SMA in Scientific Terms

DNA Strand Graphic

To see the actual medical textbook explanation of each of these types, click on the hyperlinks below associated with each type.

Spinal Muscular Atrophy (SMA) is one of the neuromuscular diseases. Muscles weaken and waste away (atrophy) due to degeneration of anterior horn cells or motor neurons which are nerve cells in the spinal cord. Normally, these motor neurons relay signals, which they receive from the brain, to the muscle cells. When these neurons fail to function, the muscles deteriorate. SMA effects the voluntary muscles for activities such as crawling, walking, head and neck control and swallowing.

SMA mainly affects the proximal muscles, or in other words the muscles closest to the trunk of the body. Weakness in the legs is generally greater than weakness in the arms. Some abnormal movements of the tongue, called tongue fasciculation's may be present in patients with Type I and some patients with Type II. The brain and the sensory nerves (that allow us to feel sensations such as touch, temperature, pain etc.) are not affected. Intelligence is normal.  In fact it is often observed that patients with SMA are unusually bright and sociable.

Type 1 Severe Infantile SMA, or Werdnig-Hoffman disease

  • Infantile spinal muscular atrophy (Werdnig-Hoffman disease) is the most severe form of SMA. It usually becomes evident in the first six months of life. The child is unable to roll or sit unsupported, and the severe muscle weakness eventually causes feeing and breathing problems. There is a general weakness in the intercostals and accessory respiratory muscles (the muscles situated between the ribs). The chest may appear concave due to the diaphragmatic breathing.  These children usually do not live beyond about 24 months of age.

Type 2 Intermediate type (this does not have a hyperlink so it is spelled out below instead.)

  • What are the features of intermediate (type 2) SMA?

    A child with the intermediate form of SMA often reaches six to twelve months of age, sometimes later, and learns to sit unsupported, before symptoms are noticed. Weakness of the muscles in the legs and trunk develops and this makes it difficult for the child to crawl properly or to walk normally, if at all. Weakness in the muscles of the arms occurs as well although this is not as severe as in the legs. Usually the muscles used in chewing and swallowing are not significantly affected early on. The muscles of the chest wall are affected, causing poor breathing function. Parents notice that the child is "floppy" or limp, the medical term for this being hypotonia.  Tongue fasciculations are less often found in children with Type II but a fine tremor in the outstretched fingers is common. Children with Type II are also diaphragmatic breathers.  Physical growth continues at a normal pace and, most importantly, mental functions is not affected. The children are bright and alert and it is important that they receive all the available opportunities to develop their intellectual capacities to their fullest extent. Integration into a normal school environment gives them the best chance to mature intellectually and emotionally.

    What does the future hold?

    The course of the disease is quite variable, and difficult to precisely predict from the start.  Children with the intermediate form of SMA usually sit unsupported. Weakness of the legs and trunk usually, but not always, holds the child back from standing and walking alone. Sometimes the muscle weakness can seem to be non-progressive, but in most cases weakness and disability will increase over many years. Severe illness with prolonged periods of relative immobility, putting on excessive weight or growth spurts may contribute to deterioration in function.  Due to weakness of the muscles supporting the bones of the spinal column, scoliosis (curvature of the spine) often develops in children who are wheelchair bound. If this becomes severe it can cause discomfort and can have a bad influence on breathing function as well. An operation can be done to straighten the spine and prevent further deterioration.  Recurrent chest infections may occur, because of decreased respiratory function and difficulty in coughing. Parents will have been shown how to encourage their child to maintain his/her maximum respiratory function as well as how they can perform postural drainage of the chest. They should start this as the first sign of any chest problem. Antibiotics and inhalation therapy may also be needed. Sometimes hospitalization is required to best manage and care for the child.  The long term outlook depends mainly on the severity of weakness of the muscles of the chest wall and on the development of scoliosis. Lifespan is always difficult to predict. Mildly affected children may live into adult years. The more severely affected children may die, due to pneumonia and other chest problems, before or in their teens.

Type 3 Mild Juvenile SMA, or Kugelberg-Welander disease

  • Juvenile spinal muscular atrophy (Kugelberg-Welander disease) usually has its onset after 2 years of age. It is considerably milder than the infantile or intermediate forms. In juvenile spinal muscular atrophy children are able to walk, although with difficulty.  The patient with Type III can stand alone and walk, but may show difficulty with walking and/or getting up from a sitting or bent over position. With Type III, a fine tremor can be seen in the outstretched fingers but tongue fasciculations are seldom seen.

Type 4 Adult Onset

  • Typically in the adult form symptoms begin after age 35. It is very rare for Spinal Muscular Atrophy to begin between the ages of 18 and 30. Adult SMA is characterized by insidious onset and very slow progression. The bulbar muscles, those muscles used for swallowing and respiratory function, are rarely affected in Type IV.

Type 5 Kennedy's Syndrome or Bulbo-Spinal Muscular Atrophy

  • This form also known as Adult Onset X-Linked SMA, occurs only in males, although 50% of female offspring are carriers. This form of SMA is associated with a mutation in the gene that codes for part of the androgen receptor and therefore these male patients have feminine characteristics, such as enlarged breasts. Also noticeably affected are the facial and tongue muscles. Like all forms of SMA the course of the disease is variable, but in general tends to be slowly progressive or non-progressive.


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