This case report highlights that in autosomal recessive myotonia congenita, marked muscle weakness can correlate with a decline in compound muscle action potential during exercise and repetitive stimulation.
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CMAP decline may mark weakness in recessive myotonia congenita; leaves open generalizability beyond this single case.
A 16-year-old boy had a 10-year history of stiffness in leg muscles. There was marked weakness of neck flexors, shoulder abductors, and ankle dorsiflexors, with hypertrophy of most muscle groups and both action and percussion myotonia. The parents were normal. Motor unit potential mean duration was reduced in the weakest muscle (tibialis anterior), and a biopsy of the same muscle showed only minimal abnormalities. Exercise and repetitive stimulation (30 Hz) of the tibialis anterior disclosed a decline in the compound muscle action potential that appeared to correlate with the muscular weakness.
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Miller et al. (1992) studied this question.
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