Spinal Muscular Atrophy
NORD is very grateful to Barry Russman, MD, Professor of Pediatrics and Neurology, Oregon Health Sciences University and Shriners Hospital for Children, for assistance in the preparation of this report.
Synonyms of Spinal Muscular Atrophy
- SMA type 0
- SMA type 1
- SMA type 2
- SMA type 3
- SMA type 4
Spinal muscular atrophy (SMA) is a group of inherited disorders characterized by a loss of certain nerve cells in the spinal cord called motor neurons or anterior horn cells. Motor neurons receive the nerve impulses transmitted from the brain to the spinal cord (brainstem) and, in turn, transmit the impulses to the muscle via the peripheral nerves. The loss of motor neurons leads to progressive muscle weakness and muscle wasting (atrophy) in muscles closest to the trunk of the body (proximal muscles) such as the shoulders, hips and back. These muscles are necessary for crawling, walking, sitting up and head control. The more severe types of SMA can affect muscles involved in feeding, swallowing and breathing.
SMA is divided into subtypes based on age of onset and maximum function achieved. SMA types 0, 1, 2, 3 and 4 are inherited as autosomal recessive genetic disorders and are associated with abnormalities (mutations) in the SMN1 and SMA2 genes which are located on chromosome 5.
SMA type 0 is the most severe form of the disease and is characterized by decreased fetal movement, joint abnormalities, difficulty swallowing and respiratory failure.
SMA type 1 is the most common type of SMA and is also a severe form of the disease. Infants with SMA type 1 experience severe weakness before 6 months of age and never sit independently. Muscle weakness, lack of motor development and poor muscle tone are the major clinical manifestations of SMA type I. Infants with the gravest prognosis have problems sucking or swallowing. Some show abdominal breathing in the first few months of life. Muscle weakness occurs on both sides of the body and the ocular muscles are not affected. A twitching of the tongue is often seen. Intelligence is normal. Most affected children die before two years of age but survival may be dependent on the degree of respiratory function. For more information about SMA type 1, chose "Werdnig Hoffman" disease as your search term in the Rare Disease Database.
The onset of weakness in SMA type 2 patients is usually between 6 and 12 months. Affected children are able to sit independently early in development but are unable to walk even 10 feet independently. A trembling (tremor) of the fingers is almost always seen in SMA type 2. Approximately 70% of those affected do not have deep tendon reflexes. Those affected with SMA type 2 are usually not able to sit independently by the mid-teens or later.
Patients with SMA type 3 (Kugelberg-Welander syndrome) learn to walk but fall frequently and have trouble walking up and down stairs at 2-3 years of age. The legs are more severely affected than the arms. The long-term prognosis depends on the degree of motor function attained as a child. For more information about SMA3 chose "Kugelberg Welander syndrome" as your search term in the Rare Disease Database.
The onset of muscle weakness for those with SMA type 4 is after age 10 years; these patients usually are ambulatory until age 60 years.
Complications of SMA include scoliosis, joint contractures, pneumonia and metabolic abnormalities such as severe metabolic acidosis and dicarboxylic aciduria.
SMA types 0, 1, 2, 3 and 4 are inherited as autosomal recessive genetic disorders and are associated with abnormalities (mutations) in the SMN1 and SMA2 genes on chromosome 5 at chromosomal locus 5q11-q13. SMA1 is thought to be the primary disease-causing gene. Approximately 95-98% of affected individuals have deletions in the SMA1 gene and 2-5% have specific mutations in the SMA1 gene that result in a decreased production of the SMN protein. When three or more copies of the SMA2 gene are also present, the disease may be milder.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
Biological or surrogate markers for SMA are under development. These potential surrogate markers studied to date include:
1) Measuring the amount and ratio of full-length and truncated SMN2 RNA transcripts as well as the amount of SMN protein from white blood cells or fibroblast culture.
2) Counting motor units (Motor Unit Number Estimation, or MUNE) which have shown correlation with the SMN2 copy number, age, and function.
3) Quantitative ultrasound in assessing muscle changes in patients with SMA.
4) Electrical impedance myography.
5) The compound action potential determined by EMG in children with SMA. The authors point out that MUNE is difficult and not as reliable as initially thought. A longitudinally study of the surrogate markers will be necessary before any particular one can be used in an intervention study.
The estimated frequency of spinal muscular atrophy is 1 in 4,000 to 1 in 7,000 people.
Symptoms of the following disorders can be similar to those of spinal muscular atrophy. Comparisons may be useful for a differential diagnosis.
The following spinal muscular atrophies are not linked to chromosome 5q but have certain signs and symptoms in common:
Finkel type SMA: manifest usually after age 30 years, is inherited as an autosomal dominant genetic disorder and is associated with mutations in the VAPB gene.
Scapuloperoneal spinal muscular atrophies are usually autosomal dominant conditions, starting in childhood or adulthood, the weakest muscles being the scapula and peroneal muscles. The progression is usually slow, and ambulation is not lost until the fifth decade of life. Linkage analysis of 1 large kindred has identified a locus on chromosome 12q24.1-q24.31.
Distal spinal muscular atrophies are characterized by weakness and wasting that starts with distal muscles of the upper and lower limbs and spreads later to other muscle groups. Phenotypically, affected individuals resemble patients with Charcot-Marie-Tooth disease. More than 10 Distal SMAs have been reported, each associated with a different gene.
A form of distal spinal muscular atrophy with respiratory distress secondary to diaphragmatic paralysis (SMARD) in the early months of life was first described in 1974 and linked to chromosome 11q12-q14.1.
Conditions with distinctive presenting features other than muscle weakness include:
X-linked spinal and bulbar muscular atrophy, also known as Kennedy disease, is a gradually progressive neuromuscular disorder in adult men in whom degeneration of lower motor neurons results in proximal muscle weakness, muscle atrophy, and fasciculations beginningbetween the ages of 20 and 50 years.
Hexosaminidase A deficiency results in lysosomal storage of the pecific glycosphingolipid,
GM2 ganglioside. The juvenile, chronic, and adult-onset variants have onset after infancy, slow progression, and variable neurologic findings, including dystonia, spinocerebellar degeneration, and lower motor neuron disease. Diagnosis is by biochemical or molecular testing.
Monomelic muscular atrophy is predominantly a cervical form of spinal muscular atrophy.
Goutieres and colleagues reported 5 patients with a form of spinal muscular atrophy that
presents in the first few months of life, predominantly with weakness of the neck flexors and
Fazio-Londe disease, a motor neuron disease limited to the lower cranial nerves, starts in the second decade of life and progresses to death within 1 to 5 years.
Other diseases involving both the anterior horn cell and other neurologic systems include spinal muscular atrophy associated with brain atrophy and olivopontocerebellar atrophy. A mutation in the VRK1 gene in consanguineous members of a family has been described with this condition.
Spinal muscular atrophy associated with congenital bone fractures was initially reported in a
boy, and the disorder was speculated to be X-linked, but the latest report described a girl with the same problem. The gene for spinal muscular atrophy, 5q13, was found to be normal.
The following conditions should be considered when concerns about SMA 0 or 1 are present:
Prader-Willi syndrome is a genetic disorder characterized by diminished muscle tone (hypotonia), feeding difficulties, and failure to grow and gain weight (failure to thrive) during infancy; short stature; genital abnormalities; and mental retardation. In addition, beginning at approximately age six months to six years, affected individuals may develop excessive body weight (obesity), especially in the lower regions of the body (e.g., lower abdomen, thighs, buttocks). Progressive obesity results from lack of physical activity and excessive intake of food, which may be associated with no feeling of satisfaction (satiety) after completing a meal, an obsession with eating, unusual food rituals, and binge-type eating habits. Individuals with Prader-Willi syndrome may also have a characteristic facial appearance due to certain features, including almond-shaped eyes, a thin upper lip, and full cheeks. For more information on this disorder, choose "Prader-Willi syndrome" as your search term in the Rare Disease Database.
Adrenoleukodystrophy is a rare inherited metabolic disorder characterized by the loss of the fatty covering (myelin sheath) on nerve fibers within the brain (cerebral demyelination) and the progressive degeneration of the adrenal gland (adrenal atrophy). Adrenoleukodystrophy that is inherited as an X-linked genetic trait may begin in childhood or adulthood. However, Adrenoleukodystrophy that is inherited as an autosomal recessive genetic trait typically begins during infancy (neonatal period). For more information on this disorder, choose "adrenoleukodystrophy" as your search term in the Rare Disease Database.
Pompe disease is a hereditary metabolic disorder caused by the complete or partial deficiency of the enzyme acid alpha-glucosidase (also known as lysosomal alpha-glucosidase or acid maltase). This enzyme deficiency causes excess amounts of glycogen to accumulate in the lysosomes of many cell types but predominantly in muscle cells. The resulting cellular damage manifests as muscle weakness and/or respiratory difficulty. For more information on this disorder, choose "Pompe disease" as your search term in the Rare Disease Database.
Congenital myasthenia is caused by genetic defects of muscle and nerve communication (neuromuscular transmission). This condition usually occurs in infants but may become evident in adulthood. Associated features may vary in severity from case to case. Such abnormalities may include feeding difficulties, sudden episodes of absence of spontaneous breathing (apnea), failure to grow and gain weight at the expected rate, muscle weakness and fatigue, weakness or paralysis of eye muscles (ophthalmoplegia), and/or other abnormalities. For more information on this disorder, choose "myasthenia gravis" as your search term in the Rare Disease Database.
Myotubular myopathy is a rare muscle wasting disorder that occurs in three forms. The most severe form is present at birth, inherited as an X- Linked genetic trait, and presents with severe respiratory muscle weakness. A less severe form is present at birth or early childhood, progresses slowly and is inherited as an autosomal recessive genetic trait. The least severe of the three forms of is inherited as an autosomal dominant genetic trait, presents between the first and third decades of life and is slowly progressive. For more information on this disorder, choose "myotubular myopathy" as your search term in the Rare Disease Database.
Nemaline myopathy is a rare inherited neuromuscular disease that is usually apparent at birth (congenital) and characterized by extreme muscle weakness (hypotonia). Laboratory examination of muscle tissue samples from people with Nemaline Myopathy reveal the presence of fine fibrous threads known as nemaline rods that interfere with the muscle function. For more information on this disorder, choose "nemaline myopathy" as your search term in the Rare Disease Database.
Central core disease (CCD) is a relatively rare genetic disorder of infancy and childhood characterized by abnormalities of skeletal (voluntary) muscle (congenital myopathy). Associated symptoms and findings may include abnormally diminished muscle tone (hypotonia), potentially resulting in unusual "floppiness" of muscles; muscle weakness; delays in motor development, such as in walking; and/or, in some cases, associated musculoskeletal problems, such as dislocation of the hips at birth (congenital). CCD may also be associated with susceptibility to malignant hyperthermia, a potentially life-threatening reaction to certain anesthetics or skeletal muscle relaxants. For more information on this disorder, choose "central core disease" as your search term in the Rare Disease Database.
Myotonic muscular dystrophy is an inherited disorder involving the muscles, vision, and endocrine glands. It may also cause mental deficiency and loss of hair. Onset of this disorder usually occurs during early adulthood. However, it may occur at any age and is extremely variable in degree of severity. For more information on this disorder, choose "dystrophy, myotonic" as your search term in the Rare Disease Database.
Congenital muscular dystrophy (CMD) is characterized by slowly progressive muscle weakness and changes in the white matter of the brain (where nerve tracts are located) that can be seen with magnetic resonance imaging. Many children with CMD are able to walk, and remain ambulatory for varying amounts of time. The disease typically progresses slowly but the severity can vary dramatically from one child to the next. The severe types of CMD, which typically have extensive brain involvement with mental retardation in addition to muscle weakness, include Fukuyama CMD, Walker-Warburg disease and muscle-eye-brain disease (MEB).
Duchenne muscular dystrophy, a hereditary degenerative disease of skeletal (voluntary) muscles, is considered the most prevalent form of childhood muscular dystrophy. SMA 3 may be confused with this condition. The disorder typically is recognized from approximately age three to six years and has a relatively rapid, progressive disease course. Duchenne Muscular Dystrophy is initially characterized by muscle weakness and wasting (atrophy) within the pelvic area that may be followed by involvement of the shoulder muscles. With disease progression, muscle weakness and atrophy affect the trunk and forearms and gradually progress to involve most major muscles of the body. For more information about this disorder, choose "muscular dystrophy, Duchenne" as your search in the Rare Disease Database.
Becker muscular dystrophy is a rare inherited muscle wasting disease usually beginning during the second or third decade of life and may be considered when the diagnosis of SMA 4 is being entertained. This slowly progressive disorder affects males almost exclusively. Muscles of the hips and shoulders are weakened, walking abnormalities develop, and mild mental retardation may be present. Eventually, other more severe symptoms may involve the heart and lungs. For more information on this disorder, choose "muscular dystrophy, Becker" as the search term in the Rare Disease Database.
The diagnosis of SMA is suspected when symptoms are present and the diagnosis can be confirmed with molecular genetic testing. Molecular genetic testing is used to determine if a mutation is present in the SMN1 gene. SMA types 0, 1, 2, 3 and 4 are caused by a partial or complete loss of the SMN1 gene and about 95% of those affected will show a deletion of both copies of a specific portion (exon 7 or exon 8) of the gene. About 5% of those affected will show a deletion of exon 7 in one copy of the SMN1 gene and a different mutation in the other copy of the SMN1 gene. Molecular genetic testing can also be used to determine the number of copies of the SMN2 gene.
Prior to the availability of molecular testing, neurophysiologic studies and muscle biopsy were used for diagnosis, but these tests are no longer necessary unless SMN gene testing is normal.
Carrier testing for SMA is available using a molecular genetic test in which the number of copies of the SMN1 gene is determined.
Molecular genetic testing for the VAPB gene is available to diagnose Finkel type SMA.
Care for individuals with SMA is symptomatic and includes physical therapy, occupational therapy, monitoring of respiratory function and nutritional status, orthotics and adaptive equipment. Respiratory support for SMA1 using a breathing machine called BiPAP (bi-level positive airway pressure) has been shown to increase comfort and life expectancy in some affected children.
Genetic counseling is recommended for affected individuals and their families.
The management of children with spinal muscular atrophy starts with the diagnosis and
classification into 1 of the 5 categories. Health issues specific to spinal muscular atrophy are as follows:
Pulmonary management: Children with SMA1 can survive beyond 2 years
of age when offered tracheostomy or noninvasive respiratory support.
An intermittent positive-pressure breathing device (mechanical in-exsufflator) has proven effective.
Nutrition: Bulbar dysfunction is universal in SMA1 patients. Early gastrostomy should be
considered as part of the management of such patients. The bulbar dysfunction eventually
becomes a serious problem for spinal muscular atrophy II patients and only very late in the
course of disease for spinal muscular atrophy III patients.
Scoliosis: Scoliosis is a major problem in most SMA2 patients and in half of SMA3 patients.
The Vertical Expandable Prosthetic Titanium Rib (VEPTR) was approved by the FDA in 2004 as a treatment for thoracic insufficiency syndrome (TIS) in pediatric patients. TIS is a congenital condition where severe deformities of the chest, spine, and ribs prevent normal breathing and lung development. The VEPTR is an implanted, expandable device that helps straighten the spine and separate ribs so that the lungs can grow and fill with enough air to breathe. The length of the device can be adjusted as the patient grows. The titanium rib was developed at the University of Texas Health Science Center in San Antonio. It is manufactured by Synthes Spine Co.: http://www.synthes.com/sites/NA/Products/Spine/Screw_Hook_Rod_and_Clamp_System/Pages/VEPTR_and_VEPTR_II.aspx
For more information, please contact:
1302 Wrights Lane East
West Chester, PA 19380
Hip dislocation: Hip dislocation is another orthopedic concern in patients with spinal muscular atrophy. If the hip dislocation is asymptomatic, surgery is not indicated.
Behavior issues: Compared to siblings and normal controls, patients with spinal muscular
atrophy were quite well adjusted. Concern was, however, raised about unaffected siblings, who had a 2 to 3-fold higher rate behavioral problems than normal children.
Sleep disorders: Sleep-disordered breathing may develop prior to respiratory failure. Night-time use of continuous positive airway pressure with a nasal mask may be helpful.
Medication treatment: Specific medical treatments for spinal muscular atrophy do not exist. The following medications have studied and the results have been disappointing: Gabapentin, phenylbutyrate, valproic acid, hydroxyura, riluzole and myostatin.
An open-label, escalating dose study to assess the safety, tolerability and dose-range finding of a single intrathecal dose of Isis 396443 in patients with spinal muscular atrophy has just commenced; the experimental drug has shown efficacy in the SMA mouse model.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
For information about clinical trials sponsored by private sources, contact:
Contact for additional information about spinal muscular atrophy:
Barry Russman, MD
Professor of Pediatrics and Neurology
Oregon Health Sciences University
Spinal Muscular Atrophy Resources
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Online Mendelian Inheritance in Man (OMIM). The Johns Hopkins University. Spinal Muscular Atrophy, Type IV; SMA4. Entry No: 271150. Last Edited August 21, 2007. Available at: http://www.ncbi.nlm.nih.gov/omim/. Accessed February 9, 2012.
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