Nystagmus is repeated movements of the eyeballs following one after the other. There are two types of nystagmus:
- pendulum nystagmus (smooth sinusoidal oscillations)
- jerky nystagmus (alternation of a slow phase and a corrective fast phase)
In healthy individuals, nystagmus occurs in response to vestibular and optokinetic stimulation. To find out the cause of nystagmus, the patient's anamnesis is collected (information about taking medications, alcohol, etc.) and a complete examination of the movements of his eyeballs is carried out.
Pathological nystagmus occurs when the mechanisms that ensure gaze fixation are damaged. The vestibular, optokinetic systems and the system for tracking targeted eye movements interact in such a way as to maintain a stable image on the retina. The neural integrator makes it possible to keep the object under consideration in the field of vision. Damage to these systems leads to nystagmus.
Types of nystagmus
In clinical practice, four types of nystagmus are distinguished in neurological patients:
- Congenital nystagmus
- Labyrinth-vestibular nystagmus
- Periodic alternating nystagmus
- Dissociated vertical nystagmus
For congenital nystagmus, long-term horizontal pendulum-shaped or jerky movements of the eyeballs are characteristic. In some cases, Congenital nystagmus is accompanied by damage to the visual pathway and visual impairment in the patient. The mechanism and localization of the lesion in congenital nystagmus are not known.
The defeat of the vestibular apparatus leads to the appearance of a slow smooth phase and a corrective fast phase, which together form jerky nystagmus of the "sawtooth" type. This unidirectional movement of the slow phase of nystagmus reflects the instability of the tonic neuronal activity of the vestibular nuclei. Damage to the semicircular canals of the inner ear leads to a slow deviation of the eyeball towards the lesion of the semicircular canal, followed by a rapid compensatory movement in the opposite direction.
Slow deviations of the eyeballs in the same direction are also pathological, however, according to the rules, the side of nystagmus is determined by the direction of the fast corrective impulse (fast phase of nystagmus). This instability of vestibular tone usually leads to systemic dizziness and oscillopsia (illusory movements of the surrounding objects) in the patient.
The defeat of the peripheral parts of the vestibular system is almost always accompanied by damage to several semicircular canals simultaneously. This leads to an imbalance between signals entering the brain from individual semicircular canals of the inner ear. In such a case, nystagmus is often of a mixed nature:
- with benign postural nystagmus, the patient usually develops mixed vertical-rotational nystagmus
- with one-sided destruction of the labyrinth, the patient has a mixed horizontal-rotational nystagmus
Peripheral vestibular nystagmus decreases when the head is fixed and increases with changes in its position (tilt, turn, rotation).
In cases of damage to the central vestibular connections, a central imbalance occurs between the signals coming from the various semicircular canals, and the ascending vestibular or cerebellar-vestibular connections are also interrupted. Central vestibular nystagmus may visually resemble nystagmus that develops with a lesion of the semicircular canal, however, bilateral vertical (up, down), rotational or horizontal nystagmus is more common. Central vestibular nystagmus slightly decreases when fixing the patient's head and increases or is caused by a change in its position in space (tilt, turn, rotation).
Three types of labyrinth-vestibular nystagmus are important for establishing the localization of the lesion:
- Downward nystagmus is usually seen when looking straight ahead and worse when looking to the sides. Downward nystagmus is caused by anomalies of the posterior fossa such as Arnold-Chiari malformation and platybasia, as well as multiple sclerosis, cerebellar atrophy, hydrocephalus, metabolic disorders, familial periodic ataxia; downward nystagmus can also occur as a toxic reaction to anticonvulsant drugs.
- Upward nystagmus is a consequence of damage to the anterior parts of the cerebellar worm, as well as diffuse damage to the brain stem in Wernicke's encephalopathy, meningitis, or as a result of side effects of drugs.
- Horizontal (left, right) nystagmus in the initial position (when the patient is looking straight ahead) is observed when the peripheral part of the vestibular analyzer is damaged and only sometimes in tumors of the posterior cranial fossa or Arnold-Chiari malformation.
Nystagmus, which occurs with purposeful eye movements, is detected when the eyeballs deviate from the center. The ability to hold the eyes in position is impaired by damage to the neural integrator in the brainstem. Asymmetric, but friendly horizontal nystagmus with purposeful movements of the eyeballs occurs in the case of unilateral lesions of the cerebellum and tumors of the cerebellar pontine angle (acoustic neuroma or neuroma of the auditory nerve). A common cause of horizontal nystagmus is also the use of sedatives and anticonvulsants. Horizontal nystagmus, in which the rapid phase when the eyeball is brought inwardly occurs more slowly than when the eyeball is retracted outward (dissociated nystagmus), is a characteristic sign of internuclear ophthalmoplegia.
Converging pulsating nystagmus, aggravated by an attempt to raise the eyes upward, is characterized by pulsating saccadic movements of the eyeballs towards each other. As a rule, converging pulsating nystagmus is accompanied by other symptoms of damage to the posterior midbrain (Parino syndrome).
Periodic alternating nystagmus
In the case of periodic alternating nystagmus, the patient is observed to have horizontal nystagmus when looking straight ahead, periodically (every 1-2 minutes) changing its direction either to the right or to the left. There may also be nystagmus that occurs with purposeful eye movements and downward nystagmus. This form of the disease (periodic alternating nystagmus) can be hereditary and occurs in combination with craniovertebral abnormalities, as well as in multiple sclerosis and anticonvulsant drug poisoning. With the non-hereditary nature of the periodic alternating nystagmus, a positive effect is given by the appointment of baclofen to the patient.
Dissociated vertical nystagmus
In the case of dissociated vertical nystagmus, alternating movements of the eyeballs are observed: while one eyeball moves upward and inward, the other downward and outward. Dissociated vertical nystagmus indicates damage to the nuclei of the reticular formation of the midbrain, including the intermediate nucleus of Cajal. Dissociated vertical nystagmus occurs with tumors located over the area of the sella turcica (craniopharyngioma), head trauma, less often with cerebral infarctions. Dissociated vertical nystagmus is often associated with bitemporal hemianopsia.
Conditions resembling nystagmus
Nystagmus can mimic eye movement disorders such as:
- convulsive twitching of the eyeballs with a characteristic rectangular signal (small jerky movements to the side from the fixation point and back)
- trembling of the eyeballs (horizontal pulsating vibrations)
- opsoclonus (frequent saccadic fluctuations)
- myokymia of the superior oblique muscle (monocular rotator-vertical movements)
- ocular bobbing (fast deviation of the eyeballs downward with a slow return upward)
- periodic movements of the eyeballs in the horizontal direction with a change in the direction of deviation every few seconds
Visual impairments and eyeball movements are a signal of danger, the recognition of which greatly increases the knowledge of a neurologist and neurosurgeon. A neurologist or neurosurgeon who is vigilant about such visible signals that the eye can send, not only recognizes and differentiates them from each other but also understands their clinical significance.