Postural leg tremor in X-linked spinal and bulbar muscular atrophy.

X-linked spinal and bulbar muscular atrophy (SBMA) is an adult-onset neuromuscular disorder caused by a CAG repeat expansion in the androgen receptor gene. Postural hand tremor is well known as a non-motor neuron sign, but to our knowledge postural leg tremor has not been reported. We studied the occurrence and physiological features of postural leg tremor in 12 male patients (38-64 years old) with genetically proven SBMA. Three patients had postural leg tremor with a frequency of 4-7Hz. In these patients, sensory nerve action potential (SNAP) was not detected in the lower limbs. There were significant differences between the patients with postural leg tremor and those without postural leg tremor in both the SNAP of the sural nerve and the length of the CAG repeat. Phenotypical differences between shorter CAG repeats, which indicate a sensory-dominant phenotype, and longer CAG repeats, which indicate a motor-dominant phenotype, have been previously reported. In the present study, 60% of patients with shorter CAG repeats (<47) showed leg tremor and none of the patients with longer CAG repeats (≥47) did. Postural leg tremor could be a clinical feature that predicts shorter CAG repeats of the androgen receptor gene.