dissociated nystagmus

nystagmus /nys·tag·mus/ (nis-tag´mus) involuntary rapid movement (horizontal, vertical, rotatory, or mixed, i.e., of two types) of the eyeball.nystag´mic

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aural nystagmus  vestibular n.

caloric nystagmus  rotatory nystagmus induced by irrigating the ears with warm or cold water or air; see caloric test, under test.

Cheyne's nystagmus , Cheyne-Stokes nystagmus a peculiar rhythmical eye movement.

dissociated nystagmus  that in which the movements in the two eyes are dissimilar.

end-position nystagmus  that occurring in normal individuals only at extremes of gaze.

fixation nystagmus  that occurring only on gazing fixedly at an object.

gaze nystagmus  nystagmus made apparent by looking to the right or to the left.

gaze paretic nystagmus  a form of gaze nystagmus seen in patients recovering from central nervous system lesions; the eyes fail to stay fixed to the affected side with a cerebral or pontine lesion.

labyrinthine nystagmus  vestibular n.

latent nystagmus  that occurring only when one eye is covered.

lateral nystagmus  involuntary horizontal movement of the eyes.

opticokinetic nystagmus , optokinetic nystagmus the normal nystagmus occurring when looking at objects passing across the field of vision, as in viewing from a moving vehicle.

pendular nystagmus  that which consists of to-and-fro movements of equal velocity.

positional nystagmus  that which occurs, or is altered in form or intensity, on assumption of certain positions of the head.

retraction nystagmus , nystagmus retracto´rius a spasmodic backward movement of the eyeball occurring on attempts to move the eye; a sign of midbrain disease.

rotatory nystagmus  involuntary rotation of eyes about the visual axis.

spontaneous nystagmus  that occurring without specific stimulation of the vestibular system.

undulatory nystagmus  pendular n.

vertical nystagmus  involuntary up-and-down movement of the eyes.

vestibular nystagmus  that due to disturbance of the vestibular system; eye movements are rhythmic, with slow and fast components.

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dissociated nystagmus

n.

A nystagmus in which the movements of the two eyes are dissimilar in direction, extent, and periodicity.

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nystagmus

a periodic, rhythmic, involuntary movement of both eyeballs in unison. There is a slow component in one direction and a quick return. The movement may be vertical, horizontal or rotary. Common causes are lesions of the cerebellum or the vestibular apparatus, or increased intracranial pressure.

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aural nystagmus

labyrinthine nystagmus.

cerebellar nystagmus

one characterized by tremor, without fast and slow components.

Cheyne's nystagmus

a peculiar rhythmical eye movement resembling Cheyne-Stokes respiration in rhythm.

congenital nystagmus

may be a primary functional defect or secondary to lesions in the visual pathways, sometimes associated with albinism. Reported in cattle, cats (particularly Siamese), and dogs.

dissociated nystagmus

that in which the movements in the two eyes are dissimilar.

gaze nystagmus

nystagmus made apparent by looking to the right or to the left.

horizontal nystagmus

that in which the eyes move from side to side with the fast component opposite to the side of the lesion; seen with central or unilateral peripheral vestibular disease.

jerk nystagmus

vestibular nystagmus (see below).

labyrinthine nystagmus

vestibular nystagmus due to labyrinthine disturbance.

latent nystagmus

that occurring only when one eye is covered.

lateral nystagmus

involuntary horizontal movement of the eyes.

ocular nystagmus

wandering movement of the eyes as though searching for something. Associated with congenital blindness.

optokinetic nystagmus

nystagmus induced by looking at objects moving across the field of vision.

oscillatory nystagmus

pendular nystagmus.

pendular nystagmus

that which consists of to-and-fro movements of equal velocity.

positional nystagmus

that which occurs, or is altered in form or intensity, on assumption of certain positions of the head.

postrotatory nystagmus

a normal finding after the animal has been rotated, with the fast phase away from the direction of rotation.

resting nystagmus

that occurring while the head is stationary.

retraction nystagmus, nystagmus retractorius

a spasmodic backward movement of the eyeball occurring on attempts to move the eye; a sign of midbrain disease.

rotatory nystagmus

involuntary rotation of the eyes about the visual axis.

spontaneous nystagmus

that occurring without specific stimulation of the vestibular system.

vertical nystagmus

involuntary up-and-down movement of the eyes.

vestibular nystagmus

nystagmus due to disturbance of the labyrinth or of the vestibular nuclei; the movements are usually jerky.

undulatory nystagmus

an inherited disorder of Finnish Ayrshire cattle; there is a synchronous, tremor-like movement of the eyes but affected aninals are otherwise healthy.

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nystagmus

A regular, repetitive, involuntary movement of the eye whose direction, amplitude and frequency are variable. Nystagmus can be induced, acquired or congenital. (In a very small percentage of people it can even be induced voluntarily.) These eye movements typically appear as one of two types: pendular nystagmus or jerk nystagmus (Fig. N3). Jerk nystagmus is one in which there is a slow and fast phase, the nystagmus being conventionally defined by the direction of the fast phase. A feature of jerk nystagmus is the null zone (or null point) which represents the direction of gaze at which the nystagmus has the smallest amplitude. A jerk nystagmus is usually due to a motor defect that may be induced by brainstem or cerebellar lesions, drug intoxication (upbeat nystagmus in which the fast phase is in the upward direction or downbeat nystagmus in which the fast phase is downward); associated with a lesion of the central nervous system or the vestibular nerve or nuclei (central nystagmus and vestibular nystagmus); or to disease or injury to the labyrinth (labyrinth nystagmus); or to multiple sclerosis. Jerk nystagmus can also be induced physiologically, as for example optokinetic nystagmus (OKN) or train nystagmus, which occurs when watching objects that traverse the visual field rapidly, or as a result of thermal stimulation of the labyrinth of the inner ear by cold or hot water (caloric nystagmus or Barany's nystagmus), or when the eyes of a fatigued person are turned into an extreme position of gaze (end-point nystagmus), or when a person who had been spinning round is stopped (vestibular nystagmus).The other type is a nystagmus which is characterized by movements of equal velocity in each direction, the pendular nystagmus. A pendular nystagmus usually occurs as a result of poor central vision (sensory deprivation nystagmus) as in bilateral chorioretinitis, total colour blindness, albinism, congenital cataract, corneal scarring, amblyopia (amblyopic nystagmus) or in coal miners after many years of working in the dark (miner's nystagmus). In some cases one eye rotates upward and intorts while the other rotates downward, and extorts (see-saw nystagmus as a result of brainstem stroke, chiasmal lesion or multiple sclerosis). In some cases there is a mixture of the two main types; pendular in the primary position and jerk on lateral gaze (mixed nystagmus). The movements of the eyes are usually the same in both eyes (conjugate nystagmus) but in other cases they may be unrelated as a result of internuclear ophthalmoplegia (dissociated nystagmus). Examples of the latter are end-gaze nystagmus, convergence-retraction nystagmus and see-saw nystagmus. Or the eye movements are of equal amplitude and type but in opposite or different directions (disjunctive nystagmus), also commonly associated with internuclear ophthalmoplegia. There are also cases of unknown origin (idiopathic nystagmus). See hereditary spinal ataxia; Wernicke's disease; foveation period; Alexander's law; monochromat; internuclear ophthalmoplegia; oscillopsia; yoke prisms; Faden procedure; vestibulo-ocular reflex; spasmus nutans; Down's syndrome; optokinetic nystagmus test.
nystagmus blockage syndrome See nystagmus blockage syndrome.
caloric nystagmus See caloric testing.
congenital nystagmus A motor nystagmus that is present at birth or soon after. It may be inherited as X-linked recessive or autosomal dominant, or induced in the uterus, and results from decreased vision due to corneal opacity, cataract, albinism, aniridia, macular disease or optic atrophy. It is typically a horizontal jerk nystagmus and it may be associated with abnormal head movement and decreases in intensity with convergence. The visual prognosis is reasonably good, but if the head turn is excessive, extraocular muscle surgery may be needed. See spasmus nutans.
convergence-retraction nystagmus A jerk nystagmus which appears on attempted upward gaze and in which the fast phase brings the two eyes towards each other in a convergent movement with retraction of the globes into the orbit. It may result from a lesion affecting the tectum or dorsal midbrain or a pineal tumour, or form part of Parinaud's syndrome. See pinealoma.
gaze-evoked nystagmus An acquired form of horizontal nystagmus characterized by a jerk nystagmus on eccentric gaze with the fast phase towards the direction of gaze. This type of nystagmus is believed to be due to cerebellar or brainstem disease affecting the conjugate gaze centres.
occlusion nystagmus A form of nystagmus occurring when one eye is covered, or which increases in intensity when one eye is covered. The nystagmus is typically of the horizontal, jerk variety, with the fast phase occurring in the direction of the occluded eye.
physiological nystagmus See fixation movements.
rotary nystagmus A very rare form of nystagmus in which the eyeball makes a movement about the visual axis. It may result from a lesion to the vestibular nerve. See vestibular nystagmus.
sensory nystagmus A form of nystagmus thought to be due to an abnormality in the afferent mechanism. It is most often due to inadequate image stimulation of the macula, leading to abnormal development of the ocular fixation reflex. Causes include congenital cataracts, optic nerve hypoplasia, aniridia, albinism, achromatopsia, as well as Leber's congenital amaurosis.
vestibular nystagmus There are two main types of vestibular nystagmus: Peripheral vestibular nystagmus results from stimulation, injury or disease (e.g. Menière's disease) of the labyrinth or of the vestibulo-cochlear nerve (VIII). It presents as a jerk, mainly horizontal, nystagmus with a torsional component. It may be accompanied by vertigo, tinnitus and hearing loss. Fixation inhibits the nystagmus. Central vestibular nystagmus results from stimulation, injury, disease of the central vestibular pathways of the brainstem or the cerebellum, or lesion of the vestibular nuclei. It is typically a jerk nystagmus, which can be purely horizontal, vertical or torsional. It is not inhibited by fixation.

Fig. N3 The two main types of nystagmus, jerk and pendular