The detection of carriers of benign (Becker-type) X-linked muscular dystrophy.

The use of serum creatine kinase (SCK) estimations in the detection of female carriers of the severe (Duchenne-type) X-linked muscular dystrophy was first introduced by Okinaka and his colleagues in 1959, and since then the value of this test has been confirmed by many others (Dreyfus and Schapira, 1961; Hughes, 1963; Richterich, et al, 1963; Emery, 1965; Rotthauwe and Kowalewski, 1965; Wilson, Evans, and Carter, 1965; Dreyfus et al, 1966; Thompson, Murphy, and McAlpine, 1967). Approximately two-thirds of women who carry the gene for Duchenne muscular dystrophy have been found to have significantly elevated levels of SCK and this is now accepted as the most reliable single test for carrier detection (Emery, 1969b). Becker was the first to report the existence of a benign form of X-linked muscular dystrophy which was distinct from the well-recognized Duchenne type (Becker and Kiener, 1955; Becker, 1957, 1962). This form of muscular dystrophy may be defined as a comparatively benign proximal myopathy affecting several males in at least two generations of a family, the pattern of inheritance being consistent with that of an X-linked recessive trait. Although clinically similar in presentation to the Duchenne type, with pseudohypertrophy of the calves and weakness affecting first the pelvic girdle and then the pectoral girdle musculature, the Becker type is of later onset and has a more benign course. Affected individuals usually become chair-ridden only after about 20 years and most survive at least into middle age. As in the Duchenne type affected males with the Becker type of muscular dystrophy have grossly elevated levels of SCK in the precinical and early stages of the disease (Emery, 1968). The test has also been used as a method for detecting female carriers of Becker muscular dystrophy by a number of investigators with varying degrees of success. From reports in the past it would appear that approximately 50% ofknown carriers of the gene for Becker muscular dystrophy have a raised level of SCK (Emery et al, 1967). The present communication is to report the results of a study of an extended series of Becker muscular dystrophy carriers, in which the SCK was estimated by the more recent and sensitive assay described by Rosalki (1967). How the SCK level and pedigree data may be used to calculate probabilities for genetic counselling purposes is also discussed.