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Baltimore—A number of eye movement disturbances are characterized by cerebellar disease and saccadic intrusions. Michael X. Repka, MD, detailed these abnormalities and how to distinguish among them.
"Cerebellar eye movement control is predominantly organized in the region of the cerebellum," said Dr. Repka, professor of ophthalmology and a professor of pediatrics at the Johns Hopkins University School of Medicine, Baltimore. "Three areas of the cerebellum-the vestigial nucleus, the nodulus, and the flocculus-are important in modulation of eye movements."
Of these three distinct areas, the vestigial nucleus and the dorsal vermis are involved in saccadic pulse and abnormalities leading to dysmetria (lack of movement coordination characterized by under- or over-shooting the intended position) and smooth pursuit, a pursuit response to image velocity change. The nodulus is involved in the duration of vestibular response. The flocculus and the periflocculus region play a role in gaze holding, the vestibular ocular reflex, and possibly smooth pursuit, according to Dr. Repka.
A common problem
Dr. Repka showed a patient with dysmetria, the most common cerebellar eye movement disturbance. The patient made attempted saccades away from and back to primary position. On each movement back to the fixation target, the patient overshot the target, which was followed by a series of corrective saccades that finally localized on the target.
Eye movement problems such as these may originate in structural abnormalities, such as the Chiari malformation. This may be the most common abnormality of the cerebellum.
"The Chiari malformation is divided into Chiari 1, in which the tonsils are herniated through the brain and magnum, and Chiari 2, the Arnold-Chiari malformation, in which the tonsils, the fourth ventricle, and a portion of the cerebellar vermis are herniated through the brain and magnum. The latter condition is commonly associated with eye movement abnormalities," Dr. Repka said.
Other structural alterations of the cerebellum may be responsible for abnormalities in ocular movement.
The Dandy-Walker syndrome is an agenesis of the cerebellar vermis, in which a membrane line of cysts fills most of the fourth ventricle. Some patients also have agenesis of the corpus callosum.
Joubert's syndrome, familial agenesis of the vermis, is associated with tachypnea, ophthalmoplegia, nystagmus, and often an abnormal electroretinogram.
In addition, eye movement disturbances may also result from vascular lesions or stroke, according to Dr. Repka.
"The most common vascular insult is an infarction in the distribution of the posterior inferior cerebellum or artery," Dr. Repka said. "This will produce a gaze-evoked nystagmus, vertigo, and ataxia. The second most common vascular insult is an infarction in the superior cerebellar artery distribution, which is characterized by upbeating nystagmus and vertigo. A less common insult, an anterior inferior cerebellar artery infarction, has signs and symptoms of the two previous infarctions combined."
There are numerous degenerative abnormalities of the cerebellar region characterized by hypoplasia of the cerebellum, ataxia, and nystagmus.