Finally, heart block, atrial and ventricular tachycardias, unexplained cardiomyopathy with heart failure, and sudden death are well known in the disorder and require repeated EKG evaluation. Insertion of a pacemaker has become common.
We have been impressed with the variability of clinical expression. In many patients, intelligence has been unimpaired and the myotonia and muscle weakness have been so mild that the patients were unaware of any difficulty. Pryse-Philips and associates emphasized these features in their description of a large Labrador kinship in which 27 of 133 patients had only a partial syndrome and only minor muscle symptoms at the time of examination.
In addition to displaying most of the common findings of muscular dystrophy, there are several highly unusual myopathologic features. Peripherally placed sarcoplasmic masses and circular bundles of myofibrils (ringbinden) are found. There is hypertrophy of type 1 fibers with centrally placed nuclei (this may be a marked finding) and many atrophic fibers show nuclear clumping. In many of the muscle spindles there is an excess of intrafusal fibers (particularly in the congenital form; see later). Many of the terminal arborizations of the peripheral nerves are unusually elaborate and elongated.
Brief reference was made earlier to this inherited, distinctive, and potentially lethal form of myotonic dystrophy. Harper’s (1975) study of 70 personally observed patients and 56 others gathered from the medical literature suggests this disease exists in every pediatric neurology service. Profound hypotonia and facial diplegia at birth are the most prominent clinical features; myotonia is notably absent. Drooping of the eyelids, the tented upper lip (“carp” mouth), and the open jaw impart a characteristic appearance, which allows immediate recognition of the disease in the newborn infant and child. Difficulty in sucking and swallowing, bronchial aspiration (because of palatal and pharyngeal weakness), and respiratory distress (because of diaphragmatic and intercostal weakness and pulmonary immaturity) are present in varying degrees; the latter disorders are responsible for a previously unrecognized group of neonatal deaths (24 such deaths among siblings of affected patients in Harper’s study). In surviving infants, delayed motor and speech development, swallowing difficulty, mild to moderately severe mental retardation, and talipes or generalized arthrogryposis are common. Once adolescence is attained, the disease follows the same course as the later form. As stated earlier, clinical myotonia in the congenital form of the disease becomes evident only later in childhood, although EMG study may disclose myotonic discharges in early infancy. The diagnosis may be suspected by the simple test of eliciting myotonia in the mother. ECG changes occur in one-third of the patients.
In the congenital form of this disease the affected parent is always the mother with type 1 (DM1) myotonic dystrophy, in whom the disease need not be severe. Electrophysiologic testing will bring out the myotonia in the mother if it is inevident on percussion of muscle. (In cases of adult onset, transmission is maternal or paternal.) These data suggest that in addition to inheriting the myotonic dystrophy gene, the congenital cases also receive some maternally transmitted factor, possibly methylation of DNA that allows expansion of the trinucleotide repeat in oocytes. The prenatal diagnosis of myotonic dystrophy is readily accomplished by examination for CTG repeats in the amniotic fluid or in a biopsy of chorionic villi. However, it is not possible to predict whether a fetus with an expanded mutation will have congenital myotonic dystrophy or later-onset myotonic dystrophy.
Proximal Myotonic Myopathy (DM2, PROMM,