Spinal muscular atrophy (SMA) is a collective term that encompasses a range of genetic neuromuscular disorders that involve the motor neurons in the brain stem and spinal cord. It represents one of the most common autosomal recessive disorders, appearing with an incidence of one in 6,000 to 10,000 and a carrier frequency of one in 40 to 60. The rate incidence is similar in various populations.
The majority of these disorders are caused by mutations in the survival motor neuron (SMN) gene mapped to chromosome 5q13. Humans possess two nearly identical copies of the gene, labeled SMN1 and SMN2. Approximately 95 percent of individuals with spinal muscular atrophy possess a deletion in SMN1. The SMN gene is linked to the production of a protein crucial to the function of motor neurons.
Because the disorder is autosomal recessive, both parents must be carriers of the mutated gene in order to pass it to their children.
SMA is characterized by the loss of lower motor neurons in the anterior horn cells of the spinal cord, inducing the atrophy of proximal muscles in the limbs and trunk. The impact to muscles closest to the trunk of one’s body, which includes the shoulders, hips, and back, is most severe. Patients usually experience more weakness in the legs than in the arms. This weakness makes it difficult to perform such daily activities as sitting, standing, and walking. The disorder has also been implicated in a variety of lung problems, as certain respiratory muscles can also be affected.
As the disease is degenerative, patients lose function progressively, ultimately leading to paralysis. However, muscle power often remains stabilized while only functional abilities decline. The patient’s deteriorating condition is often explained by the fact that muscles experience greater demand from the body as the patient grows in size and/or weight.
With differing clinical and genetic factors, the disorders are classified into four main types based on the age of onset and severity. The four types of spinal muscular atrophy are acute, intermediate, mild, and adult—also recognized as types I, II, III, and IV, respectively. Types I to III occur during childhood, while type IV is adult onset.
Type I SMA, also known as Werdnig-Hoffman disease, is described as onset within the first six months of a child’s life. Some cases even indicate decreased movement during the final months of pregnancy. Children with this disorder experience abnormal muscle weakness, leading to the incapability for head control, walking, and sitting without assistance. Many children also have difficulty feeding and swallowing. Respiratory problems are also prevalent because the lung muscles never develop, and the child typically engages in diaphragmatic breathing. The average life span for a child with the disorder is eight months.
Type II SMA is less severe, with children able to sit unaided. However, patients still suffer respiratory problems, floppiness of the limbs, and very rarely gain the ability to stand. These children are almost always diagnosed within reaching 2 years of age. Life expectancy is variable.
Patients with type III SMA, also known as Kugelberg-Welander disease and juvenile spinal muscular atrophy, varies in age of onset between age 3 to adolescence. These children can stand and walk alone, although in an abnormal manner. Many lose the ability to walk later on, typically linked to growth spurts or illness. These patients also possess an increased risk of scoliosis and obesity.
Some clinicians also designate a type IV SMA, referring to adult onset SMA. It is the least common form. Muscle atrophy typically begins between the ages of 18 and 30, while patients do not experience symptoms until the age of 30. Respiratory muscles are rarely affected in type IV SMA.
SMA is diagnosed through a blood test that examines the SMN1 gene in addition to clinical findings. Older methods such as muscle biopsies and electrophysiologic techniques are rarely used today, although they are occasionally utilized in cases where the blood test is negative.
Because there is no cure for spinal muscular atrophy, patients receive symptomatic and supportive treatment. Physical and occupational therapy, orthotic supports, and rehabilitation are normally used to expand the patient’s physical abilities. Children with trouble swallowing are sometimes given feeding tubes. Those with trouble breathing can be put on respirators or ventilators, although this sort of treatment is rare in the United States because of the high burden of care and low quality of life. Both the quality and duration of life are factored into treatment for children with type I and type II SMA.
Genetic Disorders; Muscle Disorders.
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