Spinal Muscular Atrophy (SMA) is a motor neuron disease. The motor neurons affect the voluntary muscles that are used for activities such as crawling, walking, head & neck control, and swallowing. It is a relatively common "rare disorder": approximately 1 in 6000 babies born are affected, & about 1 in 30 people are genetic carriers. Each child born to carriers has a 25% chance of having SMA, 50% carriers, 25% uneffected. SMA affects muscles throughout the body, although the proximal muscles (those closest to the trunk of one’s body - i.e. shoulders, hips, & back) are often most severely affected. Weakness in the legs is generally greater than in the arms. Sometimes feeding & swallowing can be affected. Involvement of respiratory muscles (muscles involved in breathing & coughing) can lead to an increased tendency for pneumonia & other lung problems. Intellectual activity is normal and it is often observed that patients with SMA are unusually bright & sociable.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 which 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.
Type IV
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.