When electrodiagnosis for tremor reveals a rare peripheral disease

Abstract

Objectives Spinal and bulbar muscular atrophy (SBMA) (or Kennedy's disease) is a rare X-linked neurodegenerative disorder associated with limb and bulbo-facial paresis, sensory symptoms and tremor. Tremor usually precedes peripheral motor signs by a decade. It is supposed to result from several peripheral mechanisms [1] . Results Here we report a case of a 42 years-old patient who was addressed to the department for exploration of isolated bilateral upper limb tremor. Surface polymyographic examination revealed numerous spontaneous and short (7–12 ms) bursts at rest consistent with fasciculations and associated with a postural myoclonic tremor as well as with exaggerated physiological tremor of 8–10 Hz. Detailed clinical examination showed a generalized areflexia, cramps, mild paresis of upper limbs, gynecomastia and facial myokymia. Conventional electromyography (EMG) showed a chronic motor denervation in the muscles of the four limbs and the face with normal motor nerve conduction; however, fasciculations were not detected by needle EMG. In sensory nerves of four limbs, a homogeneous decrease of action potentials amplitudes was found consistent with axonal sensory neuropathy. Altogether, these findings were strongly suggestive of a SBMA. Genetic testing for CAG repeats in the androgen receptor gene is under investigation. As a reference, we show typical surface EMG recordings of upper limbs and neurophysiological characteristics of tremor of a patient with genetically documented SBMA (48 CAG triplets) at a more severe stage of the disease. Conclusion Spontaneous surface EMG activity compatible with fasciculations might be a valuable marker of a neuropathic tremor and potentially a revealing sign of SBMA.