Pendular nystagmus is a sinusoidal oscillation. The waveform of pendular nystagmus may occur in any direction; it can be torsional, horizontal, vertical, or a combination of these, resulting in circular, oblique, or elliptical trajectories. It may be different in the two eyes, sometimes even monocular. Diagnosis is an exercise in pattern recognition since pendular nystagmus is subdivided into a number of types, some characterized by specific trajectories:
- Acquired pendular nystagmus
- Pendular nystagmus from visual loss
- Specific variants: spasmus nutans; oculopalatal myoclonus; see-saw nystagmus; oculomasticatory myorhythmia (Whipple's)
- Congenital nystagmus
ACQUIRED PENDULAR NYSTAGMUS
Causes — Most patients with acquired pendular nystagmus have multiple sclerosis [1,2]; the incidence of pendular nystagmus in this population is about 2 to 4 percent [3,4]. Less commonly it follows strokes, encephalitis, or vascular malformations in the brainstem [1,5]. Rare causes include chronic toluene encephalopathy , Pelizaeus-Merzbacher leukodystrophy , unusual familial syndromes , and orbital myositis . One unique case with monocular nystagmus had a chiasmal glioma .
Pathophysiology — The pathophysiology of acquired pendular nystagmus is unclear. It is associated with cerebellar signs, so it may be a "cerebellar tremor" . The association with internuclear ophthalmoplegia suggests a brainstem site ; dysfunction of the vergence system is a possibility in some patients .
Pendular nystagmus can be asymmetric or even monocular. How this arises is an interesting problem. It may reflect asymmetric brainstem disease ; however, asymmetries in nystagmus do not correlate with asymmetries in other brainstem signs [4,12]. It may be related to optic nerve function since larger oscillations correlate with greater optic neuropathy . Taking all this together, pendular nystagmus is most likely caused by abnormal brainstem feedback circuits for eye position , which are calibrated by visual factors [14,15].