Nystagmus

Nystagmus, is involuntary eye movement that can be part of the vestibulo-ocular reflex (VOR) or it can be part of a pathological process. It is characterized by alternating smooth pursuit in one direction and a saccadic movement in the other direction.

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General overview

Nystagmus can be caused by subsequent foveation of moving objects, pathology, sustained rotation, or substance abuse.

The direction of nystagmus is defined by the direction of its quick phase (e.g. a right-beating nystagmus is characterized by a rightward-moving quick phase). The oscillations may occur in the vertical, horizontal or torsional planes, or in any combination. The resulting nystagmus is often named as a gross description of the movement, e.g. downbeat nystagmus, upbeat nystagmus, seesaw nystagmus, periodic alternating nystagmus.

These descriptive names can be misleading however, as many were assigned historically, solely on the basis of subjective clinical examination, which is not sufficient to determine the eyes' true trajectory.

Over the past forty years, however, objective eye movement recording techniques have been applied to the study of nystagmus, and the results have led to a greater accuracy and understanding of the condition.

Nystagmus is not to be confused with other superficially similar-appearing disorders of eye movements (saccadic oscillations) such as opsoclonus or ocular flutter that are composed purely of fast-phase (saccadic) eye movements, while nystagmus is characterised by the combination of a smooth pursuit, which usually acts to take the eye off the point of regard, interspersed with the saccadic movement that serves to bring the eye back on target. Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.

In medicine, the presence of nystagmus can be benign, or it can indicate an underlying visual or neurological problem. Over forty types of nystagmus have been classified.^{[citation needed]}

Nystagmus and alcohol

In police work, testing for horizontal gaze nystagmus is one of a battery of field sobriety tests used by officers in the field to determine whether a suspect is driving under the influence of alcohol. The test involves observation of the suspect's pupil as it follows a moving object, noting (1) lack of smooth pursuit, (2) distinct and sustained nystagmus at maximum deviation, and (3) the onset of nystagmus prior to 45 degrees. A general rule of thumb is that a person's blood alcohol concentration can be estimated by subtracting the angle of onset from 50 degrees. Therefore, a person with an angle of onset of nystagmus at 35 degrees has a blood alcohol concentration of approximately 0.15%.

Pathological nystagmus

When nystagmus occurs without filling its normal function, it is pathologic (deviating from the healthy or normal condition). Pathological nystagmus is the result of damage to one or more components of the vestibular system, including the semicircular canals, otolith organs, and the vestibulocerebellum.

Pathological nystagmus generally causes a degree of vision impairment, although the severity of such impairment varies widely. Sometimes it is the other way around — many blind people have nystagmus, which is one reason that some wear dark glasses.^[1]

Prevalence

Nystagmus is a relatively common clinical condition, affecting one in every 5,000 to 10,000 individuals.^{[citation needed]} One survey in Oxfordshire, England identified one in every 670 children by the age of two as manifesting nystagmus. ^[2]

Variations

• Peripheral nystagmus occurs as a result of either normal or diseased functional states of the vestibular system and may combine a rotational component with vertical or horizontal eye movements and may be spontaneous, positional or evoked.
  • Positional nystagmus occurs when a persons head is in a specific position. An example of disease state in which this occurs is Benign paroxysmal positional vertigo (BPPV).
  • Gaze Induced nystagmus occurs or is exacerbated as a result of changing one's gaze toward or away from a particular side which has an affected vestibular apparatus.
  • Post Head Shake nystagmus occurs after an imbalance is created between a normal side and a diseased side by stimulation of the vestibular system by rapid shaking of the head.
  • Spontaneous nystagmus is nystagmus that occurs randomly, regardless of the position of the patient's head.
• Central nystagmus occurs as a result of either normal or abnormal processes not related to the vestibular organ. For example lesions of the midbrain or cerebellum can results in up and down-beat nystagmus.

Causes

The cause for pathological nystagmus may be congenital, idiopathic, secondary to a pre-existing neurological disorder or may be induced temporarily by certain drugs (alcohol and other central nervous system depressants and stimulants, such as lithium salts, phenytoin and ecstasy).

Congenital

Congenital nystagmus occurs more frequently than acquired nystagmus, is not associated with other disorders (such as refraction errors or diplopia) and is usually mild and non-progressive. The affected persons are not aware of their spontaneous, small-amplitude eye movements.

• Infantile:

• Idiopathic
• Albinism
• Aniridia
• Leber's congenital amaurosis
• Bilateral optic nerve hypoplasia
• Bilateral congenital cataracts
• Rod monochromatism
• Optic nerve or macular disease
• Persistent tunica vasculosa lentis

• Latent nystagmus
• Nystagmus blockage syndrome

Acquired

Diseases

Some of the diseases which present nystagmus as a pathological sign are:

• Benign Paroxysmal Positional Vertigo
• Head trauma
• Stroke (the most common cause in older people)
• Ménière's disease and other balance disorders
• Multiple sclerosis
• Brain tumors
• Wernicke-Korsakoff syndrome
• Encephalopathy
• Lateral medullary syndrome
• Aniridia
• Optic nerve hypoplasia
• Albinism
• Noonan syndrome
• Pelizaeus-Merzbacher disease

Toxic/metabolic

Nystagmus from toxic or metabolic reasons could be the result of e.g.:

• Alcohol intoxication (see above)
• Lithium
• Barbiturates
• Phenytoin (Dilantin)
• Salicylates
• Benzodiazepines
• Lysergic acid diethylamide (LSD)
• Phencyclidine (PCP)
• Other anticonvulsants or sedatives
• Methylenedioxymethamphetamine
• Wernicke's encephalopathy
• Thiamine deficiency

Central nervous system disorders

If the pathologic nystagmus is based in the central nervous system (CNS), such as with a cerebellar problem, the nystagmus can be in any direction including horizontal. Purely vertical nystagmus is usually central in origin.

Causes include e.g.:

• Thalamic hemorrhage
• Tumor
• Stroke
• Trauma
• Multiple sclerosis

Other causes

• Vestibular Pathology (Ménière's disease, BPPV, Labyrinthitis)
• Trochlear nerve malfunction^[3]
• Non-physiologic

Diagnosis

Nystagmus is very noticeable, but little recognised. Nystagmus can be clinically investigated by using a number of non-invasive standard tests. The simplest one is Caloric reflex test. In a caloric reflex test, one external auditory meatus is irrigated with warm or cold water. The temperature gradient provokes the stimulation of the vestibulocochlear nerve and the consequent nystagmus.

The resulting movement of the eyes may be recorded and quantified by special devices called electronystagmograph (ENG), which is a form of electrooculography (an electrical method of measuring eye movements using external electrodes) or even less invasive devices called videoonystagmograph (VNG), which is a form of videooculography(VOG) (a video-based method of measuring eye movements using external small cameras built into head masks). Special swinging chairs with electrical controls are also used in this test to induce rotatory nystagmus.

Treatment

Congenital nystagmus has traditionally been viewed as non-treatable, but medications have been discovered in recent years that show promise in some patients. In 1980, researchers discovered that a drug called baclofen could effectively stop periodic alternating nystagmus. Subsequently, gabapentin, an anticonvulsant, was found to cause improvement in about half the patients who received it to relieve symptoms of nystagmus. Other drugs found to be effective against nystagmus in some patients include memantine, levetiracetam, 3,4-diaminopyridine, 4-aminopyridine, and acetazolamide.^[4] Several therapeutic approaches, such as contact lenses, drugs, surgery, and low vision rehabilitation can also be used in order to improve visual function.

Clinical trials of a surgery to treat nystagmus (known as tenotomy) concluded in 2001. Tenotomy is being performed regularly at the University of Pittsburgh Children's Hospital and by a handful of surgeons around the world. The surgery developed by Louis F. Dell'Osso Ph.D aims to reduce the eye shaking (oscillations) which in turn tends to improve visual acuity.

Research

Several universities are researching Nystagmus and are looking for volunteers to take part in research activities.

Current UK Research Projects

See also

• Optokinetic nystagmus

References