• 10 SECTION Nystagmus

Sidebeat, Upbeat, and Downbeat Nystagmus

Key Facts

•Horizontal or vertical jerk nystagmus that reflects brainstem vestibulocerebellar dysfunction

•May be caused by congenital malformations or acquired structural or metabolic abnormalities, toxins, or medications

•Common acquired causes:

•tumors

•demyelination

•infections

•neurodegenerative disorders

•metabolic disturbances (hypomagnesemia, thiamine or cyanocobalamin deficiency)

•medication toxicity (phenytoin, lithium, carbamazepine)

•Medical treatment rarely effective in relieving oscillopsia

Clinical Findings

•Horizontal or vertical jerk nystagmus

•Occlusion or Frenzel lenses do not affect nystagmus amplitude

•Sidebeat nystagmus: usually first appears on sidegaze and beats in direction of gaze

•Upbeat nystagmus: may be present in primary position and increases in amplitude in upgaze

•Downbeat nystagmus: may be present in primary position and increases in amplitude on down-and-sidegaze

•Periodic alternating nystagmus (PAN):

•beats to one side in primary position with increasing and then decreasing amplitude for about 90 s

•eyes are stationary for about 10 s

•the nystagmus begins in the opposite direction with increasing and then decreasing amplitude as before

•each full cycle lasts nearly 3 min

•Ataxia, skew deviation, alteration in consciousness may be present

Ancillary Testing

•Brain MRI (concentrating on brainstem, cerebellum, and cervical cord) often shows structural abnormalities but often normal in toxic, metabolic, or degenerative conditions

•Full neurologic evaluation recommended

Differential Diagnosis

•Congenital nystagmus

•Peripheral vestibular jerk nystagmus

•Ocular flutter or opsoclonus

•Ocular dysmetria

Fig. 10.11 Sidebeat nystagmus. The eyes display no nystagmus in the primary position. In right gaze, there is a right-beating (jerk) nystagmus. In left gaze, there is a left-beating (jerk) nystagmus.

Fig. 10.12 Upbeat nystagmus. The eyes display an upbeat nystagmus in all positions of gaze.

Nystagmus Downbeat and Upbeat, Sidebeat,

• 10 SECTION Nystagmus

Sidebeat, Upbeat, and Downbeat Nystagmus (Continued)

Treatment

•Directed at underlying cause

•Surgical treatment, including Chiari malformation repair, does not reliably relieve nystagmus

•Medical treatment, indicated if oscillopsia is disabling, is disappointing and may exacerbate other neurologic manifestations

•Some physicians report oscillopsia reduction in PAN with baclofen, in downbeat with clonazepam, baclofen, scopolamine, and gabapentin

•Intramuscular or retrobulbar botulinum toxin injection does not reduce oscillopsia

•Large extraocular muscle recessions do not reduce oscillopsia

Prognosis

• Depends on underlying cause

Fig. 10.13 Downbeat nystagmus. The eyes display a jerk nystagmus with a fast downward component. The nystagmus is often most obvious when the eyes are in downgaze.

Fig. 10.14 Chiari malformation in a patient with downbeat nystagmus. Precontrast sagittal T1 MRI shows that the cerebellar tonsil has descended into the foramen magnum (arrow). The low signal in the center of the cervical spinal cord (arrowhead) is a syrinx, a common accompaniment of Chiari malformation.

(continued) Nystagmus Downbeat and Upbeat, Sidebeat,