History The disorder was first brought to the attention of the medical profession in 1876 by Julius Thomsen, a Danish physician who himself suffered from the disease, as did 20 members of his family over 4 generations. His designation of ataxia muscularis was not correct, but his description left no doubt as to the nature of the condition in that it featured “tonic cramps in voluntary muscles associated with an inherited psychical indisposition.” The latter aspect of the condition was not borne out by subsequent studies and is now thought to be his erroneous speculation as to causality. In 1881, Strümpell assigned the name myotonia congenita to the disease and in 1883, Westphal referred to it as Thomsen’s disease. Erb provided the first description of its pathology and called attention to 2 additional unique features: muscular hyperexcitability and hypertrophy. In 1923, Nissen, Thomsen’s great-nephew, extended the original genealogy to 35 cases in 7 generations, and in 1948, Thomasen’s monograph is still a useful clinical reference.
Myotonia, a tonic spasm of muscle after forceful voluntary contraction, stands as the cardinal feature and is best represented in this disease. As emphasized in Chaps. 2 and 45, this phenomenon reflects electrical hyperexcitability of the muscle membrane. The electrophysiologic representation of the spasm is shown in Fig. 2-19 and a description of the physiology can be found in Chap. 2. It is most pronounced after a period of inactivity. Repeated contractions “wear it out,” so to speak, and the later movements in a series become more swift and effective. Rarely, the converse is observed—where only the later movements of a series induce myotonia (myotonia paradoxica); usually this is a feature of another condition, cold-induced paramyotonia congenita (see further on). Unlike cramp, the myotonic spasm is painless but after prolonged activity, nocturnal myalgia (a pinching-aching sensation in the overactive muscles) may develop and prove distressing. Close observation reveals a softness of the muscles during rest and the initial contraction appears not to be significantly slowed.
The disease, myotonia congenital, as mentioned above is usually inherited as a dominant trait so that most often, other members of the family have been affected.
Its congenital nature may be evident even in the crib, where the infant’s eyes are noted to open slowly after it has been crying or sneezing and its legs are conspicuously stiff as the child tries to take its first steps. In other cases, the myotonia becomes evident only later in the first or second decade. The muscles are generously proportioned and may become hypertrophied but seldom to the degree observed in the recessive form of the disease described further on.