The course of the disease is quite variable, and difficult to precisely predict from the start. Often the person who is affected can give the best prognosis, by taking into account the rate of progression and severity of their condition. In SMA Type III, weakness is slow, symmetric and progressive, and there is noted weakness in leg, hip, shoulder, arm and sometimes respiratory muscles. Patients with SMA type 3 walk independently, but may fall frequently by age two or three due to limb weakness. Sensation is normal in affected individuals.

Patients with SMA Type III who could never climb stairs without using a rail, lose walking ability by the mid-teens, however, patients who develop normal walking skills prior to the onset of weakness can maintain this ability until the third or fourth decade of life. As a rule, the later the onset of symptoms, the less severely affect an individual is.

General muscle weakness has an impact on daily living. An assessment may be helpful to identify where the person needs additional aids or adaptations to enable them to live as independently as possible. Children with SMA have enhanced intelligence and usually attend main stream school.

SMA Type III is not expected to affect an individual’s life span.

Pregnancy can cause women to experience increased muscle weakness.

A child or adult with SMA who must undergo surgery (for example, to correct scoliosis) needs to take special precautions. The surgical team, particularly the anesthesiologist, must thoroughly understand SMA

Every tissue in the body is made up of cells. Each cell has genetic material that determines how that cell (and hence our body) functions. The basic units of genetic material which are responsible for hereditary or inheritance are called genes. Individual genes cannot be seen even with powerful microscopes but the genes are located like beads on a string, on structures called chromosomes.
 

Most cases of spinal muscular atrophy are thought to be inherited in what is called an autosomal recessive manner. This means that one defective genetic message (gene) that causes the disease has come from each parent. Boys or girls can be affected.

Every person has two copies of each gene (or genetic message), one coming from each parent. People who have only one abnormal gene for a particular condition, along with a normal gene, are called carriers. They do not develop the disease, as the normal gene counteracts the effects of the abnormal gene. If a person has two abnormal genes for a particular disease, that disease will develop.

If two unsuspecting carriers of the abnormal gene for SMA have children, the chance of a child inheriting the abnormal gene from each parent and thus developing the disease, is one in four (25%). Parents who carry the abnormal gene will not be aware of this risk before their first affected child is born but thereafter they will have to consider the one in four risk of recurrence for each subsequent pregnancy.

Apart from the physical examination, in which muscle weakness and hypotonia (poor muscle tone) are found, certain other investigations have to be done to distinguish SMA from other disorders. EMG (electromyography). In this investigation a small, thin needle is put through the skin into a muscle. This needle detects the electrical currents of the muscle during both rest and activity and the result is recorded on a screen and loudspeaker.

This helps to determine if the weakness of the muscle is due to a disease in the muscle itself or if it is the consequence of lack of nerve supply (denervation), like that which occurs in SMA. Nerve conduction studies (also known as an NCV) is performed at the same time. These are performed to make sure that the sensory nerve function is normal. This involves giving individual nerves a small electric shock and recording the responses at various places along the nerve. The EMG and NCS tests are uncomfortable but not harmful.

Muscle biopsy. In this test a small piece of muscle is removed, usually from the thigh, and the specimen is examined in the laboratory for changes that are recognised to occur in SMA. This test is usually done under a general anaesthetic. Because there are complications with an anaesthetic, a punch muscle biopsy is also available for children and is becoming more readily available.

A combination of the results from these investigations along with the history and clinical examination establishes the diagnosis of spinal muscular atrophy. The distinction between the different types of SMA is not made from the tests but the age at which symptoms begin and the progress of the disorder over time gives clues

Whilst there is currently no cure for SMA, proper management of the condition is very important. Particularly for children who are still growing, so that any potential problems can be picked up promptly and any necessary treatment put into practice.

Physiotherapy is very important, it is possible to build up an exercise routine that can be carried out at home regularly between visits to a clinic. Swimming is also a good way of getting gentle exercise. Your GP should be able to refer you to a physiotherapist, or ideally if possible one of the neuromuscular centres where there are specially trained physiotherapists, Occupational Therapists and other health care professionals.

Scoliosis (curvature of the spine) occurs in some children with Type III SMA. The degree of the scoliosis will be a factor in deciding how to treat it. Again it is important that children are monitored regularly so that any potential problems are picked up early. (If in doubt ask for referral to an orthopaedic specialist). Respiratory infections should be treated promptly, or prevented where possible. As the chest muscles are also affected excess weight will add to any problems so it is important to follow a sensible diet and exercise as much as you are able. Excessive weight puts an unnecessary strain on muscles and makes mobility more difficult. Good eating habits help contribute to strong minds and strong bodies. Ask your GP to refer you to a dietician if you are worried or want advice.

There is no reason why a child or young adult with SMA Type III should not be encouraged to lead as full and active a life as their friends who do not have the condition. They may tire easier or find games and physical activity difficult but should be encouraged to do as much as they feel able. Some adaptations may be required both in the home and school to make life easier. As with all children, it is very important that children with SMA are assisted in reaching their utmost potential.