Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy definition and facts*

Spinal muscular atrophy facts medically edited by Melissa Conrad Stöppler
  • Spinal muscular atrophy (SMA) is an inherited (hereditary) disease that results in destruction of lower motor neurons, nerve cells in the brain stem and spinal cord that control essential voluntary muscle activity.
  • This genetic disorder is related to defects in a gene known as SMN1.
  • Symptoms of spinal muscular atrophy include problems with all kinds of movements including speaking, walking, breathing, and swallowing as well as movement in the arms, legs, chest, face, throat, and tongue.
  • Spinal muscular atrophy disorders in children are inherited in an autosomal recessive manner, meaning each parent must possess a copy of the defective gene for the child to develop the condition.
  • SMA is classifies into 3 types (types I-III) based on symptoms, age of onset, and severity.
  • Kennedy's disease is an x-linked inherited, adult form of spinal muscular atrophy in which the mother carries the defective gene on one of her X chromosomes and passes the disorder along to her sons.
  • Blood testing is available for diagnosis of SMN1 genetic mutations.
  • Prognosis and life expectancy vary according to the type of spinal muscular atrophy. Some forms of spinal muscular atrophy are fatal.

What is spinal muscular atrophy?

Spinal muscular atrophy (SMA)is one of several hereditary diseases that progressively destroy lower motor neurons-nerve cells in the brain stem and spinal cord that control essential voluntary muscle activity such as speaking, walking, breathing, and swallowing. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue.

When there are disruptions in the signals between lower motor neurons and muscles, the muscles gradually weaken and may begin wasting away and develop uncontrollable twitching (called fasciculations). When there are disruptions in the signals between the upper motor neurons (located in the brain) and the lower motor neurons, the limb muscles develop stiffness (called spasticity), movements become slow and effortful, and tendon reflexes such as knee and ankle jerks become overactive. Over time, the ability to control voluntary movement can be lost.

What causes spinal muscular atrophy?

Spinal muscular atrophy is caused by defects in the gene SMN1, which makes a protein that is important for the survival of motor neurons (SMN protein). In spinal muscular atrophy, insufficient levels of the SMN protein lead to degeneration of the lower motor neurons, producing weakness and wasting of the skeletal muscles. This weakness is often more severe in the trunk and upper leg and arm muscles than in muscles of the hands and feet.

How is spinal muscular atrophy inherited?

Spinal muscular atrophy disorders in children are inherited in anautosomal recessive manner. Autosomal recessive means the child must inherit a copy of the defective gene from both parents. These parents are likely to be asymptomatic (without symptoms of the disease). Autosomal recessive diseases often affect more than one person in the same generation (siblings or cousins).

Kennedy's disease, an adult form of spinal muscular atrophy is X-linked inherited, which means the mother carries the defective gene on one of her X chromosomes and passes the disorder along to her sons. Males inherit an X chromosome from their mother and a Y chromosome from their father, while females inherit an X chromosome from each parent. Daughters have a 50 percent chance of inheriting their mother's faulty X chromosome and a safe X chromosome from their father, which would make them asymptomatic carriers of the mutation.

What are the types and symptoms of spinal muscular atrophy?

Spinal muscular atrophy in children is classified into three types, based on ages of onset, severity, and progression of symptoms. All three types are caused by defects in the SMN1 gene.
  • Spinal muscular atrophy type I, also called Werdnig-Hoffmann disease or infantile-onset spinal muscular atrophy, is evident by the time a child is 6 months old. Symptoms may include hypotonia (severely reduced muscle tone), diminished limb movements, lack of tendon reflexes, fasciculations, tremors, swallowing and feeding difficulties, and impaired breathing. Some children also develop scoliosis (curvature of the spine) or other skeletal abnormalities. Affected children never sit or stand and the vast majority usually die of respiratory failure before the age of 2. However, the survival rate in individuals with spinal muscular atrophy type I has increased in recent years, in relation to the growing trend toward more proactive clinical care.
  • Symptoms of spinal muscular atrophy type II, the intermediate form, usually begin between 6 and 18 months of age. Children may be able to sit without support but are unable to stand or walk unaided, and may have respiratory difficulties, including an increased risk of respiratory infections. The progression of disease is variable. Life expectancy is reduced but some individuals live into adolescence or young adulthood.
  • Symptoms of spinal muscular atrophy type III (Kugelberg-Welander disease)appear between 2 and 17 years of age and include abnormal gait; difficulty running, climbing steps, or rising from a chair; and a fine tremor of the fingers. The lower extremities are most often affected. Complications include scoliosis and joint contractures-chronic shortening of muscles or tendons around joints, caused by abnormal muscle tone and weakness, which prevents the joints from moving freely. Individuals with spinal muscular atrophy type III may be prone to respiratory infections, but with care may have a normal lifespan.

Other forms of spinal muscular atrophy include:

  • Congenital spinal muscular atrophy with arthrogryposis (persistent contracture of joints with fixed abnormal posture of the limb) is a rare disorder. Manifestations include severe contractures, scoliosis, chest deformity, respiratory problems, unusually spinal muscular small jaws, and drooping of the upper eyelids.
  • Kennedy's disease, also known as progressive spinobulbar muscular atrophy, may first be recognized between 15 and 60 years of age. The onset of symptoms varies and includes weakness and atrophy of the facial, jaw, and tongue muscles, leading to problems with chewing, swallowing, and changes in speech. Early symptoms may include muscle pain and fatigue. Weakness in arm and leg muscles closest to the trunk of the body develops over time, with muscle atrophy and fasciculations. Individuals with Kennedy's disease also develop sensory loss in the feet and hands. Nerve conduction studies confirm that nearly all individuals have a sensory neuropathy (pain from sensory nerve inflammation or degeneration). Affected individuals may have enlargement of the male breasts or develop noninsulin-dependent diabetes mellitus.

How is spinal muscular atrophy diagnosed?

A test is available that can indicate whether there are deletions or mutations of the SMN1 gene. This test identifies at least 95 percent of spinal muscular atrophy Types I, II, and III. Other diagnostic tests may include electromyography (which records the electrical activity from the brain and/or spinal cord to a peripheral nerve root found in the arms and legs that controls muscles during contraction and at rest), nerve conduction velocity studies (which measure electrical energy by assessing the nerve's ability to send a signal), muscle biopsy (used to diagnose neuromuscular disorders and may also reveal if a person is a carrier of a defective gene that could be passed on to children), and laboratory tests of blood, urine, and other substances.

Medically Reviewed by a Doctor on 1/4/2017

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