Spinal Muscular Atrophy


Spinal Muscular Atrophy (SMA) is a rare, genetic neuromuscular condition causing progressive muscle wasting (atrophy) and weakness leading to loss of movement. Approximately one in every 10,000 babies worldwide are born with a Type of SMA.


Medical text written September 2011 by Dr M Scoto Clinical Research Associate, and Dr A Manzur, Consultant Paediatric Neurologist, Dubowitz Neuromuscular Centre, UCL Institute of Child Health and Great Ormond Street Hospital, London, UK. Text updated July 2022 by Spinal Muscular Atrophy UK (SMA UK).

What are the symptoms?

Spinal Muscular Atrophy (SMA) may affect crawling and walking ability, arm, hand, head and neck movement, breathing and swallowing. There are different forms of SMA and a wide spectrum of how severely children are affected.

The most common form is known as ‘5q SMA’; the term ‘5q’ refers to its genetic cause. When a child is first diagnosed with 5q SMA, the doctor gives a clinical classification – SMA Type 1, 2, 3 – which reflects the age of onset of symptoms and the motor milestones that someone would be expected to achieve. The summaries below briefly describe how the condition impacts if untreated. Before 2007, without intervention for breathing difficulties, most children were expected to live for less than two years.  Since late 2016, the gradual worldwide introduction of drug treatments has been changing outcomes positively, especially for newly diagnosed infants.

Type I (sometimes called Werdnig Hoffman disease)

Symptoms of muscle weakness usually begin between 0 and 6 months. Infants are unable to sit without support and may be described by clinicians as ‘non-sitters’. They also often have feeding and breathing difficulties due to their weak muscles. Generally, the earlier the onset of symptoms, the more severe the condition.  In the past, without intervention for breathing difficulties and drug treatments infants rarely survived their second birthday.

Type 2 (sometimes called SMA Type II or Dubowitz disease or intermediate SMA)

Symptoms usually begin between 7 and 18 months of age. Children and adults are unable to stand without support and may be described by clinicians as ‘sitters’. Their swallowing and breathing muscles may or may not be affected. If someone’s breathing muscles are affected, this can make it difficult to cough effectively, which can make them more vulnerable to chest (respiratory) infections. Though this is a serious complication associated with reduced life expectancy, improvements in healthcare standards mean that the majority of people live long lives.

Type 3 (sometimes called SMA Type III or Kugelberg-Welander disease)

Type 3a: symptoms usually begin between 18 months and 3 years of age. Children can stand and walk, although this becomes more difficult with age, and they will need more support over time.

Type 3b, symptoms usually begin after 3 years. Difficulties with standing and walking usually occur later than they do for children with SMA Type 3a.

Depending upon the individual impact of their condition, children with SMA Type 3 may be described as ‘sitters’ or ‘walkers’.

Fewer people with SMA Type 3 have breathing or swallowing symptoms as swallowing and breathing muscles are not usually affected. Life expectancy isn’t usually affected and most live long lives.

What are the causes?

The most common form, 5q SMA, is caused when an individual inherits two faulty copies of the survival motor neurone 1 gene (SMN1) on chromosome 5 – one from each parent. 1 in 40 people are ‘carriers’ of the faulty gene.

How is it diagnosed?

Any child with suspected SMA will be physically examined and a blood sample for DNA testing will be arranged. The sample is tested for a deletion mutation in the Survival Motor Neuron 1 (SMN1) gene on chromosome 5. The result is usually available within 2 – 4 weeks.

How is it treated?

Although there is currently no cure for SMA, this doesn’t mean that nothing can be done. There are a range of options aimed at managing symptoms, reducing complications of muscle weakness and maintaining the best quality of life. These are outlined in the internationally agreed Standards of Care for SMA.

There have also been huge developments in the field of research and drug treatment in recent years. There are currently three treatments funded by the NHS, however not all of these treatments are suitable or possible for all children and young people who have SMA. For more information about the drugs and who may have access visit: https://smauk.org.uk/treatments-nhs-funded-uk

Inheritance patterns and prenatal diagnosis

Inheritance patterns
5q SMA is passed from parents to their children through faulty SMN1 genes. It usually follows an autosomal, recessive pattern of inheritance. This means that people who have inherited two faulty copies of the SMN1 gene (one from each parent) have SMA

In around 2% of cases of SMA, the mutation is new in the affected person. This is called a de novo mutation and means that at least one parent is not a carrier.

Prenatal diagnosis
If the genetic fault in a family affected by 5q SMA has been identified, the following options may be possible: Preimplantation Genetic Diagnosis (PGD) – pre-conception; Non-Invasive Prenatal Diagnosis (NIPD) – from around the 8th week of pregnancy; Chorionic Villus Sampling (CVS) – between the 11th and 14th week of pregnancy; Amniocentesis – between the 15th and 20th week of pregnancy.

Is there support?

Spinal Muscular Atrophy UK (SMA UK)

Tel: 01789 267 520
Email: office@smasupportuk.org.uk
Website: smauk.org.uk

SMA UK is a Registered Charity in England and Wales No. 1106815. It offers support and information to families and individuals affected by all forms of SMA, raises awareness of the condition and supports research-related initiatives. It has a small team of outreach workers who give emotional support, practical advice and guidance for anyone affected by SMA by email, phone, text or virtual meeting (e.g., zoom) and, when they have capacity, may be able to home visit. It can provide multi-sensory toy packs free of charge in the UK for newly diagnosed infants up to 12 months of age. 

Group details last reviewed July 2022.

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