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Spinal muscular atrophy Content Supplied by NHS Choices

Spinal muscular atrophy (SMA) is a genetic disease that causes muscle weakness and progressive loss of movement.

SMA is caused by deterioration in the nerve cells (motor neurones) connecting the brain and spinal cord to the body's muscles.

As the link between the nerves and muscles breaks down, the muscles used for activities such as crawling, walking, sitting up and moving the head become progressively weaker and shrink (atrophy). Mental abilities are unaffected by SMA.

The term spinal muscular atrophy can be applied to a number of different related conditions.

Classification of spinal muscular atrophy

SMA is classified according to the age symptoms appear and how much physical mobility a person has:

  • Type I - the most severe type. Symptoms appear in babies less than six months old, and they never develop the ability to sit unsupported.
  • Type II - less severe than type I. Symptoms usually appear in babies aged 7 to 18 months. Children with Type II can sit unsupported and some are able to stand, but they can't walk independently.
  • Type III - the mildest type affecting children. Symptoms usually appear after 18 months of age, and children are usually able to reach all the major motor milestones, including independent walking.
  • Type IV - affects adults.

In the most severe cases (Type I), severe respiratory problems mean children rarely survive beyond two years of age.

Type II SMA may shorten life expectancy, but improvements in care standards mean the majority of people can live long, fulfilling and productive lives. Survival into adulthood is now expected.

Life expectancy is usually unaffected in Types III and IV.

Read more about the symptoms of the different types of spinal muscular atrophy.

Management and support

It's not currently possible to treat the underlying genetic fault that causes SMA, although a number of experimental therapies are currently being evaluated in early clinical trials.

However, treatment and support is available to help manage the symptoms and provide people with SMA with the best possible quality of life.

Depending on its severity, treatment may involve:

  • exercises and equipment to improve mobility and breathing
  • feeding tubes and nutrition advice
  • bracing or surgery to treat curvature of the spine (scoliosis)

A range of healthcare professionals are involved in the care of a person with SMA, including:

  • doctors who specialise in neurology
  • physiotherapists
  • occupational therapists
  • speech and language therapists
  • if appropriate, respiratory physicians and orthopaedic surgeons

Read more about treating spinal muscular atrophy.

Testing for spinal muscular atrophy

If you have a family history of SMA, you should be able to access genetic testing. This involves checking a sample of your blood for the genetic fault responsible for the condition.

If there's a chance your baby could have SMA, it's also possible to check for this genetic fault during pregnancy, using tests such as chorionic villus sampling (CVS) and amniocentesis. Pre-implantation diagnosis (PGD) is also possible. Families should be referred to a geneticist to discuss all these options.

If a child shows typical signs of SMA, such as obvious muscle weakness, a blood test can usually confirm a diagnosis.

Read more about diagnosing spinal muscular atrophy.

Further information and support

The most common types of spinal muscular atrophy (SMA) affect an estimated 2,000 to 2,500 people in the UK. There are several charities for people with the condition, including:

Information about you

If you or your child has spinal muscular atrophy, your clinical team will pass information about you/your child on to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS).

This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.

Find out more about the register.

Types of spinal muscular atrophy

There are several different types of spinal muscular atrophy (SMA) - the condition is classified according to the age at which symptoms develop and the level of muscle weakness it causes.

In general, SMA affects a person's physical abilities, such as moving, walking and breathing, but doesn't affect their mental development.

It causes the muscles throughout the body to become weak and shrink (atrophy). The muscles closest to the centre of the body, such as the shoulders, hips and back (proximal muscles) are usually affected first and most severely.

The most common forms of SMA are usually divided into four types, described below.

Type I

Type I SMA (also known as Werdnig-Hoffmann disease) is believed to be the most common form. It causes severe muscle weakness, which can result in problems moving, eating, breathing and swallowing.

These symptoms are usually apparent at birth or during the first few months of life.

The muscles of babies with Type I SMA are thin and weak, which makes their limbs limp and floppy. They're usually unable to raise their head or sit without support.

Breathing problems can be caused by weakness in the baby's chest muscles, and difficulty swallowing can be made worse by weakness of the muscles in the tongue and throat.

Because of the high risk of serious respiratory problems, most children with Type I SMA die in the first few years of life.

Type II

Symptoms of Type II SMA usually appear when an infant is 7-18 months old. The symptoms are less severe than Type I, but become more noticeable in older children.

Infants with Type II SMA are usually able to sit, but cannot stand or walk unaided. They may also have the following symptoms:

  • breathing problems
  • weakness in their arms and, particularly, their legs
  • swallowing or feeding problems
  • a slight tremor (shaking) of their fingers

In some cases, deformities of the hands, feet, chest and joints develop as the muscles shrink.

As they grow, many children with Type II SMA develop scoliosis. This is an abnormal curvature of the spine caused by the muscles supporting the bones of the spine becoming weaker.

A child with Type II SMA has weak respiratory muscles, which can make it difficult for them to cough effectively. This can make them more vulnerable to respiratory infections.

Although Type II SMA may shorten life expectancy, improvements in care standards mean most people can live long, fulfilling and productive lives. The majority of children with Type II SMA are now expected to survive into adulthood.

Type III

Type III SMA (also known as Kugelberg-Welander disease) is the mildest form of childhood SMA. Symptoms of muscle weakness usually appear after 18 months of age, but this is very variable and sometimes the symptoms may not appear until late childhood or 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
  • difficulty walking
  • difficulty running or climbing steps
  • a slight tremor (shaking) of their fingers

Over time, the muscles of children with Type III SMA become weaker, resulting in some children losing the ability to walk when they get older.

Breathing and swallowing difficulties are very rare and the condition doesn't usually affect life expectancy.

Type IV (adult-onset)

Type IV SMA is a less common form that begins in adulthood. The symptoms are usually mild to moderate, and may include:

  • muscle weakness in the hands and feet
  • difficulty walking
  • muscle tremor (shaking) and twitching

Type IV SMA doesn't affect life expectancy.

Rarer types

As well as the four more common forms of SMA, there are a number of rare types, which have slightly different characteristics and causes.

Spinal muscular atrophy with respiratory distress

Spinal muscular atrophy with respiratory distress (SMARD) is a very rare form of SMA that severely affects the muscles used in breathing. It's usually diagnosed within the first year of life.

Kennedy's syndrome

Kennedy's syndrome, or spinobulbar muscular atrophy (SBMA), is a rare type of adult SMA.

SBMA only affects men. It usually develops very gradually between the ages of 20 and 40. Rarely, it can affect teenage boys or sometimes only become obvious after 40.

The initial symptoms of Kennedy's syndrome may include tremor (shaking) of the hands, muscle cramps on exertion, muscle twitches and weakness of the limb muscles.

As the condition progresses, it may cause other symptoms, including:

  • weakness of the facial and tongue muscles, which may cause difficulty swallowing (dysphagia) and slurred speech
  • recurring pneumonia (infection of lung tissue)

Some people with Kennedy's syndrome also develop enlarged male breasts (gynaecomastia), diabetes, and a low sperm count or infertility.

Kennedy's syndrome doesn't usually affect life expectancy.

Distal SMA

Distal spinal muscular atrophy (DSMA) is a rare form of SMA that affects the distal muscles, such as the hands, feet, lower arms and lower legs. This leads to reduced mobility and range of movement.

Some types of DSMA can affect the muscles used for speaking or swallowing.

Causes of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by faulty genes, usually passed on to a child by their parents.

The most common types of spinal muscular atrophy - types I, II and III - are caused by a fault with a gene called SMN1.

The SMN1 gene produces a protein needed by certain cells in the spinal cord (motor neurones), which connect the brain and spinal cord to the muscles. If there's a problem with the gene, this protein is only produced in very low levels.

This causes the motor neurone cells in the spinal cord to deteriorate. Messages sent through the spinal cord to the nerves and muscles are reduced or, in the most severe forms, disappear, and the muscles can't work effectively. The muscles eventually start to shrink from a lack of use (muscular atrophy).

How it's inherited

People who have the faulty gene that causes SMA but don't have SMA themselves are known as carriers. Both parents must be carriers of the faulty gene to pass it on to their child.

If two people who are SMA carriers have a child, there's a:

  • 50% chance the child will be a carrier of SMA
  • 25% chance the child will develop SMA
  • 25% chance the child will be completely unaffected by SMA

Approximately 1 in every 40-60 people is a carrier of a defective SMN1 gene.

Other types of SMA

The causes of some rarer types of SMA can be different from those of types I, II and III.

Spinal muscular atrophy with respiratory distress

A type of SMA called spinal muscular atrophy with respiratory distress (SMARD) is inherited in the same way as types I, II and III, but it's not related to a fault with the SMN1 gene. Instead, a fault with a gene called IGHMBP2 is responsible for the condition.

Adult-onset SMA

SMA that develops in adults (type IV) is linked to the SMN1 gene in some cases, although not all cases are thought to be inherited.

In cases where adult-onset SMA is passed on, the way it's inherited can be different from types I, II and III. For example, it's sometimes possible for someone to develop adult-onset SMA if only one parent has the faulty gene.

In Kennedy's syndrome, the condition is passed on by the mother and only affects male children, although female children can become carriers.

Read more about genetic inheritance.

Testing for SMA

If there are concerns a child may inherit SMA, tests can be carried out before and during pregnancy to check for the defective gene that causes the condition.

See diagnosing spinal muscular atrophy for more information.

Diagnosing spinal muscular atrophy

Tests related to spinal muscular atrophy (SMA) can be carried out before, during and after pregnancy.

Tests before pregnancy

If one partner in a couple has a family history of SMA or is known to carry the faulty gene, the other partner can have a blood test to check if they also carry the faulty gene.

If both partners carry the faulty gene, every pregnancy carries a one in four chance that the child will have SMA.

Couples at risk of having a child with SMA should be offered information and advice, known as genetic counselling, to help them understand their options.

Read more about genetic testing and counselling.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) may be an option for couples who are both carriers of SMA.

PGD involves using in-vitro fertilisation (IVF), where eggs are removed from a woman's ovaries before being fertilised with sperm in a laboratory. After a few days, the resulting embryos can be tested for SMA and a maximum of two unaffected embryos are transferred into the uterus.

Funding for PGD is decided on an individual basis. For example, a couple may not be considered for PGD on the NHS if they already have unaffected children or if the chances of success are thought to be low. In these cases a couple can choose to fund PGD privately, although it's likely to cost between 6,000 and 10,000.

Tests during pregnancy

Tests can also be carried out during pregnancy to check if an unborn child has SMA. These are offered to people who already have a child with SMA as, in the majority of cases, there's a one in four chance with every pregnancy that the child will also have the condition.

There are two different tests that can be used at different points in a pregnancy, including:

  • chorionic villus sampling (CVS) - where a sample of cells from the placenta are removed and tested, usually between weeks 11 and 14 of pregnancy
  • amniocentesis - where a sample of amniotic fluid is removed and tested, usually during weeks 15 to 20 of pregnancy

Both these tests can slightly increase the chance of having a miscarriage.

If tests show a child is likely to have SMA, a couple can decide to either continue the pregnancy or have an abortion. Support will be offered to couples facing this decision.

Tests after birth

If SMA isn't diagnosed before birth and a child or adult has typical symptoms of SMA, genetic testing can be used to confirm the condition. This involves analysing a blood sample for the faulty gene that causes SMA.

The doctor will also ask about the family's medical history to see if anyone has had a condition affecting the nerves and muscles (neuromuscular conditions).

A physical examination will also be carried out to look for signs of SMA, such as:

  • muscle weakness and wastage
  • reduced or absent tendon reflexes
  • twitching of individual muscle fibres

In some cases, other tests may be used to confirm the diagnosis. For example:

  • electromyography - where a thin needle is inserted into a muscle to detect its electrical currents at rest and during activity
  • muscle biopsy - where a small sample of muscle tissue is taken for analysis

Treating spinal muscular atrophy

Treatment and support is available to help manage the symptoms of SMA and provide the best possible quality of life. It's not currently possible to treat the genetic fault that causes SMA.

As well as doctors and nurses, a number of other healthcare professionals are often involved in treatment for SMA, including:

Some of the methods used to help manage the condition are outlined below. Treatment can vary depending on the type of SMA.


For someone with SMA, exercise is very important for maintaining circulation, preventing joint stiffness, and improving flexibility and range of movement.

The amount of exercise someone with SMA is able to do depends on the severity of their condition, but most healthcare professionals recommend as much exercise as possible.

Your occupational therapist or physiotherapist should be able to help design an exercise routine to help maintain joint mobility, prevent shortening of the muscles, and maintain muscle strength.

The exercises may incorporate elements of hydrotherapy, which involves exercising in water, and games for young children.

Assistive equipment

If someone with SMA has difficulty moving, an occupational therapist will be able to offer advice and support.

For example, they can provide advice about equipment, such as walking frames and powered wheelchairs.

Nutrition and feeding

It's important for people with SMA, especially children, to get the right nutrition to avoid problems such as dehydration and ensure healthy development.

But this can be difficult because some people have problems feeding and swallowing. A nutritionist can offer advice about diet, such as which formulas to use for babies with SMA.

A feeding tube may be required if feeding and swallowing problems are severe. Several different types of tube may be used, such as a tube attached directly to the stomach (gastrostomy tube) or a tube passed into the stomach through the nose (nasogastric tube).

Breathing support

Many people with SMA experience potentially fatal breathing problems caused by a weakening of the respiratory muscles, but there are a number of treatments that can help reduce this risk.

Breathing exercises are sometimes used to help reduce the risk of problems developing from respiratory tract infections and improve difficulties coughing.

In more severe cases, breathing may need to be assisted using a special machine that supplies air to the lungs through a mask or tube.

A special suction machine may also be used to help with any difficulties clearing the throat. This involves passing a thin plastic tube to the back of the throat to suck away any mucus.

For people with SMA - as well as those in frequent contact with someone who has SMA - immunisations against respiratory tract infections, such as flu and pneumonia, are sometimes recommended because of the risk of serious complications.

Spine problems

For children with SMA, the risk of developing a sideways curve in the spine (scoliosis) is high as a result of the progressive weakness in the muscles supporting the spine.

Scoliosis may be treated with physiotherapy and braces or with surgical intervention. Braces are usually used first to give stability while sitting. This allows people to use their hands and arms for daily activities such as eating and washing. They can provide control of the curve during growth until surgery is required.

There are several types of surgery for scoliosis, depending on your child's age. Younger children may be treated with growing rods, which allow the spine to grow while partially correcting the scoliosis.

Teenagers and young adults may be treated with a spinal fusion. This is where the spine is straightened using metal hooks and rods, before being fused into place using bone grafts. Spinal fusion is the only way to correct the problem permanently.

Read more about treating scoliosis in children.

Research into treatments

Research is currently being carried out into possible treatments for spinal muscular atrophy (SMA).

You can check the database of clinical trials for spinal muscular atrophy or read more general information about medical research and clinical trials.