Information on the following diseases can be found in the Related Disorders section of this report:

• Prader-Willi syndrome
• adrenoleukodystrophy
• Pompe disease
• nemaline myopathy
• central core disease
• congenital myotonic dystrophy
• congenital muscular dystrophy
• congenital myasthenia gravis

The symptoms and progression of SMA1 or Werdnig-Hoffman disease varies among affected individuals. Affected infants are weak before 6 months of age. The early signs include a generalized muscle weakness, diminished muscle tone (hypotonia) resulting in "floppiness," abnormal flexibility (hypermobility) of the joints, absent tendon reflexes, twitching (fasciculation) of the tongue, a frog-like position with the hips moved apart (abducted) and knees bent or flexed, and an alert appearance. Muscles of the face are not affected initially. Mental development is usually normal. Typically, the child does not gain head control, cannot turn over and is unable to sit or stand. In addition, children with SMA may develop difficulties sucking, swallowing, and breathing; have an increased susceptibility to respiratory infections, or develop other complications that may lead to potentially life-threatening abnormalities within the first months or years of life.

For infants who appear to have normal development for several months prior to the onset of muscle weakness, the disorder may tend to have a more slowly progressive course. Muscles of the lower extremities appear to be disproportionately affected. With disease progression, diminished muscle tone and weakness may gradually spread to affect almost all voluntary muscles, with the exception of certain muscles controlling movements of the eyes.

The rate of progression of Werdnig-Hoffman disease varies. Within a few months, breathing (respiratory) and bowel (constipation) difficulties may develop. The infant may be unable to swallow. Respiratory failure may occur or food inhaled into the lungs (aspiration) may cause choking. Most affected children die before 2 years of age but survival may be dependent on the degree of respiratory function.

All forms of spinal muscular atrophy are caused by mutations in the SMN1 (survival motor neuron 1) gene at chromosomal locus 5q11-q13. A second gene, known as the SMN2 (survival motor neuron 2) gene, plays a role in the development of SMA. The SMN2 gene is next to (adjacent) to the SMN1 gene on chromosome 5. While the mutations of the SMN1 cause SMA, evidence has been developed that SMN2 influences disease severity; individuals with more copies of the SMN2 gene tend to have a milder form of SMA.

Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated "p" and a long arm designated "q". Chromosomes are further sub-divided into many bands that are numbered. For example, "chromosomal locus 5q11-q13" refers to bands 11-13 on the long arm of chromosome 5. The numbered bands specify the location of the thousands of genes that are present on each chromosome.

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. All SMAs are inherited in an autosomal recessive manner. 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.

Parents of several individuals with Werdnig-Hoffman disease have been closely related by blood (consanguineous). All individuals carry 4-5 abnormal genes. Parents who are consanguineous have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.

The specific underlying cause of Werdnig-Hoffmann disease is unknown. In SMA, it appears that the SMN1 and SMN2 genes produce (encode) a protein, which is essential for the proper function of motor neurons. Mutation of the SMN1 causes the gene to produce a defective protein that cannot perform its intended function. It is believed that SMN2 gene produces a partially-effective protein required by motor neurons to function. This is why individuals with more copies of SMN2 have a milder form of SMA.

Additional genes may influence the development of SMA. Deletion of the NAIP (neuronal apoptosis inhibitory protein) gene that is close to the SMN gene may also be associated with SMA. More patients with Werdnig-Hoffman disease (SMA1) than other types of SMA have NAIP deletions. Some researchers suggest that loss of the NAIP gene and/or different mutations of the SMN gene may play a role in affecting the disorder's severity. Some investigators also indicate that other genetic factors may contribute to the variable clinical expression of the disorder.

Werdnig-Hoffman disease is a rare disorder that affects males and females in equal numbers. The prevalence of all types of spinal muscular atrophy has been estimated to be 4-7.8 per 100,000 live births. Approximately 80% of SMA patients have the Werdnig-Hoffman form.

Symptoms of the following disorders can be similar to those of Werdnig-Hoffman disease. Comparisons may be useful for a differential diagnosis:

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. The diagnosis is established by chromosomal analysis. (For more information on this disorder, choose "Prader-Willi syndrome" 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. Pompe disease is a single disease continuum with variable rates of disease progression. The infantile form is characterized by severe muscle weakness and abnormally diminished muscle tone (hypotonia), and usually manifests within the first few months of life. Additional abnormalities may include enlargement of the heart (cardiomegaly), the liver (hepatomegaly), and/or the tongue (macroglossia). Progressive cardiac failure usually causes life-threatening complications by the age of 12 to 18 months. The childhood form usually begins during late infancy or early childhood. The extent of organ involvement may vary among affected individuals; however, skeletal muscle weakness is usually present with minimal cardiac involvement. Specific treatment for Pompe’s disease is now available. (For more information on this disorder, choose "Pompe disease" as your search term in the Rare Disease Database.)

Congenital muscular dystrophy (CMD) is a general term for a group of genetic muscle diseases that occur at birth (congenital) or early during infancy and have similar findings on microscopic examination of the muscle tissue. CMDs are generally characterized by diminished muscle tone (hypotonia), which is sometimes referred to as “floppy baby”; progressive muscle weakness and degeneration (atrophy); abnormally fixed joints that occur when thickening and shortening of tissue such as muscle fibers cause deformity and restrict the movement of an affected area (contractures); and delays in reaching motor milestones such as sitting or standing unassisted. Some forms of CMD may be associated with structural brain defects and, potentially, mental retardation. The severity, specific symptoms, and progression of these disorders vary greatly. Almost all known forms of CMD are inherited as autosomal recessive traits. (For more information on these disorders, choose "congenital muscular dystrophy" as your search term in the Rare Disease Database.)

Congenital myopathies are a group of muscle disorders (myopathies) that are present at birth (congenital). These disorders are characterized by muscle weakness, loss of muscle tone (hypotonia), diminished reflexes, and delays in reaching motor milestones (e.g., walking). In some disorders, muscle weakness is progressive and may result in life-threatening complications. This group of disorders includes central core disease, nemaline rod myopathy, hyaline body myopathy, centronuclear myopathy, congenital fiber type disproportion, and minimulticore myopathy. Congenital myopathies are usually apparent in the newborn (neonatal) period, but may present much later in life even in adulthood. In most cases, inheritance of these disorders is either autosomal recessive or autosomal dominant. The diagnosis is established by microscopic examination of muscle tissue. (For more information on these disorders, choose the specific disorder name as your search term in the Rare Disease Database.)

Additional disorders are included in the differential diagnosis of spinal muscular atrophy including arthrogryposis multiplex congenita, adrenoleukodystrophy, and congenital myasthenia gravis.

Diagnosis
A diagnosis of SMA may be suspected based upon a detailed patient history, a thorough clinical examination and identification of characteristic findings. A diagnosis may be confirmed through molecular genetic testing, which can determine whether a mutation is present in the SMN gene. SMA is caused by a partial or complete loss of the SMN gene and about 95 percent of those affected will show a deletion of both copies of a specific portion (exon 7 or exon 8) of the gene. About 5 percent of those affected will show a deletion of exon 7 in one copy of the SMN gene and a different mutation in the other copy of the SMN gene.

Prior to the availability of molecular testing, electromyogram testing (neurophysiologic study of muscles) and microscopic study of samples of affected muscle tissue (biopsy) were used for diagnosis. These tests are no longer necessary unless SMN gene testing is normal.

Carrier testing for SMA is a molecular genetic test in which the number of copies of the SMN gene in which exons 7 and 8 are present is determined.

Treatment
No curative treatment exists for infants with Werdnig-Hoffman disease. Treatment is aimed at the specific symptoms that are present in each individual. Treatment may require a team of specialists.

Feeding difficulties may cause nutritional concerns and may require a gastrostomy, a procedure in which a feeding tube is inserted directly into the stomach through a surgical opening.

Breathing (respiratory) function declines in individuals with Werdnig-Hoffman disease. Treatment options range from offering no respiratory support to using noninvasive procedures or long-term invasive procedures such as a tracheotomy. A tracheotomy is a procedure in which a tube is inserted through a surgical opening in the windpipe (trachea). A non-invasive option is intermittent positive pressure ventilation (NIPPV), in which breathing is mechanically assisted without the creation of an artificial airway as is done with a tracheotomy. The specific treatment for the life-threatening respiratory complications of Werdnig-Hoffman disease is controversial. Any decisions for treatment should be made after consultation between the parents and the entire medical team and taken on an individual case basis.

Physical and occupational therapy is helpful to minimize contractures and help the patient/caretakers develop compensatory strategies; muscle strengthening is not a reason for therapy. Orthopedic devices (e.g., braces) and surgery to correct scoliosis may become necessary. Genetic counseling may be of benefit for affected individuals and their families. Other treatment is symptomatic and supportive.

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 NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:
www.centerwatch.com.

In 2004, Phase I and II clinical trials investigating the drug, hydroxyurea, were initiated. They are still in progress (2006). More information on this study, which is sponsored by the Muscular Dystrophy Association, is listed on www.clinicaltrials.gov or may be obtained by contacting the research team at Stanford University (sma@stanfordmed.org). For contact information for the Muscular Dystrophy Association, see the Resources section of this report.

TEXTBOOKS
Russman BS. Spinal Muscular Atrophy. In: The NORD Guide to Rare Disorders, Philadelphia: Lippincott, Williams and Wilkins, 2003:637.

Zerres K, Rudnik-Schoneborn S. Spinal Muscular Atrophies. IN: Rimoin D, Connor JM, Pyeritz RP, Korf BR. Eds. Emory and Rimoin’s Principles and Practice of Medical Genetics. 4th ed. Churchill Livingstone. New York, NY; 2002:3349-3372.

JOURNAL ARTICLES
Petrone A, Pavone M, Testa MB, et al. Noninvasive ventilation in children with spinal muscular atrophy types 1 and 2. Am J Phys Med Rehabil. 2007;86:222-4.

Bush A, Fraser J, Jardine E, et al. Respiratory management of the infant with type I spinal muscular atrophy. Arch Dis Child. 2005;90:709-11.

Chung BY, Wong VC, Ip P. Spinal muscular atrophy: survival pattern and functional status. Pediatrics. 2004;114:e548-53.

Hardart MKM, Truog RD. Spinal muscular atrophy-type I. Arch Dis Child. 2003;88-848-50.

Russman, BS, Iannaconne ST, Samaha FJ. A phase 1 trial of riluzole in spinal muscular atrophy. Arch Neurol. 2003;60:1601-3.

Gozal D. Pulmonary manifestations of neuromuscular disease with special reference to Duchenne muscular dystrophy and spinal muscular atrophy. Pediatr Pulmonol. 2000;29:141-50.

Strober JB, Tennekoon GI. Progressive spinal muscular atrophies. J Child Neurol. 1999;14:691-95.

Andersson PB, Rando TA. Neuromuscular disorders of childhood. Curr Opin Pediatr. 1999;11:497-503.

Iannaccone ST, Browne RH, Samaha FL, Buncher CR. DCN/SMA group: A prospective study of SMA before age six years. Pediatr Neurol. 1993;9:187-193.

Russman BS, Iannaccone ST, Buncher CR, et al. Spinal muscular atrophy: new thoughts on the pathogenesis and classification of schema. J Child Neurol. 1992;7:347-353.

FROM THE INTERNET
Prior TW, Russman BS, Spinal Muscular Atrophy in GeneReviews at GeneTests/GeneClinics: Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2003. Available at http://www.geneclinics.org. Accessed 08/07.

McKusick VA., ed. Online Mendelian Inheritance in Man (OMIM). Baltimore. MD: The Johns Hopkins University; Entry No:253300; Last Update:08/21/2007. Available at: http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=253300 Accessed on: August, 2007.