Chronic proximal spinal muscular atrophy

Abstract

Chronic proximal spinal muscular atrophy is one of the motor neuron diseases characterized by weakness and wasting of proximal muscles of the limbs, and a prolonged course of the disease. The manifestations of this disease in a total of 382 reported patients, including the authors' experience of 7 patients, are summarized. The disease has been reported from many parts of the world in various races. The age of onset of the disease has ranged from infancy to 63 years. Infantile onset (age 0–2 years) occurred in 36% of patients; juvenile onset (age 3–18 years) in 49%, and adult onset (age 19 or more) in 15%. The ratio of male to female patients was 6:4, but it was 5:5 in patients with adult onset. In 62% of patients, the disease affected more than one member of a family with various modes of inheritance. Initial symptoms were mostly due to weakness of the pelvic girdle muscles, and many patients had been diagnosed as having muscular dystrophy. Proximal muscle weakness of the legs was found in 98% of patients, and proximal muscle weakness of both arms and legs in 82% of patients. Diminished or absent deep tendon reflexes were described in 77% of patients, particularly the knee jerk, and muscle fasciculations in 50% of patients. Extensor plantar reflex(es) was found in 6% of patients, and ankle clonus in 1 patient. Pseudohypertrophy, mainly of calf muscles, was reported in 15% of patients. In 29% of patients there was weakness of muscles innervated by the cranial nerves. The distribution indicated that the motor portion of any of the cranial nerves may be involved, although life-threatening bulbar symptoms were rare. Sensory and sphincter impairment were exceptional. Because of long-standing weakness of muscles many patients had contractures and deformity of the spine and the limbs. Serum creatine phosphokinase (CPK) activity was increased in 37% of patients and aldolase activity in 21%. Electrophysiologic examination established a diagnosis of neuropathy in 97% of patients. Spontaneous discharges occurred in 47% of patients, including fasciculation potentials in 30%, fibrillation potentials in 29% and positive sharp waves in 7%. Motor unit potentials of increased amplitude and long duration occurred in 90% of patients, polyphasic motor unit potentials in 35%, and incomplete interference patterns in 79%. Conduction velocity in motor nerves was normal in 97% of patients. Muscle biopsy showed neuropathic changes in 68%, both neuropathic and myopathic changes in 19%, myopathic changes alone in 5%, and non-specific changes or normal findings in 13%. Therefore this disease resembles myopathy not only in the distribution of involved muscles, but also in the presence of pseudohypertrophy, elevated serum enzymes, and microscopic findings in skeletal muscle. Grouped atrophic muscle fibers, characteristic of neuropathy, were found in 86% of patients. Electrophysiologic examination was therefore more helpful than muscle biopsy in the diagnosis of this disease. In postmortem examination of 3 patients, degeneration of anterior horn cells, and of motor nuclei of the cranial nerves was present. The median duration of the disease was 12 years. The duration was over 20 years in 23%, and over 10 years in 56% of patients. In 13 deceased patients, the mean duration of the disease was 30 years and the mean age at death was 51 years. Recognition of chronic proximal spinal muscular atrophy as a form of motor neuron disease facilitates understanding of this group of disorders.