About SMA

Spinal Muscular Atrophy (SMA) refers to a group of inherited diseases of the motor nerves that cause muscle weakness and atrophy (wasting). The motor nerves arise from the spinal cord and control the muscles that are used for activities such as breathing, crawling, walking, head and neck control, and swallowing. SMA is a rare disorder affecting approximately 1 out of every 6,000 births.

SMA affects muscles throughout the body. In the most common types, weakness in the legs is generally greater than in the arms. Usually feeding, swallowing, and respiratory function (e.g., breathing, coughing, and clearing secretions) can be affected. When the muscles used for breathing and coughing are affected and weakened, this can lead to an increased risk for pneumonia and other respiratory infections, as well as breathing difficulty during sleep. The brain’s cognitive functions are not affected. People with SMA are generally grouped into one of four types (I, II, III, IV) based on their highest level of motor function or ability.

Understanding SMA:

SMA is an autosomal recessive genetic disease. One out of every 40 people are genetic carriers of the disease (meaning that they carry the mutated gene but do not have SMA). In order for a child to be affected by SMA, usually both parents are carriers of the abnormal gene and pass this gene on to their child. Thus the child has 2 abnormal copies of the gene, one from each parent, and this is termed a recessive genetic disease. When both parents are carriers, the possibility of a child inheriting the disorder is 1 in 4, with each pregnancy.

SMA is caused by a missing or abnormal (mutated) gene known as survival motor neuron gene 1 (SMN1). In a healthy person, this gene produces a protein in the body called survival motor neuron (SMN) protein. In a person with mutated genes, this protein is absent or significantly decreased, and causes severe problems for motor neurons. Motor neurons are nerve cells in the spinal cord which send out nerve fibers to muscles throughout the body. Since SMN protein is critical to the survival and health of motor neurons, nerve cells may shrink and eventually die without this protein, resulting in muscle weakness. As a child with SMA grows, it is difficult for his/her weakened muscles to keep up with the demands of daily activities. The resulting weakness can also lead to bone and spine changes that may cause breathing problems and further loss of function.

There are four types of SMA: Type I, II, III, and IV. The determination of the type of SMA is based upon the physical milestones achieved.

Type I

SMA Type I is also called Werdnig-Hoffmann Disease. The diagnosis of children with this type is usually made before 6 months of age, depending on the severity of their disease. Usually children with SMA Type I have poor head control and are not able to accomplish developmentally-expected motor skills. The hallmark feature of SMA Type I is that children are unable to sit or stand without help. Swallowing and feeding will become difficult for children with this type of SMA and they will eventually lose the ability to swallow safely without aspirating (choking or inhaling secretions and food particles into the lungs). Children will eventually require a feeding tube.

Children with SMA Type I will experience weakness of the muscles used for breathing, those that help expand the chest and fill the lungs with air. The chest is smaller than usual in these children and they begin to breathe using primarily their stomach muscles (belly breathing). The lungs do not fully develop due to this type of breathing, and coughing becomes very weak.

Type II

The diagnosis of SMA Type II is almost always made before 2 years of age. Children with this type have delayed motor milestones and display a range of physical abilities. The hallmark feature of SMA Type II is the ability to maintain a seated position unsupported. With assistance and bracing these children may be able to stand, but are unable to walk and require a wheelchair to get around. Children with SMA Type II usually do not have swallowing problems, but this can vary from child to child. Some children may have difficulty eating enough food by mouth to maintain their weight and grow, and a feeding tube may become necessary. Children with SMA Type II may also develop weakness of the muscles used for breathing and experience difficulty coughing. Some of these children may require equipment to help them breathe easier at night.

Type III

SMA Type III is typically diagnosed by 3 years of age, but can be diagnosed as late as the teenage years. The hallmark feature of SMA Type III is the ability to stand and walk independently. Affected individuals may have difficulty walking, running, and climbing stairs as they get older; some will lose the ability to walk independently in childhood, while others may remain ambulatory into adolescence or adulthood. Problems with the spine (scoliosis) may develop at various rates and ages. Swallowing and coughing difficulties, along with breathing difficulty at night, may occur but do so less commonly and later in the disease course than in SMA Type II. Children and adults with SMA Type III are at risk of becoming overweight, as they are not usually able to be extremely physically active. Fine shaking of the fingers and hands (tremors) can be seen in this type of SMA, and symptoms of joint aches and overuse frequently occur. Curvature of the spine may occur and can be treated with a brace and in some instances surgery. As in all SMA Types, bones become weak and may break easily, and a variety of therapeutic supports are available to help position individuals and maintain mobility.

Type IV (Adult Onset)

In the adult form of SMA, mild to moderate symptoms usually begin in the second or third decade of life, typically after the age of 35, although they may occur as early as 18 in some cases. Adult onset SMA is much less common than the other forms. It is typically characterized by mild motor impairment such as muscle weakness, tremor, and twitching, with or without respiratory problems. Weakness is gradual and the muscles used for swallowing and breathing are rarely affected in Type IV. Life expectancy is normal and therapeutic supports are available to help maintain optimal function for individuals with SMA Type IV