Spinal Muscular Atrophy

[heidi1439]

Spinal Muscular Atrophy (SMA) is a hereditary disease characterized by wasting of the skeletal muscles caused by progressive degeneration of the anterior horn cells found in the spinal cord. It is the second most common childhood neuromuscular disorder and is caused by a deletion or mutation on chromosome 5q. Muscle atrophy and weakness occur in the voluntary muscles that are used for walking, crawling, head and neck control, and swallowing.

SMA is an autosomal recessive disorder, which means that both parents must be carriers of the defective gene and both parents much each pass the SMA gene to the child (receiving one defective gene will not cause SMA). The likelihood of parents passing the gene on to their offspring and having an affected child is 25%, or a 1 in 4 chance. One in 40 people are carriers of SMA and the occurrence rate is estimated to be 1 in every 6,000 to 10,000 live births (with some forms more common than others).

The first substantive descriptions of SMA occurred at the end of the 19^th century when Drs. Werdnig and Hoffman characterized the features of autosomal recessive SMA. Later, in 1950, Dr. Brandt reported 112 cases of progressive SMA and noted that an EMG and a muscle biopsy were useful in establishing a diagnosis. Then, in 1965, Drs. Kugelberg and Welander reported several patients with a later onset of SMA and a more prolonged course. Finally, in 1973, Dr. Pearn published a series of papers that were considered the most important documentation of the clinical, epidemiologic, and genetic

 

Features and Characteristics

There are three types of SMA that appear in childhood. The major differences between the types are the age of onset and the severity of the condition.

Type I SMA (also known as Werdnig-Hoffman Disease) is the most severe form of SMA and is characterized by the following:

• Decreased fetal movement in utero
• Inability to lift head, sit without support, and reach other typical physical milestones
• Symmetric limb and trunk weakness
• Hypotonia
• Tongue fasciculation
• Swallowing and feeding difficulties
• Absent tendon reflexes
• General weakness in respiratory muscles
• Labored breathing
• May appear to have a concave (sunken) chest due to abdominal breathing
• Repeated respiratory infections
• Scoliosis
• Normal to above-average intelligence
• Onset is in utero or the first couple months of life

Type II SMA is the intermediate form of SMA and is characterized by the following:

• Difficulty with gross motor skills (i.e., can sit unsupported, however, may have difficulty transitioning to a sitting position) and may need braces and/or a stander for weight bearing
• Weakness of the muscles in the arms, legs and trunk
• Normal to above-average intelligence
• May have weakened chest and respiratory muscles resulting in infections and pneumonia
• Hypotonia
• Decreased or absent deep tendon reflexes
• Scoliosis
• Fine tremors in fingers
• Tongue fasciculation
• Onset is typically between 3 and 15 months of age

Type III SMA (also known as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy) is the most mild form of SMA and is characterized by the following:

• May have difficulty with walking and getting up from a sitting position
• Abnormal gait
• Fine tremors in fingers
• Scoliosis
• Normal to above-average intelligence
• Onset is typically between two and 17 years of age

 

Diagnosis

Genetic testing is available to check for the defective gene found in individuals with SMA. The test, known as the SMN gene test, can detect deletions in 95% of those individuals with SMA. Another test that may be used in conjunction with the genetic blood test is an electromyography (EMG), which measures the electrical activity of the muscles. If, however, the defect is not detected with the genetic test, a muscle biopsy may be necessary to confirm the diagnosis.

 

Treatment

While gene replacement therapy and/or medication will likely one day be available for individuals with SMA, treatment at this time consists of managing the associated symptoms of the disease. In SMA Type I, a physical therapist can work to maintain range of motion and assist with other physical difficulties the child may experience. In addition, the therapist can recommend necessary equipment for positioning the child in order to keep the child comfortable. A respiratory therapist should also be consulted with, so that parents can learn how to do chest physiotherapy (CPT), which assists in clearing the child’s lungs. For children who are aspirating (either their secretions or food), a feeding tube can be used to maintain proper nutrition and protect the child from aspirating his or her meals. For those who are having difficulty breathing, a ventilator may be used.

For children with Type II and III, assistance from a physical therapist will be needed if the child is in need of a stander, orthotics (braces), or a wheelchair. Occupational therapy may be useful to assist with daily living activities including self care, skills used in school (writing, cutting, pasting), and play and leisure activities. (Of course, if the child is more severely affected, treatment methods may be similar to what was described for children with Type I.)

 

What to Expect

The prognosis for individuals with SMA varies depending on the type of SMA and the degree of respiratory function. Children with Type I, the most severe form of SMA, experience progressive weakness and encounter many respiratory infections. They typically do not live past 2 years of age.

The prognosis for children with Type II SMA varies greatly. Some individuals will learn to walk and survive into adulthood, while others may encounter similar issues to those with Type I SMA.

The prognosis varies for those with Type III as well, however, it is usually quite good. Most individuals will be able to walk and will be fully functional into adulthood.

Regardless of the type of SMA a child may have, children with SMA all have normal to advanced intelligence and it is important that they receive all available opportunities to develop their intellectual capacities to the fullest extent.

[heidi1439]

Resources

If you are interested in meeting other parents and individuals who are involved in raising a child with Spinal Muscular Atrophy, the following listserv is available:

SMA.net Public e-mail list - To subscribe, send an e-mail message to [e-mail] with any information about you and your child that you would like to share.

For more information on Spinal Muscular Atrophy, please see the following helpful references:

• Families of Spinal Muscular Atrophy http://www.fsma.org
• National Institute of Neurological Disorders and Stroke http://www.ninds.nih.gov/patients/Disorder/SMA/spinal-mus-atrophy.htm
• SMAnet http://fortuna.italia.com/smanet
• Muscular Dystrophy Association http://www.mdausa.org/research/munsat.html
• Muscular Dystrophy Association of Australia SMA Fact Sheet http://www.mda.org.au
• National Center for Biotechnology Information
  http://www3.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?253300 ;
  http://www3.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?253550 ;
  http://www3.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?253400
• Vanderbilt Medical Center http://www.mc.vanderbilt.edu/peds/pidl/neuro/spineatr.htm
• Andrew’s Buddies http://www.fightsma.com