Spinal Muscular Atrophy
What is spinal muscular atrophy (SMA)?
Spinal muscular atrophy is a degenerative problem that affects the motor nerves, resulting in muscle wasting and weakness. Spinal muscular atrophy occurs in approximately one in 6,000-10,000 live births.
What causes spinal muscular atrophy?
SMA is an autosomal recessive disease. This means that two abnormal copies of the gene, one inherited from each parent, are necessary to have the condition. Boys and girls are affected with equal frequency. A child who inherits only one abnormal gene copy is a carrier and is not at risk to develop symptoms. When both parents are carriers, there is a one in four, or 25 percent, chance with each pregnancy, to have a child with SMA. Carrier testing of parents can help determine the recurrence risk in a specific family.
A gene called survival motor neuron (or SMN) is found to have an abnormal area (called a deletion) in over 95 percent of cases of SMA. Symptomatic individuals of all ages can be tested through DNA studies typically done from a blood sample.
What are the symptoms of spinal muscular atrophy?
Spinal muscular atrophy is sometimes difficult to diagnose, as symptoms can resemble other conditions or medical problems. Each child may experience symptoms differently. There are several types of spinal muscular atrophy based on symptoms and age of onset.
Type I (Werdnig-Hoffman)
This is the most severe type of SMA, and unfortunately, the most common. Symptoms may be present at birth or develop within the first few weeks or months after birth. Infants have difficulty holding up their head, sucking, feeding, swallowing and often move very little. The legs are more severely affected than the arms. The muscles of the chest that help to expand the lungs are affected, and the chest may appear small or "bell-shaped". They have a weak cough, and are prone to respiratory infections. The tongue may demonstrate "worm-like" movements, and they may demonstrate a tendency to choke while feeding. Complications from breathing problems often lead to death or dependence on some form of respiratory support by 2 to 3 years of age.
Type II (Intermediate Form)
This form of SMA most commonly becomes evident in children between 6 months to 2 years of age. They may show delays in acquiring motor skills such as rolling, sitting or crawling. They are unable to walk independently without support. They typically have generalized muscle weakness and may require braces, walkers or a wheelchair for assistance. Life expectancy varies greatly in this group of children, since they demonstrate a very wide range in degree of weakness. However, complications commonly include weakness of chest muscles involved in breathing, resulting in a weak cough and tendency for pneumonia. Scoliosis develops in virtually all children at some point, and they are prone to bone fractures.
Children who are unable to bear weight often develop hip dislocation. Contractures of the muscles and joints can limit function over time. Children in this group may also demonstrate difficulties in swallowing and chewing and require close monitoring of nutrition. Lifespan in this group depends on the severity of respiratory muscle weakness, but many children survive well into adulthood.
Type III (Kugelberg-Welander)
This form of SMA most commonly becomes evident in children between 2 and 17 years of age. These children may show delays in motor development, difficulty walking, trouble getting up from the floor, mild muscle weakness and frequent falls. Fatigue can be a significant problem, which limits the ability to walk long distances. A tremor involving the hands is common. Scoliosis is frequent in later childhood. Respiratory muscle involvement is much less often a problem, and difficulty swallowing is rare.
This form of SMA includes those individuals who don't develop symptoms of weakness until they reach adulthood. Usually, this results in muscle weakness predominantly affecting the legs and manifests as a walking disability. The symptoms of spinal muscular atrophy may resemble other problems or medical conditions and can be confused with other muscle or nerve conditions, including muscular dystrophy, myopathy, other spinal muscular atrophy variants or even forms of amyotrophic lateral sclerosis (ALS). Another closely related condition, known as spinobulbar muscular atrophy (Kennedy's disease), can also present in late childhood or adulthood.
How is spinal muscular atrophy diagnosed?
The diagnosis of spinal muscular atrophy may be suspected if you or your child demonstrate specific symptoms or demonstrate signs on examination that are consistent with the pattern of weakness seen in this disorder. During the physical examination, your child's physician will obtain a complete medical history, and he/she may also ask if there is a family history of any medical problems.
Diagnostic tests that can help to confirm the diagnosis of spinal muscular atrophy include the following:
Genetic testing can confirm a suspected diagnosis in most cases. Sometimes the initial genetic test is negative, and additional genetic testing or other testing may be needed to confirm a diagnosis.
An EMG is a test that measures the electrical activity of a muscle or a group of muscles. An EMG can detect abnormal electrical muscle activity due to diseases and neuromuscular conditions.Muscle biopsy
A small sample of the muscle is removed and examined to determine and confirm a diagnosis or condition.MRI or imaging studies
Imaging studies of the brain or spine to help rule out other conditions
While there are as yet no specific, confirmed pharmaceutical therapies that can either extend lifespan or increase strength in SMA subjects, the identification of compounds that can increase SMN protein in cells from SMA subjects and in genetic animal models of SMA is promising. Moreover, proactive management strategies to optimize lung function, physical mobility and nutrition can help preserve motor function, improve quality of life and extend survival, particularly in more severely affected SMA infants and young children.
Following diagnosis in children with milder forms of SMA, including the infant with an as yet uncertain prognosis, it is important to work closely with parents in order to anticipate problems and pursue management aggressively to optimize outcomes. Because of the tremendous variability in severity of muscle weakness, an individualized approach is often necessary. Far too often, respiratory, nutritional and even physical rehabilitation interventions are reactive rather than proactive. Treatment for the intermediate or milder forms of SMA should be focused on preserving mobility and minimizing respiratory complications, particularly restrictive pulmonary disease or respiratory compromise due to progressive scoliosis. Specific interventions can be helpful in optimizing the individual's health and helping to maintain motor function.
Find information, guides and more resources on each of the following topics to aid you in planning a management strategy:
- Exercise & Mobility
- Special Considerations in SMA Infants
- Dental Care
Proven treatments for SMA may soon become a reality as we gain a better understanding of disease pathogenesis. Clinical trials to assess compounds that increase SMN protein levels in cell and animal models have begun. In the meantime, a proactive management strategy can help to limit disease progression and optimize outcome.
Undoubtedly, intervening as early as possible in the disease process will prove most effective as additional therapeutic strategies are identified. Participation in clinical trials can provide patients and families with additional clinical care and monitoring that can help to ensure that proactive care is the rule, rather than the exception. Participation is also vital to the community to help demonstrate the effectiveness of potential treatments. However, a careful consideration of the potential risks as well as the time commitment involved should be carefully considered in each case.
View SMAoutcomes.org, an online interactive manual for physical therapists performing the Modified Hammersmith and Modified Hammersmith Extend physical therapy assessments, fully funded and made possible by Families of SMA.