Nystagmus in Wikipedia
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(Source - Retrieved 2006-09-07 14:25:38 from https://en.wikipedia.org/wiki/Nystagmus)
Nystagmus is rapid involuntary rhythmic eye movement, with the eyes moving quickly in one direction (quick phase), and then slowly in the other (slow phase). The direction of nystagmus is defined by the direction of its quick phase (e.g. right nystagmus is due to a right moving quick phase). Nystagmus may occur in the vertical or horizontal directions, and also in a semicircular movement, and thus are called downbeat nystagmus, upbeat nystagmus, seesaw nystagmus, periodic alternating nystagmus, and pendular nystagmus. There are other similar alterations in periodic eye movements (saccadic oscillations) such as opsoclonus or ocular flutter. One can accurately think of nystagmus as the combination of a slow adjusting eye movement (slow phase) like would be seen with the vestibulo-ocular reflex, followed by a quick saccade (quick phase) when the eye has reached the limit of its rotation.
In medicine, the clinical importance of nystagmus is that it indicates that the patient's spatial sensory system perceives rotation and is rotating the eyes to adjust. Thus it depends on the coordination of activities between two major physiological systems: the vision and the vestibular apparatus (which controls posture and balance). This may be physiological (or normal) or pathological.
An easy way of inducing physiological nystagmus is by having the person close her or his eyes and spin around. After a few spins, there is a distinct jerking of the eyes from side to side when they are reopened: this is rotatory-induced nystagmus. The degree of physiological nystagmus varies greatly between people and even in the same person at different times.
Another type of physiological nystagmus is the optokinetic nystagmus (OKN). It can be induced by presenting a moving pattern. The eyes tend to track the pattern, but snap back regularly. Nystagmus is distinguished from normal involuntary eye activity by the rapidity and repetitive pattern of the movement.
Nystagmus is a relatively common clinical condition, affecting one in every 5,000 to 10,000 individuals.
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). Nystagmus generally causes a degree of vision impairment, although the severity of such impairment varies widely.
If the pathologic nystagmus is based in the central nervous system (CNS), such as with a cerebellar problem, the nystagmus will be in any direction including horizontal. Vestibular nystagmus is always horizontal, and may be spontaneous or positional. Spontaneous vestibular nystagmus is nystagmus that occurs spontaneously, regardless of the position of the patient's head. In milder cases, the patient is often asked to fixate on an object, or wear fresnel lens glasses, which blur vision, to bring out the nystagmus. Positional nystagmus is the opposite of spontaneous nystagmus in that it occurs when the patient's head is in a specific position (e.g., benign paroxysmal positional vertigo; BPPV).
Horizontal nystagmus is also classified into three degrees as follows:
- First degree nystagmus is present only on lateral gaze, and has the fast phase in the direction of gaze;
- Second degree nystagmus is present in the primary (neutral) position of gaze;
- Third degree nystagmus is present in the same direction in all gaze positions
Such distinctions help to identify the anatomical source of the nystagmus. First degree nystagmus usually originates in the brainstem or cerebellum, while second and third degree nystagmus are usually vestibular in origin.
Other (extremely) rare pathologic nystagmuses are gaze paretic, rebound, fixation, congenital and dissociated nystagmus.
Diseases presenting nystagmus
Some of the diseases which present nystagmus as a pathological sign are:
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.
- Latent nystagmus
- Nystagmus blockage syndrome
- Visual loss (e.g. dense cataract, trauma, cone dystrophy)
- Toxic/metabolic (e.g. alcohol intoxication, lithium, barbituates, phenytoin(Dilantin), salicylates, benzodiazepines, phencyclidine, other anticonvulsants or sedatives, Methylenedioxymethamphetamine, Wernicke's encephalopathy, thiamine deficiency)
- Central nervous system disorders (e.g. thalamic hemorrhage, tumor, stroke, trauma, multiple sclerosis)
Diagnosis and therapy
Nystagmus can be clinically investigated by using a number of non-invasive standard tests. The simplest one is to irrigate an external auditory meatus 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) (an 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.
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, $4-aminopyridine, andacetazolamide$.$$ Several therapeutic approaches, such as contact lenses, drugs, surgery, and low vision rehabilitation can also be used in order to improve visual function.
Pathological acquired nystagmus is mostly a temporary condition and stops spontaneously. When it is secondary to a neurological disorder, this must be treated accordingly.
- Gareth Grainger
- the Tennessee Beast
- Groves, Nancy. Many options to treat nystagmus, more in development. Ophthalmology Times, March 15, 2006. 
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