The syndrome of infantile spinal muscular atrophy (Werdnig-Hoffmann disease) is the leading example of flaccid paralysis of lower motor neuron type. Perceptive mothers may be aware of a paucity of fetal movements in utero; in most cases the motor defect becomes evident soon after birth or the infant is born with arthrogrypotic deformities. Several other types of familial progressive muscular atrophies have been described in which the onset is in early or late childhood, adolescence, or early adult life. Weakness, atrophy, and reflex loss without sensory change are the main features and are discussed in detail in Chap. 38. A few patients suspected of having infantile or childhood muscular atrophy prove, with the passage of time, to be merely inactive “slack” children, whose motor development has proceeded at a slower rate than normal. Others may remain weak throughout life, with thin musculature. These and several other congenital myopathies—central core, rod-body, nemaline, mitochondrial, myotubular, and fiber-type disproportion and predominance—are described in Chap. 45. Unlike Werdnig-Hoffmann disease, the effects of many of them tend to diminish as the natural growth of muscle proceeds. Rarely, polymyositis and acute idiopathic polyneuritis manifest themselves as a syndrome of congenital hypotonia.
Infantile muscular dystrophy and lipid and glycogen storage diseases may also produce a clinical picture of progressive atrophy and weakness of muscles. The diagnosis of glycogen storage disease (usually the Pompe form) should be suspected when progressive muscular atrophy is associated with enlargement of the tongue, heart, liver, or spleen. The motor disturbance in this condition may be related in some way to the abnormal deposits of glycogen in skeletal muscles, although it is more likely the result of degeneration of anterior horn cells that are also distended with glycogen and other substances. Certain forms of muscular dystrophy (myotonic dystrophy and several types of congenital dystrophy) may also be evident at birth or soon thereafter. The latter may have led to arthrogryposis and clubfoot (see Chap. 45 for discussion of the congenital neuromuscular disorders).
Brachial plexus palsies, well-known complications of dystocia, usually result from forcible extraction of the fetus by traction on the shoulder in a breech presentation or from traction and tipping of the head in a shoulder presentation. The effects of such injuries are sometimes lifelong. Their neonatal onset is betrayed later by the small size and inadequate osseous development of the affected limb. Either the upper brachial plexus (fifth and sixth cervical roots) or the lower brachial plexus (seventh and eighth cervical and first thoracic roots) suffer the brunt of the injury. Upper plexus injuries (Erb palsy) are about 20 times more frequent than lower ones (Klumpke palsy). Sometimes the entire plexus is involved. Further details are found in Chap. 43.
Facial paralysis, because of forceps injury to the facial nerve immediately distal to its exit from the stylomastoid foramen, is another common (usually unilateral) peripheral nerve affection in the newborn. Failure of one eye to close and difficulty in sucking make this condition easy to recognize. It must be distinguished from the congenital facial diplegia that is often associated with abducens palsy, that is, the Möbius syndrome discussed earlier in the chapter. In most cases of facial paralysis caused by physical injury, function is recovered after a few weeks; in some, the paralysis is permanent and may account for lifelong facial asymmetry.