Downbeat nystagmus, which is always of central origin, is characteristic of lesions in the medullary–cervical region such as syringobulbia, Chiari malformation, basilar invagination, and demyelinating plaques. It has also been seen with Wernicke disease and may be an initial sign of either paraneoplastic brainstem encephalitis or cerebellar degeneration with opsoclonus. Downbeat nystagmus has also been observed in patients with lithium intoxication or with profound magnesium depletion (Saul and Selhorst). Halmagyi and coworkers, who studied 62 patients with downbeat nystagmus, found that half were associated with Chiari malformation and various forms of cerebellar degeneration; in most of the remainder, the cause could not be determined. Cases associated with antibodies against glutamic acid decarboxylase (GAD), a substance that has a documented relationship to the stiff man syndrome, have been reported by Antonini and colleagues and by other groups. Whether this antibody explains the idiopathic cases of downbeat nystagmus is not known.
Spontaneous upbeat nystagmus can be observed in patients with demyelinating or vascular disease, tumors, or Wernicke disease. There is still uncertainty about the anatomic basis of coarse upbeat nystagmus, but it has been associated with lesions of the midbrain and cerebellum (particularly the anterior cerebellar vermis). Kato and associates also cite cases with a lesion at the pontomedullary junction involving the nucleus prepositus hypoglossi, which receives vestibular connections and projects to all brainstem and cerebellar regions concerned with ocular motor functions.
Nystagmus of several types—including gaze-evoked nystagmus, downbeat nystagmus, and “rebound nystagmus” (gaze-evoked nystagmus that changes direction with refixation to the primary position)—occurs with cerebellar disease, particularly with lesions of the vestibulocerebellum or with brainstem lesions that involve the nucleus prepositus hypoglossi and the medial vestibular nucleus. Also characteristic of cerebellar disease are several closely related disorders of saccadic movement that appear as nystagmus (opsoclonus, flutter, dysmetria) described in the following text. Tumors situated in the cerebellopontine angle may cause a coarse bilateral horizontal nystagmus that is higher amplitude to the side of the lesion (Brun’s nystagmus).
Nystagmus that occurs only in the abducting eye is referred to as dissociated nystagmus and is a common sign of internuclear ophthalmoplegia, as discussed earlier.
Infantile (Congenital, Pendular) Nystagmus
This nystagmus can occur in association with profound visual loss or as an isolated abnormality with relatively preserved visual function. When accompanied by visual loss, it may be associated with albinism, Leber’s congenital amaurosis, and various other diseases of the retina and refractive media. Occasionally it is observed as a congenital abnormality, even without poor vision. The defect in infantile nystagmus is postulated to be instability of smooth pursuit or gaze-holding mechanisms. A cardinal feature of this type of nystagmus is that it is in one plane; that is, it remains horizontal even during vertical movement. It is mainly pendular (sinusoidal) except in extremes of gaze, when it comes to resemble jerk nystagmus. Eye movement recordings demonstrate an exponentially increasing velocity of the slow phase that is unique among nystagmus.
Infantile nystagmus is often suppressed during convergence. Many individuals have a “null position,” where the nystagmus is dampened in a particular direction of gaze. These patients, therefore, adopt a compensatory head turn in order to utilize the null position, where the retinal image is most stable, to its maximum effect. Also characteristic is a paradoxical response to optokinetic testing (see later), in which the quick phase is in the same direction as the drum rotation.