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A 28 year old male presented to our neurology service with stiffness of a few years’ duration in all muscles. The stiffness was maximum at the onset of exercise and his muscles loosened up after some time. He had no difficulty in releasing his grip after holding an object. He had very well developed muscles in spite of a sedentary lifestyle. There was no family history of similar problems. On examination he showed percussion myotonia. There was no grip myotonia or eyelid myotonia. Nerve conduction studies were normal. Electromyogram showed myotonic potentials with a characteristic dive bomber sound on the loudspeaker. A diagnosis of myotonia congenita was made. He most likely had the recessive form of myotonia congenita because both parents were unaffected clinically. He was started on Mexiletene 200 mg three times daily with resultant improvement in stiffness.
A dramatic increase in the bulk and definition of voluntary muscles is common in patients with myotonia congenita, presumably because the skeletal muscles are in an almost continuous state of muscle contraction. In most myotonias, repetitive contractions reduce muscle stiffness except for paramyotonia, where there is a paradoxical increase in stiffness with repetitive exercise. Myotonia, defined as delayed relaxation of muscle after contraction, is seen in a group of genetic disorders that includes autosomal dominant myotonia congenita (Thomsen’s disease) and autosomal recessive myotonia congenita (Becker’s disease). Both diseases are caused by mutations in the skeletal muscle voltage gated chloride channel gene.1 Becker’s disease resembles Thomsen’s disease but there are also differences, eg later onset and transient muscle weakness experienced by many patients during muscle exertion after rest. The criterion for the differential diagnosis of Thomsen or Becker type myotonia is the mode of inheritance.2
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