The related condition of latent nystagmus refers to nystagmus that occurs when one eye is covered. The fast phase of the nystagmus is in the direction of the covered eye. The finding may occur when either is covered, or it may be asymmetric and occur only with covering one of the eyes but not the other. Latent nystagmus is considered to be a result of impaired development of binocular stereoscopic vision. In some individuals with this condition who then lose vision in one eye later in life, the latent nystagmus becomes unmasked constantly and is termed manifest latent nystagmus.
Even in adulthood, severe acquired blindness can produce nystagmus of pendular or jerk variety. Both horizontal and vertical components are evident and the characteristic feature is a fluctuation over several seconds of observation in the dominant direction of beating. The oscillations of the eyes are usually very rapid, increase on upward gaze, and may be associated with compensatory oscillations of the head. The formerly common syndrome of “miner’s nystagmus” is an associated condition that occurs in patients who have worked for many years in comparative darkness.
Spasmus nutans, a specific type of pendular nystagmus of infancy, is accompanied by head nodding, and occasionally by wry positions of the neck. Most cases begin between the 4th and 12th months of life, never after the 3rd year. The nystagmus may be horizontal, vertical, or rotatory; it is usually more pronounced in one eye than the other (or limited to one eye) and can be intensified by immobilizing or straightening the head. Most infants recover within a few months or years. Most cases are idiopathic, but symptoms like those of spasmus nutans may betray the presence of a perichiasmal or third ventricular tumor (see also seesaw nystagmus below in “Other Types of Nystagmus”); rare cases accompany childhood retinal diseases. Although there is no direct connection to the rare childhood condition of bobble-head syndrome, which is caused by lesions in or adjacent to the third ventricle, they are similar in the rhythmic head movements as described in Chap. 29.
Acquired forms of pendular nystagmus may occur with leukodystrophies, including Pelizaeus-Merzbacher syndrome (see Chap. 36), multiple sclerosis (see Chap. 35), and toluene intoxication. In the oculomasticatory myorhythmia of Whipple disease, the nystagmus is conjoined to rhythmic jaw movements (see Chap. 31).
Other Types of Nystagmus
Convergence nystagmus refers to a rhythmic oscillation in which a slow abduction of both eyes is followed by a quick movement of adduction, usually accompanied by quick rhythmic retraction movements of the eyes (retraction nystagmus) and by one or more features of the Parinaud–dorsal midbrain syndrome discussed earlier in the chapter. There may also be rhythmic movements of the eyelids or a maintained spasm of convergence, best brought out on attempted elevation of the eyes on command or downward rotation of an OKN drum (see below for discussion of optokinetic nystagmus). These unusual phenomena all point to a lesion of the upper midbrain tegmentum and are usually manifestations of vascular disease, traumatic damage, or tumor, notably pinealoma that compresses this region.
Seesaw nystagmus is a torsional-vertical oscillation in which the intorting eye moves up and the opposite (extorting) eye moves down, then both move in the reverse direction. It is occasionally observed in conjunction with chiasmatic bitemporal hemianopia caused by sellar or parasellar masses and after pituitary surgery.