Spinal muscular atrophy

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


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

It occurs due to 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 that are 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.

There are a number of different conditions which are called SMA. The term is usually used for the most common form, which is caused by a genetic problem where one copy of the genetic error is inherited from each parent.

Read more about the causes of spinal muscular atrophy

Classification of spinal muscular atrophy

SMA is classified according to the age at which symptoms develop and how severe they are:

  • Type I – the most severe type, develops in babies less than six months old
  • Type II – less severe than type I, affects babies 6-18 months old
  • Type III – the mildest type affecting children, usually affects children around the age of three
  • Type IV – affects adults

In the most severe cases (Types I and II), fatal respiratory problems often develop during childhood. Life expectancy is usually unaffected in more mild cases (Types III and IV).

Read more about the types of spinal muscular atrophy.

Treatment and support

There is no cure for SMA, but treatment and support can help to manage the condition. 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 SMA, including doctors who specialise in neurology, physiotherapists, occupational therapists, and speech and language therapists.

Read more about treating spinal muscular atrophy.

Testing for spinal muscular atrophy

If there are concerns your children may inherit SMA, for example if you have a family history of the condition, genetic testing may be recommended. This involves checking a sample of your blood for the genetic problem responsible for the condition.

It's also possible to check an unborn child for this genetic problem during pregnancy, using tests such as chorionic villus sampling (CVS) and amniocentesis.

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.


There are several different types of spinal muscular atrophy (SMA) – the disease is classified according to the age at which symptoms develop and how severe they are.

In general, SMA affects a person’s physical abilities, such as moving, walking and breathing, but does not affect their mental development.

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

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

Type I

Type I SMA (previously known as Werdnig-Hoffmann disease) is believed to be the most common form of SMA. 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 are 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.

Unfortunately, due to the high risk of serious respiratory problems, most children with Type I SMA die before they reach one year old.

Type II

Type II SMA usually develops when an infant is 6–18 months old. The symptoms are less severe than Type I, but usually get worse over time.

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

  • breathing problems
  • floppy arms and legs
  • twitching of the muscles in the arms, legs or tongue

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 which occurs when the muscles supporting the bones of the spine become weaker.

Like Type I SMA, there is a high risk of fatal respiratory problems, although most people with Type II SMA live into their teenage years or early adulthood.

Type III

Type III SMA (previously known as Kugelberg-Welander disease) is the mildest form of childhood SMA. It's usually diagnosed around the age of three, but it can be as late as the teenage years.

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, 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 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's usually diagnosed before a baby is six months old.

Infants with SMARD have very weak breathing muscles, resulting in severe breathing difficulties that are often fatal. Like Type I SMA, most children with SMARD die before their first birthday.

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, although 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 muscles of the limbs.

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 (gynecomastia), diabetes and a low sperm count or infertility.

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


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

The most common types of spinal musclar atrophy – types I, II and III – are caused by a problem with a gene called SMN1.

The SMN1 gene is responsible for the production of a protein needed by the nerves (motor neurones) that connect the brain and spinal cord to the muscles. If there is a problem with the gene, this protein is only produced in very low levels, causing the motor neurone cells in the spinal cord to deteriorate, meaning that the connection between the nerves and the muscles disappears and the muscles cannot work effectively.

The muscles will eventually start to shrink due to a lack of use. This is known as muscular atrophy.

How it is inherited

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

If two people who are SMA carriers have a child, there is 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 one 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 to 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 problem with the SMN1 gene.

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


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

These can determine if a person has SMA, or if a couple is at risk of having a child with the condition.

Tests before pregnancy

If there are concerns your children may inherit spinal muscular atrophy (SMA), tests can be carried out to assess your risk of having a child with the condition.

This will usually involve having a blood test to check for the faulty gene that can cause SMA (see causes of spinal muscular atrophy for more information).

This can be particularly useful if a partner has a family history of SMA or if it's already known that one partner carries the gene, and the couple would like to know if the other partner is affected.

If the test shows you may be at risk of having a child with SMA, you should be offered information and advice known as genetic counselling.

Read more about genetic testing and counselling.

Pre-implantation genetic diagnosis

For couples at risk of having a child with SMA, pre-implantation genetic diagnosis (PGD) may be an option.

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.

However, funding for PGD is decided on an individual basis. For example, you may not be considered for PGD on the NHS if you already have unaffected children or if the chances of success are thought to be low.

Tests during pregnancy

Tests can also be carried out during pregnancy to check if an unborn child will develop SMA. These are often used for people who already have a child with SMA, as there is a chance any other children they have will also have the condition.

There are several different tests which 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 10 and 13 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 your chances of having a miscarriage.

If tests show your child is likely to have SMA, you can choose to either continue the pregnancy or have an abortion.

Tests after birth

If SMA is not diagnosed before birth and a child has typical symptoms of SMA, there are a number of tests that can check for the condition.

Most cases can be confirmed with genetic testing, which involves analysing a blood sample for the faulty gene that causes SMA.

If you or your child is referred for genetic testing, the doctor will also ask about your family's medical history to see if there are any conditions that affect the nerves and muscles (neuromuscular conditions) in your family.

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

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

If a diagnosis is not entirely certain, several other tests – such as an electromyography test or a muscle biopsy – may be carried out. These are described in more detail below.


During an electromyography 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.

Due to the availability of genetic tests, EMG is now very rarely carried out in typical cases of 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.


There is currently no cure for spinal muscular atrophy (SMA), but treatment can help improve the lives of people with the condition.

As well as being cared for by doctors and nurses, a number of other healthcare professionals are often involved in treatment for SMA. These can include physiotherapists, occupational therapists, and speech and language therapists.

Some of the methods used to help manage SMA are outlined below.


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 that someone with SMA is able to do will largely depend on the severity of their condition, but most healthcare professionals recommend people with SMA should do as much exercise as they are comfortable with.

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

The exercises may incorporate elements of hydrotherapy (exercises 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 motorised wheelchairs.

Nutrition and feeding

It's important for people with SMA, especially children, to get appropriate nutrition to avoid problems like dehydration and ensure healthy development. However, this can be difficult because some people with SMA have problems feeding and swallowing.

A nutritionist can offer advice about diet, such as which formulas to use for babies with SMA.

However, if feeding and swallowing problems are severe, a feeding tube may be required. Several different types of tube may be used, such as a tube that is 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 which 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 due to the risk of serious complications.

Spine problems

For children with SMA, the risk of developing a sideways curve in the spine (scoliosis) is high due to the progressive weakness in the muscles that support the spine. There are various treatments for scoliosis in children, including back braces and surgery.

A specially made back brace can be used to help support the back and encourage the spine to grow correctly. However, this cannot correct the curve.

Spinal fusion is the only way to correct the problem permanently. This is where the spine is straightened using metal hooks and rods, before being fused into place using bone grafts.

Read more about treating scoliosis in children.

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