Myorhythmia: phenomenology, etiology, and treatment.

Myorhythmia is defined as repetitive, rhythmic, slow (1-4 Hz) movement affecting chiefly cranial and limb muscles. When occurring in the limbs it may be oscillatory and jerky, whereas oculo-masticatory myorhythmia, typically associated with Whipple's disease, is a slow, repetitive, often asymmetrical, facial and ocular movement. Thus, myorhythmia overlaps phenomenologically with tremor and segmental myoclonus. Although often present at rest, it must be differentiated from parkinsonian or dystonic tremor. Recognition of this movement disorder is important because it is usually associated with lesions involving the brainstem, thalamus, or other diencephalic structures with potentially treatable etiologies. In addition to Whipple's disease, myorhythmia has been described in patients with cerebrovascular disease, listeria encephalitis, anti-N-methyl-d-aspartate receptor encephalitis, steroid-responsive encephalopathy associated with autoimmune thyroiditis, multiple sclerosis, and other disorders. In addition to our own experience, we have systematically reviewed the medical literature, focusing on the phenomenology, pathophysiology, and etiology of this poorly recognized movement disorder. In this review, we aim to highlight the clinical features that differentiate myorhythmia from other movement disorders. Treatment should be directed against the underlying etiology.