Overuse weakens CMT muscles

Abstract of the 9th Annual Symposium of the European Charcot-Marie-Tooth Consortium, held in Antwerpen, June 30-July 1, 2000

Vinci P, Perelli SL, Colazza GB

Abstract

The so-called overwork weakness is an overuse syndrome in which muscle fibers are damaged through exercise, so that the muscles, which undergo overload either functional or after exercise, increase in weakness.

First observed in post-polio patients, it was also reported in muscular dystrophies and amyotrophic lateral sclerosis.

We tested three intrinsic muscles in the hands of fifty-one patients affected with Charcot-Marie-Tooth (CMT) disease type 1 (demyelinating form) and type 2 (axonal form), in order to see if the functional overloading physiologically acting on the dominant side would cause reduction of strength rather than strengthening, as happens in normal subjects.

We found out that 99.3 percent of the 153 couples of muscles tested were weaker or not stronger on the dominant side and that the average difference of strength for each muscle, measured on the scale MRC, was 0.56 (0.59 in CMT1 and 0.51 in CMT2).

This phenomenon seems to be unrelated to the time of duration of the disease, as it is present since the moment the disease starts to affect the hands.

A possible explanation is that overloading exercised in that moment of vulnerability could cause degeneration of axons that otherwise would be spared by the disease.

Later in life, if the loss of nerve fibers is severe, a functional longstanding overload can hasten the physiological aging of the remaining fibers that had taken part in axonal sprouting with degeneration of the axon or of some sprouts and consequent appearance of further weakening.

*) The MRC Scale is an instrument to measure the strength of muscles. It was developed by the Medical Research Council in Britain, and its values count from 0 = no movement to 5 = normal power. – So the findings of this test with average differences from 0.51 to 0.59 can be interpreted as a significant loss of muscle strength of ca. 10 to 12%, which is, according to the explanation of the authors, caused by a degeneration of axons and therefore cannot be regenerated: Every overuse deprives cmt'ers of one more part of their muscle strength, and it is lost forever!