Spinal muscular atrophy: the intermediate form
What is spinal muscular atrophy?
Spinal muscular atrophy (SMA) is one of the neuromuscular diseases. Muscles weaken and waste away (atrophy) due to degeneration of motor neurones, which are nerve cells in the spinal cord. Normally, these motor neurones relay signals, which they receive from the brain, to the muscle cells. When these neurones fail to function, the muscles deteriorate. The brain and the sensory nerves (that allow us to feel sensations such as touch, temperature, pain etc.) are not affected. Intelligence is normal. From the description just given it can be seen why the condition is called spinal muscular atrophy — the muscles atrophy because of a problem in the nerve cells in the spine.
What are the common types of SMA?
In general we can distinguish 3 common types of SMA in childhood.
- Type 1: Severe infantile SMA, or Werdnig-Hoffmann disease.
- Type 2: Intermediate type.
- Type 3: Mild juvenile SMA, or Kugelberg-Welander disease.
Infantile spinal muscular atrophy (Werdnig-Hoffmann disease)
This is the most severe form of SMA. It usually becomes evident in the first 6 months of life. The child is unable to roll or sit unsupported, and the severe muscle weakness eventually causes feeding and breathing problems. These children usually do not live beyond about 18 months of age.
Juvenile spinal muscular atrophy (Kugelberg-Welander disease)
This 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.
Intermediate spinal muscular atrophy
This is situated somewhere between the infantile and juvenile types in its age of onset and severity.
What are the features of intermediate (type 2) SMA?
A child with the intermediate form of SMA often reaches 6 to 12 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. Physical growth continues at a normal pace and, most importantly, mental function 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 at the first sign of any chest problem. Antibiotics and inhalation therapy may also be needed. Sometimes hospitalisation 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.
Is SMA inherited?
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 heredity 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 2 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 2 abnormal genes for a particular disease, that disease will develop. If 2 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 4 (25 per cent). 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 4 risk of recurrence for each subsequent pregnancy.
What can be done?
Physiotherapy and encouragement of activity can help the child learn to move, to strengthen unaffected muscles and to prevent deformities. Physiotherapists and parents work together to achieve these aims. Swimming is an excellent and pleasant way to move all limbs and the body in an attempt to keep the muscles strong and to maintain breathing capacity. Hence parents are encouraged to give their child the opportunity to participate in and enjoy water based activities from an early age.
Some children have sufficient power to achieve a standing position with the help of callipers, while appropriate splints sometimes enable the child to walk. To use supporting sticks good arm strength is necessary and only a few children with the intermediate form of SMA are able to do this.
Parents of young children who have difficulty sitting and who are unable to walk will often need assistance in finding stable positions for play and mobility. An appropriate buggy (stroller) can be obtained while later on a small wheelchair will be more appropriate. A motorised wheelchair can be considered as early as 3 years of age and occasionally younger.
The time a child requires a motorised wheelchair is often regarded as a time of crisis. While this may be a particularly difficult time for the parents it often brings exhilaration for the children who, for the first time, have gained a degree of freedom, and are allowed independent access to places and things previously beyond their capabilities.
After becoming wheelchair bound, children are prone to develop progressive scoliosis (curvature of the spine). The physiotherapist will discuss correct posture as a preventative measure and bracing is sometimes used to minimise the progression of curvature. However surgery is often required to overcome the problem.
Contact will also be made with an occupational therapist who has an understanding of the problems associated with activities of daily living and is able to develop ways to overcome these problems.
Often problems occur in managing activities such as:
- self-care activities including dressing, bathing, toileting and car transportation;
- kindergarten and school activities including writing, cutting, pasting and managing in the school yard;
- play and leisure activities.
To help overcome any problems the occupational therapist may:
- teach the child a different way of performing an activity;
- provide adapted equipment e.g. large handled cutlery, specially designed scissors or a bath chair;
- recommend modifications to the home or school e.g. installation of ramps, widening of doorways.
The occupational therapist will also work with the kindergarten teacher or school teacher to ensure successful integration to kindergarten or school.
There is a tendency for children with muscle weakness to becomes obese (overweight) because of reduced energy requirements at a time when appetite remains normal. As obesity means that the muscles carry a larger than normal load, prevention of excessive weight gain is important.
As well as these features, when the child is examined medically there will be absence of the muscle tendon reflexes. The muscles of the tongue may show small twitchy movements called fasciculations. There is sometimes a fine tremor of the hands. These findings are useful in making a diagnosis.
Because the brain and sensory nerves are not affected the child is usually bright, alert and developing well in other respects. The condition of SMA does not cause pain. However an affected child may have the usual 'aches and pains' that are common to all children.
The disease is rather variable in its severity so that one child diagnosed as having intermediate SMA may be more or less severely affected than another child of the same age with the same diagnosis.
What investigations have to be done to establish the diagnosis?
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. 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.
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.
Can type 2 SMA be cured?
At this time there is no cure for any types of SMA but this does not mean that nothing can be done. Much can be done to help the child develop to the greatest possible extent and to assist families to manage their child's problems.
In the early stages there are not many physical demands on parents beyond those encountered in caring for a child without disability, but later the family may need extra information, support and practical assistance.
Of course the greatest source of support and encouragement usually comes from the child's own family and friends. It is the family who will develop over time their own knowledge of the child and the best ways to deal with issues that arise. However extra help is often required and is available through a variety of sources such as doctors, therapists, social workers, counsellors, teachers and the Muscular Dystrophy Association.
Brothers and sisters who are not clinically affected have a 2 in 3 chance of being carriers of one defective SMA gene. They will not pass the disease onto their own children except in the very unlikely event that their partner also is a carrier of the abnormal gene. This is more likely to happen if there is consanguinity; that is, if the partner is a relative.
The precise location on the chromosomes of the defective SMA gene has been identified and antenatal diagnosis early in pregnancy is now possible for many families. Advice should be sought on the options for antenatal testing before a pregnancy occurs. SMA can sometimes be inherited in ways other than the autosomal recessive method described but this is uncommon. Parents should discuss these issues with the neurologist or paediatrician and they may be referred to a clinical geneticist for more detailed genetic counselling.
Where to go with any questions
This is only a brief overview description of SMA type 2. Many questions and uncertainties will remain. Don't hesitate to ask your doctor, physiotherapist, occupational therapist, social worker or any other health worker concerned with your child, for further information and/or support. The Muscular Dystrophy Association (MDA) is an organisation for people with nerve and muscle disorders including the spinal muscular atrophies. The MDA supports research, parent groups and support for teachers.
Acknowledgements
The contributions to the content of this document by Ms Jane Sullivan, Ms Gail Smith, and Mrs Robyn Sheppard are gratefully acknowledged.
Helene De Vaan and Lloyd Shield, Department of Neurology, Royal Children's Hospital, Melbourne.
Last Reviewed: 13 June 2007
