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Medical treatment of nystagmus and its visual consequences.

For us to see clearly the details in our visual world, images must be held quite still upon the retina, especially the central part (the fovea). For reading, which concerns detection of high spatial frequencies, image motion must be less than about 5° / second1. If image drift substantially exceeds this limit, visual acuity will decline and we may experience the illusion that the world is moving (oscillopsia). In health, three main mechanisms hold the line of sight steady so that our view of the world is clear and stable2. The first is ‘fixation’, which has two distinct components—the visual system's ability to detect retinal image drift and programme corrective eye movements, and the ability to suppress unwanted eye movements that would take the eye away from the target. The second mechanism is the vestibulo-ocular reflex, by which the motion detectors of the inner ear promptly initiate eye movements to compensate for head perturbations, such as occur during locomotion. The third mechanism is the gaze-holding system, which makes it possible to hold the eyes at an eccentric position (e.g. in lateral gaze). 
 
If any of the three mechanisms that normally act to hold gaze steady malfunctions, the eyes may start to drift away from the object of regard, and corrective rapid eye movements may be made. Thus, nystagmus may be defined as repetitive to-and-fro involuntary eye movements that are initiated by slow drifts of the eye. It is important to realize that some forms of nystagmus are normal. Thus, nystagmus that occurs during rotation of the body in space acts to preserve clear vision. In pathological nystagmus, however, drifts of the eyes away from the target degrade vision. In one form, pendular nystagmus, the drifts consist of a to-and-fro sinusoidal oscillation. More commonly, nystagmus consists of alternation of unidirectional drifts away from the target and corrective fast movements (saccades) which momentarily bring the visual target back to the fovea; this is jerk nystagmus. 
 
Nystagmus should be distinguished from inappropriate saccades that disrupt steady fixation. Saccades produce high-speed movement of images upon the retina—too high for clear vision—and several physiological mechanisms exist to prevent perception of the smeared retinal signal due to the saccade. However, patients in whom inappropriate saccades repeatedly misdirect the fovea often complain of difficulty with reading. 
 
 
 
Box 1 Medical strategies for treating nystagmus and its visual consequences 
 
 
 
Methods that place the eye in a position in which nystagmus is minimized 
 
 
Optical and electronic methods for negating the visual consequences of the nystagmus 
 
 
Procedures for weakening the extraocular muscles 
 
 
Application of somatosensory or auditory stimuli to suppress nystagmus 
 
 
Drugs that suppress some forms of nystagmus. 
 
 
 
 
 
 
 
 
Five main approaches to treating pathological nystagmus and its visual consequences are summarized in Box 1. For a fuller discussion of the clinical features, pathogenesis and treatment of nystagmus, the reader is referred to a recent textbook on eye movements2.