From time to time in alcoholics one observes the subacute or chronic evolution of painless weakness and atrophy of the proximal muscles of the limbs, especially of the legs, with only minimal signs in the distal segments of the legs and feet. Cases such as these have been referred to as chronic alcoholic myopathy, implying a direct toxic effect of alcohol on muscle, but the data are insufficient to warrant such an assumption. Some of these cases have shown necrosis of individual muscle fibers and other signs of polymyositis; most cases seen by the authors have proved to be neuropathic in nature. This has been the experience of others as well (Faris and Reyes). Treatment follows along the lines indicated for nutritional-alcoholic neuropathy, and complete recovery can be expected if the patient abstains from alcohol and maintains adequate nutrition.
Included under this section are two sizable groups of muscle diseases: one is an assemblage of congenital deformities that involve muscle and the other is a unique class of congenital myopathies. Insofar as all of the disorders comprising these categories are congenital, it may be helpful by way of introduction to refer briefly the main facts about the natural development and aging of muscle in the introductory section of this chapter. These diseases are of particular importance in pediatric neurology, for most of them attract notice at an early age.
Congenital Deformities of Muscle
This disorder of multiple congenital contractures, now referred to as arthrogryposis (literally, curved joints), has been estimated to occur once in 3,000 births. The deformities result from a lack of movement during fetal development and are therefore produced by any disorder that immobilizes the developing embryo, whether from a lack of anterior horn cells, peripheral nerves, the motor end plate (as in an infant born to a myasthenic mother), or diseases of muscle. Often, there are associated developmental defects of the nervous system and somatic structures, low-set ears, wide and flat nose, micrognathia, and high-arched palate; less often, there are short neck, congenital heart disease, hypoplasia of the lungs, and cryptorchidism.
Of the many conditions that underlie arthrogryposis, developmental abnormalities of the anterior horn cells, mainly Werdnig-Hoffmann disease as discussed in
Chap. 38, is by far the most common. A failure in development of anterior horn cells results in an uneven smallness and paresis of limb muscles. The unopposed contraction of relatively normally innervated muscles sets the fixed deformities. In a less common group of myopathic causes of arthrogryposis, the nervous system is usually intact and the disease is that of a congenital myopathy or congenital dystrophy. It has been observed that in the myopathic variety the limbs are fixed in a position of flexion at the hips and knees and adduction of the legs, in contrast to the variable postures of the myelopathic (anterior horn cell) form. In addition to these two well-recognized causes of arthrogryposis, occasional cases are attributable to a neonatal neuropathy, neonatal myasthenia gravis, or to the Prader-Willi syndrome (causing intrauterine hypotonia).
An infant with arthrogryposis should be evaluated with an EMG that is interpreted by an experienced electromyographer, and by a biopsy of muscle for the detection of group atrophy and of the congenital myopathies described further on in this chapter. Both these tests are difficult to interpret in the incompletely formed nervous system of the premature infant. In many circumstances, it is valuable to delay the tests until several weeks of postterm development when the results are usually clearer. Sometimes, the electrophysiologic and biopsy tests may have to be repeated after several weeks or more to give a definitive diagnosis. If the initial evaluations are unrevealing, an imaging study of the brain to detect cerebral malformations (and sequencing of the PWCR locus chromosome 5 for Prader-Willi syndrome) may prove useful.