This term refers to a fixation of limb posture as a result of a developmental lack or destruction of muscles, with shortening and fibrosis of supporting tissue and ligaments. A surprising number of deformities in infants and children are traceable to this type of defect. The most common are congenital clubfoot (talipes), congenital torticollis (wryneck), congenital elevation of the scapula (Sprengel deformity), and congenital dislocation of the hips. In all these conditions, the postural distortion is produced and maintained either by a weakened, fibrotic muscle or by a normal one that is contracted and shortened because of the absence of a countervailing antagonist. Trauma to a muscle during intrauterine life or at birth may lead to fibrosis and to fibrous contracture in some cases.
Congenital clubfoot Here the deformity may be one of plantar flexion of the foot and ankle (talipes equinus), inversion (talipes varus or clubfoot), eversion (talipes valgus or splayfoot), or dorsiflexion of foot at the ankle (talipes calcaneus). Approximately 75 percent of cases are of the equinovarus clubfoot variety (i.e., the foot turns downward and inward). Usually both feet are affected. Multiple incidences may occur in one family. Several explanations of cause and pathogenesis have been offered: fetal malposition, an embryonic abnormality of tarsal and metatarsal bones, a primary defect in nerves or anterior horn cells of the spinal cord, or a congenital dystrophy of muscle. No one theory explains all cases, and available pathologic data exclude a single cause and pathogenesis. In some instances, clubfoot is the only recognizable congenital abnormality, but as often it occurs as a manifestation of generalized arthrogryposis (see later) and is an indicator of a more widespread intrauterine involvement of the CNS. (See Kakulas and Adams and also Banker for pertinent literature on the subject.)
Congenital torticollis (Wryneck) This disorder begins during the first months of life and, unlike the torticollis of adults discussed in Chap. 4, it is not the result of a dystonia but instead is caused by congenital shortening of the sternocleidomastoid muscle, which is firm and taut. The head is inclined to one side and the occiput slightly rotated to the side of the affected muscle. This disorder is nonfamilial and is ascribed to injury to the sternocleidomastoid at birth. Whether the injury is a purely mechanical one to the muscle itself or is caused by ischemia stemming from arterial or venous occlusion (or an entirely different cause) is not clear. Congenital torticollis often gives rise to a sternocleidomastoid enlargement (a pseudotumor) that appears, on exploration, as a pale, spindle-shaped swelling of the muscle belly. The histologic findings are similar to those of Volkmann contracture, that is, replacement of the muscle fibers by relatively acellular connective tissue, suggesting that an ischemic mechanism underlies the defect in at least some cases.
Congenital Absence of Muscles (Amyoplasia)
It is well known that some individuals are born without certain individual muscles. This pertains not only to certain inconstant and functionally unimportant muscles, such as the palmaris longus, but also to more substantial ones. The muscles found to be absent most frequently are the pectoralis, trapezius, serratus anterior, and quadriceps femoris, but many single muscles may be missing in isolated cases.