Postural tremor in X‐linked spinal and bulbar muscular atrophy

Abstract

Postural tremor is a common initial symptom in spinal and bulbar muscular atrophy (SBMA), but its pathophysiological mechanisms remain to be studied. This study was undertaken to examine the physiological mechanisms underlying postural tremor in SBMA. For eight patients (36-63 years old) with genetically confirmed SBMA, we recorded surface electromyograms (EMGs) from the forearm muscles and hand movements with an accelerometer (ACC) while maintaining a posture with and without a weight load. We then analyzed their power spectra and coherence. The peak tremor frequency was 6-9 Hz in seven patients and 2-3 Hz in one patient. Oscillatory movements were associated with EMG activity in five patients, but not in three patients. Weight loads and postural changes affected the tremor frequency in all patients. Tremor was classified as "reflex tremor" in five patients and "mechanical tremor" in three patients. These results suggest that peripheral factors play important roles in tremor genesis in SBMA, although its clinical features resemble essential tremor. Subclinical sensory disturbance or a decrease of motor unit numbers might be candidates for such peripheral factors contributing to tremor genesis in SBMA.

Head and Arm Tremor in X-linked Spinal and Bulbar Muscular Atrophy

“…In SBMA, two different mechanisms might contribute to tremor generation: mechanical oscillation and reflex mechanisms 7. In support of this, in SBMA tremor frequency is affected by postural changes and weight loading, suggesting that it might be considered a reflex tremor, mediated through reflex loops between peripheral nerves and the central nervous system, similar to the stretch reflex 7. In addition, the presence of sensory neuropathy in some patients might also contribute to reflex tremor 2.…”

“…Previous reports in the literature reveal that tremor is a common feature in patients with SBMA, and that while the tremor mimics that of essential tremor, the pathophysiology and degree of responsiveness to alcohol, propanolol, and other drugs such as gabapentin is different 2,7. In essential tremor, which is considered to be generated by a central mechanism, the movement is always associated with electromyography (EMG) grouping activities at the same frequency, and it is resistant to external perturbation, such as weight loading 7.…”

“…In essential tremor, which is considered to be generated by a central mechanism, the movement is always associated with electromyography (EMG) grouping activities at the same frequency, and it is resistant to external perturbation, such as weight loading 7. In SBMA, two different mechanisms might contribute to tremor generation: mechanical oscillation and reflex mechanisms 7.…”

“…In essential tremor, which is considered to be generated by a central mechanism, the movement is always associated with electromyography (EMG) grouping activities at the same frequency, and it is resistant to external perturbation, such as weight loading 7. In SBMA, two different mechanisms might contribute to tremor generation: mechanical oscillation and reflex mechanisms 7. In support of this, in SBMA tremor frequency is affected by postural changes and weight loading, suggesting that it might be considered a reflex tremor, mediated through reflex loops between peripheral nerves and the central nervous system, similar to the stretch reflex 7.…”

“…In addition, the presence of sensory neuropathy in some patients might also contribute to reflex tremor 2. On the other hand, because motor units are markedly decreased in SBMA, the muscle spindle feedback might also play an important role in the generation of tremor due to motor unit synchronization 7. Motor symptoms have been attributed to the accumulation of mutant androgen receptors in the nucleus of lower motor neurons, which is more profound in patients with a longer CAG repeat.…”