A myotonic syndrome associated with Klinefelter's syndrome.

Myotonia is a failure of voluntary muscle to relax immediately innervation ceases. The clinical and electrical manifestations reflect a primary abnormality of the muscle cell membrane. Myotonia is commonly observed in three syndromes. (1) Myotonia congenita (Thomsen, 1876), is an inherited condition, transmitted by a dominant gene. The myotonia affects all muscles which are often hypertrophied. (2) Dystrophia myotonica (Steinert, 1909; Caughey and Myrianthopoulos, 1963) is the most common of the three syndromes. The myotonia is most easily demonstrated in the forearm, tongue, and small muscles of the hand. Other features include cataract, frontal baldness (in the male), gonadal atrophy, mental retardation, weakness, and wasting of the facial muscles and sternomastoids, and a progressive muscular dystrophy affecting distal limb muscles. The disease is transmitted by an autosomal dominant gene (Klein, 1961). (3) A type of familial periodic paralysis associated with normal or high levels of serum potassium was described by Gamstorp (1956) and called 'adynamia episodica hereditaria'; these patients showed no myotonia. However, a further group of patients who have myotonia and episodic paralysis, often precipitated by the cold, have in the past been described as having paramyotonia congenita (Eulenburg, 1886). Some of these patients on investigation have proved to be examples of familial myotonic hyperkalaemic periodic paralysis (van't Hoff, 1962). A number of patients remain in whom no abnormality of potassium metabolism can be demonstrated, yet who have attacks of cold sensitive myotonia. Magee (1963, 1966) claims that these are examples of paramyotonia congenita. He suggests that this is a distinct entity, with a dominant inheritance in which both cold and repetitive muscle