SMA

Inherited diseases cause muscle weakness

Spinal muscular atrophy (SMA) is a group of three inherited diseases that affect nerves (called motor neurons) that control the voluntary muscles in the body. SMA causes weakness and wasting of these muscles. SMA occurs in about 1 in 6,000 births.

SMA is caused by a gene called SMN1, which is abnormal or missing in people with the disease. SMA is inherited in an autosomal recessive pattern, meaning that a child must inherit two copies of the defective gene, one from each parent. About 1 in 40 people carry a defective SMN1 gene that they could pass on to their children. SMA affects both males and females of all ethnic backgrounds.

Types of SMA

Three types of SMA occur in children:

• SMA Type I (also called Werdnig-Hoffman disease) is present at birth
• SMA Type II (also called juvenile SMA, intermediate SMA, or chronic SMA) begins between 6 to 18 months of age
• SMA Type III (also called Wolhlfart-Kugelberg-Welander disease, or mild SMA) can begin as early as the toddler years or as late as adolescence

There is an SMA Type IV that begins in adulthood (in the twenties or thirties).

Symptoms

All types of SMA have weakness in the arms and legs, but the severity of the weakness is related to the type of SMA.

• SMA Type I – symptoms include low muscle tone (hypotonia) throughout the body, feeble movements of the arms and legs, swallowing problems, a weak sucking reflex, a weak cry and cough, curvature of the spine (scoliosis), and difficulty breathing.
• SMA Type II – symptoms include weak legs and arms and curvature of the spine. Children with Type II are usually able to sit without support if placed in a sitting position. Some may be able to stand or walk with help.
• SMA Type III – symptoms include difficulty walking and getting up from a sitting position due to weakness in the legs.
• SMA Type IV – symptoms are similar to Type III.

Diagnosis

If a child or adult has muscle weakness that suggests SMA, a blood test is done that looks for the presence or absence of the SMN1 gene. In about 95% of individuals with SMA, the SMN1 gene is missing. In some people, the gene is not missing but is abnormal (mutated).

If the symptoms suggest SMA but the gene test is normal (SMN1 is present), additional types of tests may be done to determine if other motor neuron diseases are present.

Treatment

The treatment of SMA depends on the type and the symptoms present. The International Standard of Care Committee for Spinal Muscular Atrophy was founded in 2005 with the goal of establishing guidelines for care of individuals with SMA. The Committee identified many care issues, especially for children. These may include:

• breathing problems
• feeding and swallowing problems
• nutrition
• muscle weakness
• bone and joint problems such as spinal curvature and tightening (contracture) of joints
• emotional support
• genetic counseling

Each of these areas requires specialized care from physicians and therapists skilled in caring for people with SMA.

Outlook

Infants born with SMA Type I have a difficult time, and most die within the first two years of life due to breathing problems. The outlook for children with SMA Type II is related to how old they were when they first began to experience symptoms. The older the child was, the less severe the symptoms tend to be. Children and youth with SMA Type III and adults with Type IV have milder symptoms and less medical problems than those with other types of SMA.

_Sources:

_{Weng, Ching H., Richard S. Finkel, Enrico S. Bertini, Mary Schroth, Anita Simonds, Brenda Wong, Annie Aloysius, et al. "Consensus Statement for Standard of Care in Spinal Muscular Atrophy." Journal of Child Neurology 22(2007): 1027-1049. Print.}

_{"NINDS Spinal Muscular Atrophy Information Page." Disorders A - Z. 9 July 2008. National Institute for Neurological Disorders and Stroke. 8 Jul 2009.}

_{"Understanding SMA." FSMA Community. Families of Spinal Muscular Atrophy. 8 Jul 2009.}