Congenital absence of muscle is usually associated with congenital anomalies of neighboring nonmuscular tissues. For example, congenital absence of the pectoral muscle is accompanied by aplasia or hypoplasia of the mammary structures or of syndactyly and microdactyly. Agenesis of the pectoral muscle may also be associated with scoliosis, webbed fingers, and underdevelopment of the ipsilateral arm and hand (Poland syndrome). Another unusual constellation consists of congenital absence of portions of the abdominal muscles (“prune belly”), often in association with arthrogryposis and a defect of ureters, bladder, and genital organs. Amyoplasia also occurs in a few cases of facioscapulohumeral dystrophy.
In another group of restricted palsies, the essential abnormality is in the CNS (nuclear amyotrophies). One of the most frequent is congenital ptosis as a result of an inborn defect of the innervation of the levator palpebrae muscles. Complete paralysis of all muscles supplied by the oculomotor nerve, apparently a result of hypoplasia of the third nerve nuclei, may be observed in several members of a family and occasionally in only one member. Bilateral abducens palsy is often associated with bifacial palsy in the newborn and is known as the Möbius syndrome; this usually nonfamilial anomaly, the cause of which is thought to be a nuclear hypoplasia or aplasia, is discussed with the developmental disorders in Chap. 37. In these familial nuclear amyotrophies, the muscles develop independently of the nervous system but have no prospect of surviving because of failure of innervation. It is, therefore, a kind of congenital denervation hypotrophy. Of course, a primary dystrophy may also give rise to bifacial weakness, as in FSHD.
Beginning in 1956, with the account by Shy and Magee of a patient whose muscle fibers showed a peculiar central densification of sarcoplasm (“cores”), a new class of hereditary diseases of muscle was delineated. The more common and better-defined members of this group are the central core, nemaline (rod-body), myotubular, and centronuclear myopathies. A variety of other types of congenital myopathy have been described, but they are relatively uncommon and some are of dubious specificity; they are mentioned only briefly. As the names imply, in each of these diseases there is no loss of muscle fibers, but within each fiber there is a distinctive morphologic abnormality. These processes usually express themselves early in life by a lack of muscle bulk, hypotonia, weakness of the limbs, and often, with additional but subtle dysmorphic features of other parts of the body.
Further study has revealed that the diseases of this group are not confined to infancy and early childhood and some of them, especially those present at birth, are not as benign as their early descriptions implied. Each of the entities mentioned earlier has been observed at a later age, even in middle adult life. Indeed, if the disease is mild, there is often no way of deciding whether it has been present since birth.
The characteristic feature of most of these myopathies, in addition to usual early onset, is lack of progression or extremely slow progression, in contrast to the more rapid pace of many muscular dystrophies, Werdnig-Hoffmann disease, and of other forms of hereditary motor system disease of childhood and adolescence. Exceptionally, an example of more rapid progression of a congenital myopathy has been reported, and prior to the use of histochemical and electron microscopic techniques such patients were usually considered to have a “benign muscular dystrophy.” Familial occurrence has also been established in some types, so the clinical line of separation between this group of diseases and some of the more slowly progressive muscular dystrophies may in certain cases remain ambiguous. There is no specific treatment for any of the congenital myopathies.