Nystagmus

Overview

Nystagmus is a condition characterized by involuntary (or, in some circumstances, voluntary) eye movement. Infants can be born with it, although it is more usually acquired throughout childhood or later in life. It may result in diminished or restricted vision in many circumstances. It has been dubbed "dancing eyes" because to the involuntary movement of the eye. This page discusses many forms of nystagmus, their etiologies, and treatment options. 

Nystagmus definition

The term nystagmus is derived from the Greek words nustagmos (nodding, sleepiness) and nystazein (drowsiness) (be sleepy or doze). It is a rhythmic, involuntary, fast, oscillatory eye movement. It might be slow, rapid, or a combination of the two. It can be continuous, intermittent, or paroxysmal, with positional, gaze, or head positioning triggers. It may be separated from saccades, oscillations, and other fast-acting and mimicking aberrant involuntary eye movements.

These motions can impair vision and have an impact on depth perception, balance, and coordination. Nystagmus is frequently experienced transiently, which may suggest underlying disease. Nystagmus can also be apparent (always present), latent (occurs while one eye is covered), or both.

Classification of Nystagmus

The Classification Committee of the Bárány Society developed various categories of nystagmus for clinical and scientific reasons, with a particular emphasis on developing a worldwide language for vestibular diseases. Pathologic nystagmus is caused by diseases that affect the cortex, anterior visual pathways, brainstem, cerebellum, and peripheral vestibular system.

Physiological nystagmus - normal nystagmus variant of oculomotor function:

• Physiologic end-point nystagmus- horizontal jerks nystagmus results from testing oculomotor movements too far laterally.
• Per-rotational nystagmus- horizontal jerk nystagmus that occurs with sustained head rotations, with fast phases ipsilateral to the rotation.
• Post-rotational nystagmus- reactive horizontal nystagmus with a contralateral fast phase and a subjective rocking boat sensation that occurs with a sudden braking to a unidirectional head rotation.
• Optokinetic or pendular nystagmus- Multi-directional (vertical, torsional, or horizontal) nystagmus in reaction to moving or rotating visual fields or objects, with the slow phase ipsilateral to the visual stimuli and no rapid phase.
  • Asymmetry is abnormal, and the lesion can localize to the parietal-occipital cortex.

• Optokinetic after-nystagmus- persisting ipsilateral optokinetic nystagmus after the visual stimulus has extincted, can persist for seconds and dissipates.
• Vestibular ocular reflex- When the patient perceives a fixed picture, a quick head rotation is evoked bilaterally via semicircular canals in the inner ear.
• Caloric nystagmus- VOR reflex elicited by stimulating the tympanic membrane and horizontal semicircular canals with either warm or cold water.
  • Cold stimulation-the eyes will turn slowly towards the cold water stimulation, with fast nystagmus away. Absence might be a sign of brain death.
  • Warm stimulus- the eyes will swivel slowly away with fast nystagmus toward the stimulus's side. Absence might be a sign of brain death.

• Magnetic vestibular stimulation-induced nystagmus- The nystagmus that happens during an MRI depends on the strength and direction of the field. It is caused by a collision between the MRI field currents and the ionic currents in the endolymph fluid.

Etiology 

Pathological nystagmus can be congenital, idiopathic, or due to another neurological condition. It can also be momentarily produced by disorientation (as on roller coaster rides or when a person has been spinning in circles) or by some medicines (alcohol, lidocaine, and other central nervous system depressants, inhalant drugs, stimulants, psychedelics, and dissociative drugs).

Early-onset nystagmus

Example of congenital (early onset) nystagmus

Acquired nystagmus is more prevalent than early-onset nystagmus. It can arise alone or in conjunction with other disorders (such as micro-ophthalmic anomalies or Down syndrome). Early-onset nystagmus is usually mild and does not progress. Individuals suffering with this condition are typically unaware of their spontaneous eye movements, however depending on the degree of the eye movements, eyesight may be hampered.

Types of early-onset nystagmus include the following, along with some of their causes:

• Infantile:
• Albinism
• Aniridia
• Bilateral congenital cataract
• Bilateral optic nerve hypoplasia
• Idiopathic
• Leber's congenital amaurosis
• Optic nerve or macular disease
• Persistent tunica vasculosa lentis
• Rod monochromatism
• Visual-motor syndrome of functional monophthalmus
• Latent nystagmus
• Noonan syndrome
• Nystagmus blockage syndrome

Acquired nystagmus

Nystagmus that arises later in childhood or adulthood is known as acquired nystagmus. Idiopathic nystagmus is nystagmus induced by an unknown or idiopathic cause. Other common causes include central nervous system diseases and disorders, metabolic anomalies, and drug and alcohol abuse. Stroke is the leading cause of mortality among the elderly.

General diseases and conditions

Some of the diseases that present nystagmus as a pathological sign or symptom are as follows:

• Aniridia
• Benign paroxysmal positional vertigo
• Brain tumors (medulloblastoma, astrocytoma, or other tumors in the posterior fossa.)
• Canavan disease
• COVID-19
• Head trauma
• Lateral medullary syndrome
• Ménière's disease and other balance disorders
• Multiple sclerosis
• Optic nerve hypoplasia
• Pelizaeus–Merzbacher disease
• Superior canal dehiscence syndrome
• Tullio phenomenon
• Whipple's disease

Toxicity or intoxication, metabolic disorders and combination

Sources of toxicity that could lead to nystagmus:

• Alcohol intoxication
• Amphetamines
• Barbiturates
• Benzodiazepines
• Ketamine
• Pregabalin
• Lithium
• MDMA
• Nitrous Oxide
• Phencyclidine (PCP)
• Phenytoin (Dilantin)
• Salicylates
• Selective serotonin reuptake inhibitors (SSRIs)
• Other anticonvulsants or sedatives
• Thiamine deficiency
• Wernicke's encephalopathy
• Wernicke–Korsakoff syndrome

Thiamine deficiency

Drunkenness, dialysis, chronic diarrhea, and the use of large dosages of diuretics are all risk factors for thiamine deficiency, or beri beri. It's conceivable that it's caused by a genetic condition that makes it harder to absorb thiamine from food. Wernicke encephalopathy and Korsakoff syndrome are symptoms of dry beriberi.

Central nervous system (CNS) diseases and disorders

When there is a problem with the central nervous system, such as cerebellar dysfunction, nystagmus can occur in any direction, even horizontal. Vertical nystagmus is induced by the central nervous system in most cases, although it is also a typical side effect of phenytoin overdose. Among the other toxicity-related causes of nystagmus are:

• Brain abscess (Cerebellar)
• Cerebellar ataxia
• Chiari Malformation
• Multiple sclerosis
• Stroke
• Thalamic hemorrhage
• Trauma
• Tumor

Other causes

• Non-physiological
• Trochlear nerve malfunction
• Vestibular Pathology (Ménière's disease, SCDS (superior canal dehiscence syndrome), BPPV, Labyrinthitis)
• Strong magnetic field exposure (as in MRI machines)
• Long-term loss of light, often known as miner's nystagmus after 19th-century coal miners who got nystagmus as a result of working in the dark.
• Some people may consciously induce a somewhat different type of nystagmus.

Clinical manifestations of nystagmus

In rare circumstances, nystagmus may be asymptomatic. Most people, however, feel vertigo, oscillopsia, hazy vision, or abnormal head positioning as a result of nystagmus. Vertigo is the most common symptom, and it is usually connected with vestibular problems. Oscillopsia, or the sensation that one's surroundings are shifting, is generated by the kind of nystagmus present and can be continuous, intermittent, or gaze-evoked.

Blurred vision is frequently produced by motion affecting the retinal picture. This foggy vision may lead to improper head positioning, which happens when patients compensate for visual defects by finding gaze positions that alleviate their symptoms.

Clinical Significance

It is difficult to detect pathological nystagmus because it is difficult to determine which malfunctioning brain or ear apparatus components are damaged and what the underlying etiology is. Acquired nystagmus, which arises later in adolescence or adulthood, might indicate a central nervous system problem such as multiple sclerosis, a head injury, a brain tumor, a metabolic condition, a medication side effect, hyperventilation, or even alcohol or drug poisoning.

The differential diagnosis of nystagmus includes oculogyric crises and ocular bobbing. The absence of a clear rhythm or slow phase in the eye movements distinguishes oculogyric crises from nystagmus. Phenothiazine poisoning is the most common cause of this type of eye movement. Ocular bobbing, which is more irregular than nystagmus, is typical in locked-in syndrome.

Pathological nystagmus- aberrant nystagmus caused by injury to the vestibular-oculocephalic and/or cortical parts of the nervous system, compromising oculomotor function.

• Spontaneous nystagmus- occurs consistently with fixed central gaze position stationary, upright, and neutral positions.
• Spontaneous peripheral vestibular nystagmus- occurs due to abnormal vestibular tone between vestibular nerves and the labyrinth; it is strictly unilateral and can be subclassified based on the direction.

1. Horizontal-torsional nystagmus
2. Vertical-torsional nystagmus
3. Horizontal-vertical-torsional nystagmus
4. Peripheral vestibular nystagmus, inhibitory type
5. Peripheral vestibular nystagmus, excitatory type
6. Recovery nystagmus

• Spontaneous central vestibular nystagmus
• • Predominantly horizontal central vestibular nystagmus
    • Direction-fixed horizontal central vestibular nystagmus
    • Latent nystagmus
    • Periodic alternating nystagmus
  • Predominantly vertical or torsional central vestibular nystagmus
    • Downbeat nystagmus
    • Upbeat nystagmus
    • Torsional nystagmus
  • Infantile nystagmus
  • Acquired pendular nystagmus
• Epileptic nystagmus
• Gaze-evoked nystagmus
• Unilateral
• Bilateral
• Vertical
• First-degree vestibular
• Vestibular plus gaze-holding nystagmus
• Rebound nystagmus
• Centripetal nystagmus

• Triggered nystagmus
  • Positional nystagmus
    • Benign paroxysmal positional nystagmus (BPPN)

1. Anterior semicircular canal BPPN 
2. Posterior semicircular canal BPPN
3. Horizontal semicircular canal BPPN
4. Pseudo-spontaneous nystagmus

• • • Central positional nystagmus
• Headshaking-induced nystagmus
• Cross-coupled nystagmus
• Sound-induced nystagmus
• Valsalva-induced nystagmus
• Pressure-induced nystagmus
• Vibration-induced nystagmus
• Hyperventilation-induced nystagmus
• Pursuit-induced nystagmus

Diagnosis

Nystagmus is a noticeable but underappreciated condition. Clinically, nystagmus can be assessed using a range of non-invasive standard tests. The most basic is the caloric reflex test, in which one ear canal is irrigated with warm or cold water or air. Nystagmus is caused by the temperature gradient stimulating the horizontal semicircular canal. Chiari malformation is characterized by nystagmus.

The resulting eye movement can be recorded and quantified using an electronystagmograph (ENG), a type of electrooculography (an electrical method of measuring eye movements using external electrodes), or a less invasive device called a videonystagmograph (VNG), a type of video-oculography (VOG) (a video-based method of measuring eye movements using external small cameras built into head masks) administered by an audiologist. Special swinging chairs with electrical controllers can be used to generate rotatory nystagmus.

Objective eye-movement-recording techniques have been applied in the study of nystagmus during the last four decades, resulting in greater measurement precision and understanding of the illness. To examine a patient's eye movements, orthoptists may also employ an optokinetic drum or electrooculography.

Nystagmus can occur as a result of foveation of moving objects, illness, chronic rotation, or medication usage. Nystagmus differs from other superficially similar-appearing eye movement disorders (saccadic oscillations) such as opsoclonus or ocular flutter, which are entirely composed of fast-phase (saccadic) eye movements, whereas nystagmus is distinguished by the combination of a smooth pursuit, which usually acts to take the eye off the point of focus, interspersed with the saccadic movement that serves to bring the eye back on target. Without the use of objective recording tools, it may be difficult to distinguish between these situations.

In medicine, nystagmus can be innocuous or suggest an underlying visual or neurological disease. 

Infantile nystagmus 

The two most common types of benign nystagmus in children are congenital nystagmus (CN) and manifest latent nystagmus (MLN) (MLN). Both have conjugate, horizontal, and jerky oscillations. Differential diagnosis is based on the fact that slow phases in CN have a developing exponential velocity form, whereas slow phases in MLN have a declining or linear velocity shape.

In addition to its characteristic slow phase, MLN's quick phase always beats toward the viewer's eye. MLN is also strongly associated with the presence of strabismus and dissociated vertical divergence, is primarily visually driven, and is highly dependent on the patient's attentional state.

Both CN and MLN have been associated to a number of diseases, including albinism, optic nerve hypoplasia, and congenital cataracts. In the absence of ocular or central nervous system diseases, CN can develop.

Some CN patients have near-normal vision, especially if they have developed 'foveation periods,' which are brief epochs during which the eye remains immobile and focused on the object of attention. CN is typically decreased during convergence and at certain gaze angles, which might be used to provide optical or surgical treatments.

Several mechanisms underlying CN and MLN have been proposed throughout the years. Smooth pursuit, fixation, and optokinetic system anomalies are examples of these. To account for CN, five distinct models have been created thus far. Patients with CN who are unable to suppress their nystagmus have either an abnormal feedback loop or are unable to adjust the typical feedback loop voluntarily.

These two ideas are based on the requirement for brain miswiring. Given the variety of visual difficulties associated with CN in the absence of chiasmal misdirection, the absence of an aberrant visual evoked response in idiopathic CN, and the identification of CN in achiasmic dogs and people, this appears to be a stretch.

Fourth, Harris argued in 1995 that CN was generated by too much gain in an internal efference copy loop in the smooth pursuit system, which was focused on a leaky neural integrator. Finally, Broomhead and his colleagues suggested, using a non-linear dynamics technique, that the behavior of burst cell firing, in the form of a saccadic termination anomaly, may account for the variety of CN waveforms that previous models were unable to explain.

Because MLN is common in people with congenital or uniocular vision loss, or who have had visual deprivation, it has been postulated that egocentric localization abnormalities, as well as an aberration of extraocular proprioception, may be partially responsible for these oscillations. To recapitulate, there are no well-accepted explanations for infantile variations of nystagmus, albeit several characteristics of these oscillations suggest prospective therapeutic targets.

Management

Congenital nystagmus was long assumed to be incurable, but in recent years, therapies that show promise in specific individuals have been discovered. In 1980, researchers found that a drug known as baclofen might cure periodic alternating nystagmus. As a consequence, gabapentin, an anticonvulsant, helped around half of those who used it.

Memantine, levetiracetam, 3,4-diaminopyridine (free to qualified patients with downbeat nystagmus in the United States via an extended access program), 4-aminopyridine, and acetazolamide have also been shown to be useful against nystagmus in some people. Contact lenses, medicines, surgery, and low vision rehabilitation have all been investigated as treatment options. Miniature-telescopic glasses, for example, have been proposed as a treatment for nystagmus.

Surgery for congenital nystagmus attempts to correct head posture, imitate artificial divergence, or weaken the horizontal recti muscles. Clinical trials for a treatment to treat nystagmus (known as tenotomy) were completed in 2001. Tenotomy is now routinely done in a variety of sites throughout the world. The procedure is designed to reduce ocular oscillations, which tend to improve visual acuity.

Acupuncture research have shown conflicting results about its favorable effects on nystagmus symptoms. Acupuncture points of the neck, particularly the sternocleidomastoid muscle, have been demonstrated to be effective in treatments. Acupuncture advantages include lowered frequency and slower phase velocities, which resulted in an increase in foveation duration periods both during and after treatment.

According to evidence-based medicine standards, the quality of these studies is poor (for example, Ishikawa's study had a sample size of six subjects, was unblinded, and lacked proper controls), and given high quality studies demonstrating that acupuncture has no effect beyond placebo, the results of these studies must be considered clinically irrelevant until higher quality studies are performed.

Physical or occupational therapy can also be used to treat nystagmus. Treatment entails devising compensating measures for the weakened system.

A Cochrane Review published in June 2017 on therapy for eye movement abnormalities caused by acquired brain injury identified three studies of pharmacological interventions for acquired nystagmus but concluded that the evidence was insufficient to guide treatment decisions. 

Conclusion 

Nystagmus is defined by rhythmic, aberrant eye movements, with a "slow" eye movement pushing the eye away from the target, followed by a second movement returning the eye to the target.

The movement might be horizontal, vertical, torsional, or a combination of the three. Nystagmus can be jerk (during the fast phase) or pendular in nature, with varying amplitude and frequency, and it can be aggravated or relieved by gaze position, fixation, or covering one eye (latent).

"Saccadic incursions" or "saccadic oscillations" are defined as "rapid, back to back (without intersaccadic interval)" eye movements that push the eye away from the visual object. The neurological foundation of saccadic intrusions/oscillations is distinct from that of nystagmus and is covered in a different section.