We studied the percentage change in compound muscle action potential (CMAP)
amplitude and area during and after a 5-min maximal contraction of the mus
cle. The exercise test (ET) was performed on 64 patients with different mus
cle disorders and on 46 normal controls. The range of normal ET values was
defined as the mean + 2 SD of the control values. The mean sensitivity of t
he test was 63% in the whole group with ion channel muscle disorders, the h
ighest sensitivity being seen in primary periodic paralysis (81%) and the l
owest in chloride channelopathies (17%). In thyrotoxic periodic paralysis,
the ET was abnormal in the three of the four patients studied. in patients
with myotonic dystrophy, a smaller than normal increase in CMAP amplitude o
ccurred during and after exercise, whereas in proximal myotonic myopathy a
normal initial increase in CMAP amplitude was followed by an abnormal decre
ment. We conclude that the ET can be of use in confirming abnormal muscle m
embrane excitability in patients with calcium and sodium channelopathies an
d thyrotoxic periodic paralysis. In chloride channelopathy, the test may al
so be abnormal, but shows no, or only a small, increase in amplitude or are
a in the immediate postexercise period. The test may also be abnormal in pr
oximal myotonic myopathy, but is normal in myotonic dystrophy. (C) 2000 Joh
n Wiley & Sons, Inc.