Nystagmus
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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. Strobe lights also produce nystagmus. The degree of physiological nystagmus varies greatly between people and even in the same person at different times. Nystagmus is distinguished from normal involuntary eye activity by the rapidity and repetitive pattern of the movement.
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Pathological nystagmus
Nystagmus is 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, dilantin 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 except 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). Again, 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.
Other (extremely) rare pathologic nystagmuses are gaze paretic, rebound, fixation, congenital and dissociated nystagmus.
Some of the diseases which present nystagmus as a pathological sign are:
- Head trauma (the most common cause in young people)
- Stroke (the most common cause in older people)
- Albinism
- Multiple sclerosis
- Wernicke-Korsakoff syndrome
- Encephalopathy
- Lateral medullary syndrome
- Aniridia
- Optic nerve hypoplasia
- Noonan syndrome
- Pelizaeus-Merzbacher disease
- Ménière’s disease and other balance disorders
- Brain tumors
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.
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 a special device called electronystagmograph (ENG), which is a form of electro-oculography (an electrical method of measuring eye movements using external electrodes). Special swinging chairs with electrical controls are also used in this test to induce rotatory nystagmus.
Pathological acquired nystagmus is mostly a temporary condition and stops spontaneously. When it is secondary to a neurological disorder, this must be treated accordingly. Congenital nystagmus is usually non-treatable, but several therapeutic approaches, such as contact lenses, drugs, surgery, and low-vision rehabilitation can be used in order to improve visual function.
See also
References
- Vestibular disorders and tinnitus (http://www.med.uwo.ca/UME/Diane/Year2Postings2004-2005/Trimester%202/CNS/VestibularDisordersAndTinnitusDrParnes.pdf) - Lecture notes from a lecture given to a second year medical school class at the University of Western Ontario (http://www.uwo.ca) on 19 November 2004 by Dr. Lorne Parnes. These notes are released under the FDL.
External links
- Eyes Movements - Uncontrollable (http://www.nlm.nih.gov/medlineplus/ency/article/003037.htm). MedlinePlus.
- Hensil J, Gurwood AS: Understanding Nystagmus. Optometry 2000 Jul;71(7):439-48. Medline access (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15326897)
- American Nystagmus Network (http://www.nystagmus.org)
- German-Language Nystagmus Forum (http://www.nystagmus.de)de:Nystagmus