Proximal spinal muscular atrophies are a group of neuromuscular disorders characterized by progressive muscle weakness resulting from the degeneration and loss of the lower motor neurons in the spinal cord and the brain stem nuclei. Prevalence is estimated at around 1/30,000. Four subtypes have been defined according to the age of onset and severity of the disease: type 1 (SMA1), the most severe form, with onset before six months of age; type 2 (SMA2), with onset between 6 and 18 months of age, type 3 (SMA3), with onset between childhood and adolescence, and type 4 (SMA4), the least severe form, with adult onset (see these terms). All types are characterized by muscle weakness and atrophy of varying severity, particularly affecting the lower limbs and respiratory muscles. The weakness is almost always symmetric and progressive. Scoliosis, muscle retractions, and joint contractures may occur. Constipation and gastroesophageal reflux are frequent. Around 95% of cases of SMA are caused by homozygous deletions (either of exon 7, or of exons 7 and 8) in the SMN1 gene (5q12.2-q13.3) encoding the SMN (survival motor neuron) protein. A second SMN gene (SMN2; 5q13.2) has also been identified and contributes to the production of only 10% of the full-length SMN protein. However, although there is some variation, disease severity in SMA is inversely correlated with the number of copies of the SMN2 gene, with patients with three or four copies more frequently manifesting SMA3/4, rather than SMA1. Deletions of the NAIP (5q13.1) gene have also been identified and may play a role in modifying disease severity. Transmission of SMN1 and NAIP deletions is autosomal recessive. Around 2% of cases are caused by de novo mutations. Diagnosis is based on clinical history and examination and can be confirmed by genetic testing. Electromyography and muscle biopsy may also be performed. Differential diagnoses include amyotrophic lateral sclerosis, congenital muscular dystrophies, congenital myopathies, primary lateral sclerosis, myasthenia gravis, and carbohydrate metabolism disorders (see these terms). Antenatal diagnosis is feasible through molecular analysis of amniocytes or chorionic villus samples. Genetic counseling should be offered to patients and their families. Clinical trials are ongoing to identify potential drug treatments for SMA, mainly targeted towards increasing the levels of the full length SMN protein. However, at present, management remains symptomatic, involving a multidisciplinary approach that aims to improve quality of life. Physiotherapy and occupational and respiratory therapies are necessary. Noninvasive ventilation and gastrostomy may be required. Antibiotic therapy is used in case of pulmonary infection. The scoliosis and joint manifestations may require surgical correction. Patients may require a wheelchair, or use a corset/back brace for support. The prognosis depends on the severity of the disease, which generally correlates with the age of onset: earlier-onset forms are generally associated with a poor prognosis, whereas life expectancy may be close to normal in later-onset forms. Death may occur due to respiratory insufficiency and infections.
Last update: July 2009