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Nystagmus

Dysfunction of eye movement

  • 11 min read
  • 6 chapters

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In normal eyesight, while the head rotates about an axis, distant visual images are sustained by rotating eyes in the opposite direction of the respective axis. The semicircular canals in the vestibule of the ear sense angular acceleration, and send signals to the nuclei for eye movement in the brain. From here, a signal is relayed to the extraocular muscles to allow one's gaze to fix on an object as the head moves. Nystagmus occurs when the semicircular canals are stimulated (e.g., by means of the caloric test, or by disease) while the head is stationary. The direction of ocular movement is related to the semicircular canal that is being stimulated.

There are two key forms of nystagmus: pathological and physiological, with variations within each type. Physiological nystagmus occurs under normal conditions in healthy subjects. Nystagmus may be caused by congenital disorder or sleep deprivation, acquired or central nervous system disorders, toxicity, pharmaceutical drugs, alcohol, or rotational movement. Previously considered untreatable, in recent years several drugs have been identified for treatment of nystagmus. Nystagmus is also occasionally associated with vertigo.

Causes

The cause of pathological nystagmus may be congenital, idiopathic, or secondary to a pre-existing neurological disorder. It also may be induced temporarily by disorientation (such as on roller coaster rides or when a person has been spinning in circles) or by some drugs (alcohol, lidocaine, and other central nervous system depressants, inhalant drugs, stimulants, psychedelics, and dissociative drugs).

Early-onset nystagmus

Early-onset nystagmus occurs more frequently than acquired nystagmus. It can be insular or accompany other disorders (such as micro-ophthalmic anomalies or Down syndrome). Early-onset nystagmus itself is usually mild and non-progressive. The affected persons are usually unaware of their spontaneous eye movements, but vision can be impaired depending on the severity of the eye movements.

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

  • Infantile:
  • Latent nystagmus
  • Noonan syndrome
  • Nystagmus blockage syndrome

X-linked infantile nystagmus is associated with mutations of the gene FRMD7, which is located on the X chromosome.

Infantile nystagmus is also associated with two X-linked eye diseases known as complete congenital stationary night blindness (CSNB) and incomplete CSNB (iCSNB or CSNB-2), which are caused by mutations of one of two genes located on the X chromosome. In CSNB, mutations are found in NYX (nyctalopin). CSNB-2 involves mutations of CACNA1F, a voltage-gated calcium channel that, when mutated, does not conduct ions.

Acquired nystagmus

Nystagmus that occurs later in childhood or in adulthood is called acquired nystagmus. The cause is often unknown, or idiopathic, and thus referred to as idiopathic nystagmus. Other common causes include diseases and disorders of the central nervous system, metabolic disorders and alcohol and drug toxicity. In the elderly, stroke is the most common cause.

General diseases and conditions

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

Toxicity or intoxication, metabolic disorders and combination

Sources of toxicity that could lead to nystagmus:

Thiamine deficiency

Risk factors for thiamine deficiency, or beriberi, in turn include a diet of mostly white rice, as well as alcoholism, dialysis, chronic diarrhea, and taking high doses of diuretics. Rarely it may be due to a genetic condition that results in difficulties absorbing thiamine found in food. Wernicke encephalopathy and Korsakoff syndrome are forms of dry beriberi.

Central nervous system (CNS) diseases and disorders

Central nervous system disorders such as with a cerebellar problem, the nystagmus can be in any direction including horizontal. Purely vertical nystagmus usually originates in the central nervous system, but it is also an adverse effect commonly seen in high phenytoin toxicity. Other causes of toxicity that may result in nystagmus include:

Other causes

  • Non-physiological
  • Trochlear nerve malfunction
  • Vestibular pathology (Ménière's disease, SCDS (superior canal dehiscence syndrome), BPPV, vestibular neuritis)
  • Exposure to strong magnetic fields (as in MRI machines)
  • Long-term exposure to low light conditions or darkness, called miner's nystagmus after 19th-century coal miners who developed nystagmus from working in the dark.
  • A slightly different form of nystagmus may be produced voluntarily by some (8% of) people. Some can sustain it for up to 35 seconds, but most average around 5 seconds.

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