EYE MOVEMENT DISORDERS
Disorders of eye movements arise from damage to widespread regions of the brain, and the symptoms reflect the contributions of the affected structures to eye motor control.
One common disorder of eye motor control is strabismus, which occurs when the movements of the two eyes are not properly yoked. This disorder typically occurs in children or infants (about 2% of children are affected), but can also affect adults. Strabismus is caused by deficits in any of the several factors that are necessary to properly align the eyes, including problems with eye muscles or oculomotor nerves, the presence of a large refractive error in one or both eyes, and lesions in central structures. If strabismus occurs during infancy, while connections in the visual system are developing, it can result in amblyopia, in which case visual inputs from the affected eye are mostly ignored.
Many eye movement disorders are caused by tumors, stroke, or other damage to the pathways responsible for forming the eye motor commands. Patients with damage to structures in the brainstem or cerebellum often have difficulty maintaining fixation, and exhibit nystagmus—a pattern of involuntary drifts in eye position, interspersed with centering saccades. Nystagmus can be caused by peripheral damage to the vestibular labyrinth or the eighth cranial nerve that transmits signals about head motion to the central nervous system. The imbalance in signals received from the left and right vestibular organs causes a slow drift of the eyes, away from the damaged side. If the nystagmus is caused by peripheral damage, then voluntary saccades and smooth pursuit are mostly unaffected. However, nystagmus can also be caused by central damage to brainstem or cerebellar structures. In these cases, voluntary saccades and smooth pursuit are also impaired, because the functions of the neural integrator and final motor pathways are affected.
Because gaze movements are controlled by descending signals fromthe cerebral cortex and basal ganglia, eyemovement disorders also occur in diseases that affect higher-order brain functions, including schizophrenia, Huntington’s disease, and Parkinson’s disease. These disorders are characterized by problems with voluntary eye movements, most notably a difficulty in suppressing unwanted saccades during fixation or smooth pursuit eye movements. In Huntington’s disease, in addition to difficulty with suppressing unwanted saccades during fixation, patients also have difficulty initiating voluntary saccades. This problem can be partly overcome by blinking at the same time, because the circuits for controlling eye blinks and saccades overlap. In schizophrenia, smooth pursuit eye movements do not correctly match the speed of the moving target, and are interrupted by back-and-forth saccades that repeatedly take the eyes off the target. For reasons that are not yet understood, these deficits in smooth pursuit ability also occur in relatives of schizophrenics who are themselves asymptomatic for the disease.