Spinal muscular atrophy
Introduction
Spinal muscular atrophy (SMA) is a genetic disease that causes muscle weakness and a progressive loss of movement. There is no cure, but therapy and support are available to help manage the condition.
SMA causes the motor neurones in a certain area of the spinal cord to deteriorate. This results in progressive muscle wasting and loss of ability to move parts of the body. See SMA - symptoms for more information.
Motor neurones
Motor neurones are nerve cells that connect the brain and spinal cord to the body’s muscles. The motor neurones send out electrical signals to the muscles, which tell the muscles when to contract (shorten) and relax.
In people with SMA, the motor neurones deteriorate, causing the link between the brain and muscles to break down gradually.
As the link between the brain, spinal cord and muscles breaks down, the muscles that are used for activities such as crawling, walking, sitting up and moving the head are used less and less and become weaker or shrink (atrophy).
Treatment and support
The management of SMA currently focuses on the symptoms and providing supportive care. For example, breathing exercises and support can help to reduce the risk of chest infections, and a feeding tube may be needed for babies that are unable to swallow. See SMA - treatment for more information.
How common is SMA?
In the UK, it is estimated that there are between 5,500 and 6,000 people with SMA at any one time.
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Symptoms
Spinal muscular atrophy (SMA) affects a person’s physical abilities, such as moving, walking and breathing, but does not affect their mental development.
SMA can affect the muscles throughout the body, but it is the proximal muscles (those closest to the centre of the body, such as the shoulders, hips and back) that are usually the most seriously affected.
The proximal muscles are usually affected sooner than the muscles that are further away from the centre of the body. For example, the thigh muscles usually become weaker before the muscles of the lower legs and feet.
Weakening and atrophy (shrinking) of the muscles around the chest and abdomen can cause skeletal deformities, such as scoliosis of the spine (see below).
SMA can also affect the muscles that are involved in breathing and coughing. If they are affected, there is an increased risk of fatal respiratory problems developing, such as pneumonia.
The symptoms of the different types of SMA are outlined in more detail below.
Childhood SMA
Type 0 (develops before birth)
Type 0 SMA is a very severe form of SMA that develops before birth. It causes reduced movement of the foetus, which is usually first noticed between weeks 30 and 36 of the pregnancy.
Babies with type 0 SMA are hardly able to move and have problems breathing and swallowing.
Type I
Babies with type I SMA (known as Werdnig-Hoffmann disease) also have severe muscle weakness, which can cause problems with moving, eating, breathing and swallowing.
Symptoms may be apparent at birth or during the first few months of life. They usually occur before the baby is six months old.
The muscles of babies with type I SMA are thin and weak, which makes their limbs limp and floppy. They are usually unable to raise their head or sit without support.
Breathing problems are caused by a weakness in the baby’s chest muscles, and difficulty swallowing can be made worse by the tongue shrinking and weakness of the muscles in the throat.
Most children who have type I SMA are unlikely to live past their second birthday.
Type II
Type II SMA usually develops when an infant is 6–18 months old. Infants have less severe symptoms than those with types 0 and I, but will usually become weaker over time.
In time, infants with type II SMA will usually be able to sit, but it is unlikely that they will be able to stand or walk unaided.
Infants with type II SMA may also have the following symptoms:
- breathing problems
- floppy arms and legs
- reduced or absent tendon reflexes
- twitching of the muscles in the arms, legs or tongue
They may also develop deformities of the hands, feet and chest, and their joints may be affected by shortening of the muscles, reducing limb movement.
As they grow and develop, most children with type II SMA will develop scoliosis of their spine. Scoliosis is an abnormal curvature of the spine to one side. It occurs as a result of the muscles that support the bones of the spine becoming weaker.
Type III
Type III SMA (known as Kugelberg-Welander disease) is the mildest form of childhood SMA. However, the symptoms and their severity can vary greatly.
The condition often develops later than types I and II, with a diagnosis usually only being made after the age of two.
The symptoms of type III SMA usually develop between early childhood (after one year old) and early adulthood.
Most children with type III SMA are able to stand unaided and walk, although many find walking or getting up from a sitting position difficult. They may also have:
- balance problems
- an abnormal way of walking
- difficulty running or climbing steps
- a slight tremor (shaking) of their fingers
Over time, the muscles of children with type III SMA will become weaker, and those who are able to walk may be unable to do so when they get older.
Breathing and swallowing difficulties are not as common as in other types of SMA.
Adult SMA
Type IV
Type IV SMA begins in adulthood. The symptoms are usually mild to moderate and start to develop after the age of 35. The symptoms usually progress very slowly.
In most cases, muscle weakness initially affects just one side of the body, before eventually progressing to both sides. Symptoms may include:
- muscle weakness in the hands, feet and tongue
- muscle tremor (shaking) and twitching
- speech impairment
Kennedy’s syndrome
Kennedy's syndrome, or spinal-bulbar muscular atrophy (SBMA), is another type of adult SMA.
SBMA only affects men. It usually develops between the ages of 20 and 40, although it can sometimes affect teenage boys and those aged over 40.
The initial symptoms of Kennedy’s syndrome may include:
- tremor (shaking) of the hands
- muscle cramps on exertion
- muscle twitches
- weakness of the muscles of the limbs
As the condition progresses further, other symptoms may be experienced, including:
- weakness of the facial and tongue muscles, which may cause difficulty swallowing (dysphagia) and slurred speech
- recurring pneumonia (inflammation of lung tissue)
In some cases of Kennedy’s syndrome, the following symptoms may occur:
- enlargement of the male breasts (gynecomastia)
- a low sperm count or infertility
- diabetes mellitus, an excess of glucose (sugar) in the blood
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Causes
Spinal muscular atrophy (SMA) is caused by one or more faulty genes. It is usually inherited, which means the genes can be passed on from parents to children.
The genes that are affected normally produce a protein that is needed by the motor neurones. Motor neurones are nerves that connect the brain and spinal cord to the muscles. The protein is needed to send electrical impulses to the muscles, enabling them to work effectively.
Without this crucial protein, the motor neurone cells in the spinal cord will deteriorate and die.
The deterioration and death of the motor neurones in the spinal cord means that the muscles they control will be unable to receive vital nerve impulses. In turn, the muscles will not be able to work effectively, leading to weakness and impaired movement. Because of a lack of use, the muscles will eventually start to shrink. This is known as muscular atrophy.
How it is inherited
For a child to develop SMA, both parents must have the defective gene and pass it on to their child. These parents are known as carriers, and don't actually have SMA.
If both parents are SMA carriers and have a child, there is a:
- 50% chance that the child will be a carrier of SMA
- 25% chance that the child will develop SMA
Approximately one in every 40 people is a carrier of the defective gene that causes SMA.
Adult forms of SMA may be inherited in various ways.
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Diagnosis
A diagnosis of spinal muscular atrophy (SMA) is usually confirmed by genetic testing.
Genetic testing is used to check for faulty genes. To do this, a blood sample will be taken for analysis.
If you or your child are referred for genetic testing, the doctor will ask about your family's medical history to see if there are neuromuscular conditions in your family. Neuromuscular conditions are those that affect the nerves and muscles.
A physical examination will also be carried out. The examination will look for signs of things such as:
- muscle weakness and wastage
- reduced or absent tendon reflexes
- twitching of individual muscle fibres
Several other tests, such as an electromyography test or a muscle biopsy may be carried out to help confirm a diagnosis of SMA. These two procedures are described in more detail below.
Electromyography
During an electromyography (EMG) test, a thin needle is inserted through your skin and into your muscle.
The needle is used to detect the electrical currents of the muscle, both at rest and during activity. This can help determine whether any muscle weakness is caused by a disease in the muscle itself or is the result of a lack of nerve supply. A lack of nerve supply indicates SMA.
Muscle biopsy
During a muscle biopsy, a small sample of muscle tissue is taken for analysis. The sample, which is usually taken from the thigh, is examined under a microscope. However, with the wide availability of genetic testing, muscle biopsies are rarely done nowadays to diagnose SMA.
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Treatment
There is currently no cure for spinal muscular atrophy (SMA), but research is under way and several clinical trials have been conducted recently. Learn more about medical research and clinical trials.
Until an effective treatment for SMA can be found, the management of the condition currently focuses on relieving the symptoms and providing supportive care.
Some of the methods that are used to help manage SMA are outlined below.
Breathing
Many people with SMA experience breathing problems that are caused by a weakening of the respiratory muscles. Breathing-related problems are a common cause of death, particularly in children with types 0, I and II SMA.
In more mildly affected children, breathing exercises are used. These reduce the risk of chest infections, as well as helping to minimise symptoms if infections do develop.
More moderately affected children and adults with breathing difficulties can be given a special mask or mouthpiece to support their breathing, mainly at night and for as many hours a day as they need it. The mask or mouthpiece can usually be removed for eating, drinking, talking or breathing normally when possible.
Swallowing and feeding
Babies with SMA who are unable to suck or swallow are at risk of becoming dehydrated and poorly nourished. Not being able to swallow properly also increases the risk of choking by inhaling food and liquid.
If a baby with SMA is unable to swallow and is at risk of choking, they may need to be fed through a tube that is attached directly to their stomach. This is known as a gastrostomy tube. Some modern feeding systems allow the tube to be detached when it is not in use.
Physiotherapy
Due to the severe, progressive weakening of the proximal muscles (those closest to the centre of the body, such as shoulders, hips and back), it is likely that a person with SMA will find it difficult to move and support themselves.
Physiotherapy concentrates on problems that affect muscles, bones, the heart, the circulation and the lungs, and can be very useful for people with SMA.
For example, physiotherapy is often used to help improve a child’s posture and range of movement, as well as helping to slow the development of scoliosis (curvature of the spine).
Back brace and surgery
For children with SMA, the risk of developing scoliosis is high as a result of the progressive weakness in the muscles that support the spine. A specially made back brace, or corset, can be used to help support the back and encourage the spine to grow correctly.
However, using a back brace is not a permanent solution for spinal curvature. Spine-straightening surgery – also known as spinal fusion – is the only way to correct the problem permanently.
Spinal fusion involves fusing together the back bones of the spine in the correct position. If possible, most surgeons prefer to wait until after a child’s spine has finished growing before carrying out surgery.
Diet
There is little evidence to suggest that a particular diet is useful for people with SMA. However, as SMA is a condition that directly affects movement and mobility, it is important for someone with the condition to eat a balanced diet to maintain a healthy weight.
A nutritionist will be able to advise about the types of food that you should include in your diet, as well as providing useful advice about calories and portion size. A good nutritional plan should be balanced with an appropriate exercise programme.
Exercise
For someone with SMA, exercise is very important for maintaining circulation, preventing joint stiffness and improving flexibility and range of movement.
It is not entirely clear how much exercise someone with SMA should do, and further research is required.
Also, the amount of exercise that someone with SMA is able to do will largely depend on the severity of their condition and their level of mobility. Most healthcare professionals recommend that someone with SMA should do as much exercise as they are comfortable with.
Your occupational therapist or physiotherapist should be able to design an exercise routine that can be performed at home to help:
- maintain joint mobility
- prevent contractures (shortening of the muscles)
- maximise respiratory function
- maintain muscle strength to delay muscle wastage
Assistive equipment
An occupational therapist will also be able to offer advice and support to improve the independence and overall quality of life of someone with SMA.
For example, they will be able to provide help and advice about equipment to help with mobility, such as walking frames and motorised wheelchairs.
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Prevention
Chorionic villus sampling
For parents who have previously had a child with spinal muscular atrophy (SMA) and are concerned that their next child might also have the condition, antenatal screening can be carried out between the tenth and twelfth week of pregnancy.
Chorionic villus sampling (CVS) is a procedure that tests for serious foetal problems, including SMA.
Screening tests
As it is possible for a person to be a carrier of SMA and to pass the condition on to their children, screening tests for the defective genes are available.
These can be particularly useful if it is already known that one person of a couple carries the gene, and the couple would like to know if the other partner is affected.
Genetic counselling may be required if the screening tests indicate that the defective gene is present.
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