The phenomenon of myotonia, which expresses itself in prolonged idiomuscular contraction following brief percussion or electrical stimulation and in delay of relaxation after strong voluntary contraction, is the third striking attribute of the disease (the other two being the facial, ptotic, and limb weakness, and the cardiac-autoimmune features). Not as widespread or severe as in myotonia congenita, it is, nonetheless, easily elicited in the hands and tongue in almost all cases, and in the proximal limb muscles in half of the cases. Gentle movements do not evoke it (eye blinks, movements of facial expression, and the like are not impeded), whereas strong closure of the lids and clenching of the fist are followed by a long delay in relaxation.
Myotonia may precede weakness by several years. Indeed, Maas and Paterson have claimed that many cases diagnosed originally as myotonia congenita eventually proved to be examples of myotonic dystrophy. Of interest is the fact that in congenital or infantile cases of myotonic dystrophy, the myotonic phenomenon is not elicited until later in childhood, after the second or third year of life (see later). The child often becomes accustomed to the myotonia and does not complain about it. The relation of myotonia to the dystrophy is not direct. Certain muscles that show the myotonia best (tongue, flexors, of fingers) are seldom weak and atrophic. Moreover, there may be little or no myotonia in certain families that show the other characteristic features of myotonic dystrophy. The muscle hypertrophy that is characteristic of myotonia congenita is not a feature of myotonic dystrophy.
The fourth major characteristic of the disease is the dystrophic change in nonmuscular tissues. The most common of these is lenticular opacities, found by slit-lamp examination in 90 percent of patients. At first dust-like, they then form small, regular opacities in the posterior and anterior cortex of the lens just beneath the capsule; under the slit lamp they appear blue, blue-green, and yellow, and are highly refractile. Microscopically, the crystalline material (probably lipids and cholesterol, which cause the iridescence) lies in vacuoles and lacunae among the lens fibers. In older patients, a stellate cataract slowly forms in the posterior cortex of the lens.
Mild to moderate degrees of developmental cognitive delay are common in DM1, and the brain weight in several of our patients was 200 g less than that in normal individuals of the same age. Late in adult life, some patients become suspicious, argumentative, and forgetful. In some families, a hereditary sensorimotor neuropathy may be added to the muscle disease (Cros et al). Other nonspecific abnormalities, such as hyperostosis of the frontal bones and calcification of the basal ganglia, both readily discerned by CT, seem to be more common in patients with myotonic dystrophy than they are in healthy persons.
Progressive frontal alopecia, beginning at an early age, is a characteristic feature in both men and women with this disease. Testicular atrophy with androgenic deficiency, reduced libido or impotence, and sterility are additional frequent manifestations. In some patients gynecomastia and elevated gonadotropin excretion are found. Testicular biopsy shows atrophy and hyalinization of tubular cells and hyperplasia of Leydig cells. (Thus all the clinical characteristics of the Klinefelter syndrome may be present but without the “sex chromatin” mass [Barr body].) Ovarian deficiency occasionally develops in the female patient, but it is seldom severe enough to interfere with menstruation or fertility. The prevalence of clinical or chemical diabetes mellitus is slightly increased in patients with myotonic dystrophy, but an increased insulin response to a glucose load has proved to be a common abnormality. Numerous surveys of other endocrine functions have yielded little of significance.