Spinal muscular atrophy
Spinal muscular atrophy is a term that describes a number of different conditions, all of which have in common the gradual deterioration of the voluntary muscles.
Several different conditions fall under the name spinal muscular atrophy (SMA). These include SMA type I, also called Werdnig-Hoffmann; SMA type II; SMA type III, also called Kugelberg-Welander disease; Kennedy syndrome, or progressive spinobulbar muscular atrophy; and congenital SMA with arthrogryposis.
The autosomal recessive forms of spinal muscular atrophy are the most common inherited cause of infant death. Each type of spinal muscular atrophy has an incidence of about 10 to 15 cases in every 100,000 live births.
Causes and symptoms
All types of spinal muscular atrophy are genetic diseases. Most of the syndromes are autosomal recessive, meaning that they have no predilection for either sex. Parents of children with SMA usually carry the gene for the disease but have no symptoms themselves. A child who receives two genes (one from each parent) will express the symptoms of the disease.
Although the entire sequence of abnormalities that causes spinal muscular atrophy was not delineated as of 2004, there is thought to be an absence or deficiency of a specific protein necessary for the proper functioning of the nerve cells responsible for movement (motor neurons).
SMA type I (Werdnig-Hoffmann disease)
SMA type I is usually noted prior to birth, due to a decrease in the baby's movements in utero, or early in life. Babies with this type of SMA have decreased muscle and trunk tone, resulting in floppiness of the limbs and weak arm and leg movements. They have difficulty with swallowing and, therefore, with feeding, and they have breathing problems. These children are unable to learn to sit or to stand. The disease is usually fatal prior to the age of two.
SMA type II
Symptoms of SMA type II are usually noted in a child between three and 15 months of age. Symptoms include breathing problems; weak and floppy limbs; involuntary jerking and twitching of muscles in the arms, legs, and tongue; abnormal reflexes. Children with SMA type II may eventually be able to sit, but they are unable to learn to stand or to walk.
SMA type III (Kugelberg-Welander disease)
Children with SMA type III begin to experience symptoms between the ages of two and 17 years. Problems develop that hamper the child's ability to walk, run, climb stairs, and rise from a chair. Twitches and tremors may develop in the child's fingers.
Kennedy syndrome (progressive spinobulbar muscular atrophy)
This form of spinal muscular atrophy only affects men; it is an X-linked recessive disorder, meaning that the defective gene is passed from mother to son. Individuals with Kennedy syndrome begin to develop symptoms between the ages of 15 and 60 years. Characteristic symptoms include increasing weakness of the tongue and facial muscles, problems with swallowing, impaired speech, and increased size of the male breast (gynecomastia). The severity of the symptoms of Kennedy syndrome progress gradually.
Congenital SMA with arthrogryposis
This is one of the rarest forms of spinal muscular atrophy. It is present at birth, and children exhibit severe contractures of the joints, resulting in limb deformity; spinal curvature; deformities of the chest wall; difficulties breathing; abnormally small jaw; and upper eyelid droop (ptosis).
Diagnosis is by a combination of clinical observation; blood tests that reveal an increased level of creatine kinase (which appears in the blood when muscle tissue is being broken down); distinctive abnormalities on muscle biopsy; characteristic electromyographic and nerve conduction abnormalities; and genetic testing.
There are no cures for any of the forms of spinal muscular atrophy. The treatments involve addressing the symptoms and attempting to improve quality of life. Medical treatment may be necessary for recurrent pneumonia and other respiratory infections. Surgery may be necessary for spinal curvature and severe contractures. Physical therapy, occupational therapy, and other types of rehabilitation programs may help individuals achieve the highest level of functioning possible.
The prognosis for spinal muscular atrophy is variable. Life expectancy is dependent on the degree of respiratory impairment present. Because of the slow progression of symptoms, individuals with Types III or Kennedy syndrome may have normal life spans.
There is no way to prevent spinal muscular atrophy. However, genetic counseling is crucial so that parents can make informed decisions about having children. In general, when a family has already had a child with SMA, each subsequent pregnancy has a 25 percent chance of producing another child with SMA. Prenatal testing is available. Parents must then decide whether to use the information to help them prepare for the arrival of a baby with SMA or to terminate the pregnancy.
Caring for a child with SMA can be very challenging and emotionally draining. Support groups, respite care, and help to support other siblings in the family can be important adjunct measures.
"Disorders of Neuromuscular Transmission and of Motor Neurons." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.
Siddique, Nailah. "Degenerative Motor, Sensory, and Autonomic Disorders." In Textbook of Clinical Neurology. Edited by Christopher G. Goetz. Philadelphia: Saunders, 2003.
Muscular Dystrophy Association—USA. 3300 E. Sunrise Drive, Tucson, AZ 85718. Web site: http://www.mdausa.org.
NIH Neurological Institute. , PO Box 5801, Bethesda, MD 20824. Web site: http://www.ninds.nih.gov.
Rosalyn Carson-DeWitt, MD