What is Spinal Muscular Atrophy?
SMA is a degenerative motor neuron disease. SMA affects infants, children and adults worldwide. It is suggested that incidence of SMA occurs one in 10,000 births. It is a complex disease affecting individuals across the lifespan for which to date, there is no known cure.
SMA is a genetic condition whereby the nerves of the spinal cord (anterior horn cells) deteriorate, atrophy (waste away) and eventually die. Put simply, SMA not only wastes away the muscles you see like arms and legs, but internal muscles used for coughing, swallowing and breathing.
With SMA, the nerve cells of the spinal cord do not have the genetic programming that enables them to send messages effectively or efficiently to the muscles surrounding the spinal cord. Because the muscles are not stimulated, they become weak and the muscle cells “atrophy”, or die from lack of use. Muscle weakness becomes worse over time until the muscles no longer respond. Muscles closest to the trunk (shoulders, hips and back) are often the most severely affected, with leg weakness usually more pronounced than in the arms.
Key life functions such as swallowing and feeding can be affected. Because of the involvement of respiratory muscles used for breathing and coughing, those with SMA are at significant risk of contracting pneumonia or experiencing other lung problems. Children with SMA who contract pneumonia face a great battle for life. However the use of a cough assist machine to help clear secretions has vastly improved the recovery time in many patients, and also extended life expectancy.
Intelligence is unaffected and many health professionals are impressed that SMA children are very bright and alert, and are very interested in the people and activities going on around them. Children with SMA can experience normal feelings of touch, temperature, sensation and pain and otherwise appear normal in every way, except that they are extremely weak. They often attend mainstream schools, hold down jobs and have every chance of a normal relationship once an adult.
What causes Spinal Muscular Atrophy?
SMA is a genetic disease caused by abnormalities by the SMN1 gene (the Survival Motor Neuron 1 Gene). The SMN1 gene is located on chromosome 5. If the SMN1 gene has reduced or no function, then the motor neurons in the spinal cord and brainstem do not survive, and gradually die off.
One in 35 Australians carry the faulty gene. Being a carrier does not mean you are affected by the disease. A SMA baby is born when BOTH parents are carriers of this gene match up and have a child. In this case there is usually a 1 in 4 chance of having a baby with SMA. Parents of SMA children are usually unaware they carry the faulty genes and often have not heard of SMA until their child has been diagnosed.
If only one parent carries the faulty gene there is usually only a one in 2,500 that you will have a child affected with SMA. In some cases, an SMA Child is affected where both parents do not carry the gene.
Individuals with SMA have inherited two faulty copies of this gene, one from each parent. Two carriers of the faulty SMA gene will have a:-
- 25% chance of having a child in each pregnancy affected by SMA,
- 50% chance of a child who is an unaffected carrier, and
- 25% chance of a child who does not have SMA and is not a carrier.
How many types of Spinal Muscular Atrophy are there?
SMA is a very complex disease and each child, and each set of parents face their own unique journey and their own unique decisions, as SMA effects each person so very differently. It is important to care for the person in a way that best meets their specific needs and those of their family.
There are four clinical classifications of SMA:-
- Type 1 (Infantile SMA or Werdnig-Hoffman disease) – see Type 1 information page
- Type 2 (Intermediate SMA or chronic childhood SMA) – see Type 2 information page
- Type 3 (Juvenile SMA or Kugelberg-Welander disease) – see Type 3/4 information page
- Type 4 (Adult onset Spinal Muscular Atrophy)
- SMARD – Spinal Muscular Atrophy Respiratory Distress
How is Spinal Muscular Atrophy diagnosed?
Diagnosis of SMA is usually confirmed by blood test, which identifies the presence or absence of the SMN1 gene. The SMA type is then identified according to age and severity of symptoms, that is the younger the child the more severe the condition. Testing accurately diagnosis SMA in 95% of cases, although the rarer SMA variants that do not have deletion of SMN1 may not be diagnosed until further investigation occurs.
Parents tend to raise concerns about noticeable weakness or reduced function in their child’s limbs. A physical examination reveals loss of, or a reduction of tendon reflexes in the legs and weakness in both sides of the body (especially the legs). Other symptoms are fine twitches of the muscles in the limbs and tremor (fasciculation) of the tongue.
Classification and typical clinical features of SMA are listed in Table 1.
Apart from these; Type 4 SMA is also referred to, this is a mild form that usually presents in adulthood.
Some patients will manifest features that are at the margins between groups.
Table 1. Clinical classification of SMA
|SMA TYPE||AGE OF
|<2 years||Profound weakness and hypotonia, impaired head control, weak cry and cough, difficulty with swallowing and handling of oral secretion, early morbidity due to respiratory insufficiency and aspiration pneumonia.|
|>2 years||Delayed motor milestones, poor weight gain, weak cough, fine hand tremors, joint contractures and scoliosis.|
|Type 3 (mild)
|Adult||Variable muscle weakness and cramp, joint overuse, loss of walking ability at some point in life.|
The most severe form of SMA type I is sometimes termed SMA type 0 (or severe infantile SMA) and is diagnosed in babies that are born so weak that they can survive only a few weeks even with intensive respiratory support. SMA type 0 should not be confused with SMARD1 which may have very similar symptoms and course but has a different genetic cause than SMA.
Is there a treatment for Spinal Muscular Atrophy?
Unfortunately, there is no cure for SMA. Scientists have not yet been able to stop or reverse the nerve and muscle death or repair faulty genes.
To date there are no evidence based guidelines to inform parents/carers in the care of those affected by SMA, so the information and practise of health care professionals may vary. However, experts around the world have produced a document termed “Consensus Statement for Standard Care in Spinal Muscular Atrophy” (Wang et al 2007). This document provides guidelines on how to best care for those affected with SMA. It is written by experts who have agreed, based on their experience and the available research evidence, on what is the most appropriate management of those living with SMA.
Importantly, the consensus statement suggests that rather than treat a person based on the type of SMA they have, it is more appropriate to provide care and treatment based on what they can do or their function.
Function is described as:-
- Non sitter – cannot sit independently
- Sitter – can sit but not walk independently
- Walker – can walk independently
By managing the body’s systems (such as the respiratory or gastrointestinal system) according to the child’s level of function (as sitter, non sitter or walker) care and outcomes are better for the child.
The consensus statement recommends:-
- Coordinated multidisciplinary care is offered by skilled professionals, such as neurologists or geneticists, occupational therapists, physiotherapists, nutritionist dieticians, speech pathologists, gastroenterology and pulmonary specialists for example as soon as such information is needed.
- Parents need information that will assist them to understand the disease process, the intervention and treatment options available to them and the likely prognosis for their child.
- Care be coordinated with the family.
- Families have access to resources and information about SMA or access to support services or self help groups.
- At times, that choosing to intervene early (e.g. by inserting a nasogastric or feeding tube earlier than necessary) may be better handled by the child and prevent complications that may occur when the need for the intervention is greatest.
- Families have access to information about clinical trials,
- That the child be assessed by an experienced clinician every three months (and more frequently in clinically unstable non sitters).
- Parents and families need to discuss, agree and document the decisions they need to make should they unexpectedly face a life-threatening situation. This will allow them to respond appropriately if they are faced with an emergency.
- Parents and families are provided treatment options and the information is presented in a balanced, open, honest and fair manner and is based on the best available evidence.
- Parents and families are supported with their choices.
- Parents and families discuss, describe and document end-of-life decisions, to prevent delayed actions or a forced choice during a time of crisis.
 TREAT-NMD Neuromuscular Network. 2007. Standards of Care for Spinal Muscular Atrophy. United Kingdom.