagmus is indicated by the direction of the fast component, so jerky nystagmus can be right-sided, left-sided, superior, inferior, or rotatory. Jerky nystagmus can be subdivided into adjusting (vestibular) and gaze paresis nystagmus (slow and usually a sign of brainstem damage).
2. Pendulum-shaped nystagmus, in which both foveating and defoveating movements are slow (the speed of nystagmus is the same in both directions).
   • Congenital pendulum-like nystagmus is horizontal and tends to become jerky when looking to the side.
   • Acquired pendulum nystagmus has horizontal, vertical and torsional components.
   • If the horizontal and vertical components of pendular nystagmus are in phase (i.e. occur simultaneously), the perceived direction appears oblique.
   • If the horizontal and vertical components are out of phase, the direction appears elliptical or rotary.

Mixed nystagmus includes pendulum-like nystagmus in the primary position and jerk-like nystagmus when looking to the side.

Diagnosis of nystagmus

When examining patients with nystagmus, the results of electrophysiological studies (electroretinogram, visual evoked potentials, etc.) are important, allowing for a more precise diagnosis, determining the degree of organic damage, the presence of amblyopia, and determining treatment tactics.

In case of nystagmus, the visual acuity of each eye is examined with and without glasses, with the head in a straight and forced position. In this position, the amplitude of nystagmus usually decreases and visual acuity becomes higher. This criterion is used to decide on the advisability of performing surgical intervention on the oculomotor muscles. It is important to determine visual acuity with both eyes open (with and without glasses), since with binocular fixation, the amplitude of nystagmus also decreases and visual acuity becomes higher.

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Treatment of nystagmus

The system of measures to improve visual functions in nystagmus includes carefully selected optical correction for distance and near. In albinism, retinal dystrophy, partial atrophy of the optic nerves, it is advisable to select protective and visual acuity-enhancing color filters (neutral, yellow, orange, brown) of the density that ensures the greatest visual acuity.

With nystagmus, the accommodative ability is also impaired and relative amblyopia is observed, therefore pleoptic treatment and accommodation training exercises are prescribed. Useful are flashes through a red filter (on a monobinoscope), selectively stimulating the central zone of the retina, stimulation with contrast-frequency and color test objects (the Illusion device, computer exercises according to the Zebra, Spider, Crosses, EYE programs). These exercises can be performed sequentially for each eye and with both eyes open. Binocular exercises and diploptical treatment (the "dissociation" method, binariummetry), which also help to reduce the amplitude of nystagmus and increase visual acuity, are very useful.

Drug treatment of nystagmus is used to improve the nutrition of the tissues of the eye and retina (vasodilators, vitamin complexes).

Surgical treatment of nystagmus is performed to reduce oscillatory eye movements. In jerky nystagmus, when a forced head turn is diagnosed with increased visual acuity and decreased nystagmus amplitude in this position ("rest zone"), the goal of the operation is to move the "rest zone" to the middle position. To do this, the stronger muscles (on the side of the slow phase) are weakened and the weaker muscles (on the side of the fast phase) are strengthened. As a result, the head position is straightened, nystagmus is reduced, and visual acuity is increased.