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Correspondence |

Motor Nerve Hyperexcitability and Muscle Cramps in Machado-Joseph Disease

Kazuaki Kanai, MD, PhD; Satoshi Kuwabara, MD, PhD
Arch Neurol. 2009;66(1):138-141. doi:10.1001/archneurol.2008.515.
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We read the article by França et al1 with great interest. We thank them for confirming our previous results regarding the high frequency of and disability associated with muscle cramps2 in a larger sample of patients with Machado-Joseph disease (MJD). We would like to raise some concerns regarding the authors' interpretation of the findings.

First, França and colleagues proposed that altered excitatory inputs from the corticospinal fibers rather than the peripheral motor axonal changes might be responsible for muscle cramps in MJD. Muscle cramps and fasciculations arise from spontaneous motor unit activities frequently associated with lower motor neuron disorders, and ectopic firing usually originates from the distal motor axons, especially the intramuscular nerve terminals.3 We have shown that muscle cramp severity correlates with an increased index of peripheral axonal excitability, suggesting that cramps in MJD are associated with underlying peripheral axonal loss and resulting collateral sprouting by the surviving neurons. We consider muscle cramp a symptom of motor nerve sprouting but not of neuronal degeneration. In our experience, disabling muscle cramps in patients with MJD are most prominent during the early stages of the disease and then gradually disappear with the progression of amyotrophy.2 This phenomenon would possibly be associated with the high ability of sprouting in the early stages of disease. Therefore, the subgroup of patients without cramps might include patients in the early and advanced stages of MJD. We believe that this fact accounts for the seemingly insignificant difference in the neurophysiological parameters between groups with and without cramps.

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