Spinal Muscular Atrophy

Background

Spinal muscular atrophy (SMA) is a rare, inherited disease, occurring in 1 in 6,500 live births. It results in loss of nerve cells in the spinal cord and weakness of the muscles connected with those nerves.

Credits

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.

What are the symptoms?

SMA affects the proximal muscles (those closer to the body) more severely than the distal ones (those closer to the hands and feet). There are four types of childhood SMA.

Type 0
This is a severe and develops before birth. Babies with SMA type 0 are hardly able to move and have problems breathing and swallowing.

Type I (Werdnig-Hoffmann disease)
Babies with type I SMA have severe muscle weakness, limp and floppy limbs and are unable to raise their head or sit without support. They develop breathing and swallowing difficulties. SMA type I is diagnosed at around three to six months of age.

Type II (Dubowitz disease)
Usually occurs between 6 to 18 months of age. Babies will be able to sit, but it is unlikely that they will be able to stand or walk unaided. They may experience breathing problems, reduced or absent tendon reflexes, floppy arms and legs, twitching of some muscles, deformities of the hands, feet and chest, and scoliosis.

Type III (Kugelberg-Welander disease)
This is usually diagnosed after two years of age. Most children are able to stand unaided but find walking or getting up from sitting difficult. They may experience balance problems, difficulty running or climbing steps and a slight shaking in their fingers. Over time, the muscles of children with type III SMA become weaker.

What are the causes?

SMA is a genetic condition, which is caused by a mutation in the survival motor neurone 1 gene (SMN1) on chromosome 5.

How is it diagnosed?

Physical examination can identify muscle wastage, reduced or absent tendon reflexes and twitching of individual muscle fibres. Other tests used for diagnosis include an electromyography test (which studies electrical currents in muscle) and muscle biopsy. Rarely, a diagnosis is made by testing for the SMN1 mutation.

How is it treated?

There is no cure for SMA, but supportive care can help minimise the symptoms. Oxygen therapy is useful in children who have trouble breathing and breathing exercises can help to minimise the risk of chest infections. If a baby with SMA is unable to swallow and is at risk of choking, they may need a gastrostomy tube, which feeds them directly into their stomach. Physiotherapy is used to improve a child’s posture, range of movement and for helping to slow development of scoliosis. Bracing may be needed to help reduce spinal curvature, but, in severe cases, spinal surgery may be needed.

Although there is no specific cure for SMA, several centres specialised in neuromuscular disorders offer a drug called salbutamol to children with SMA II and III aged over three years. This drug has been shown to improve muscle strength in children and enhances SMN protein levels in laboratory experiments.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
SMA is inherited in autosomal recessive manner. Cases of SMA often occur sporadically.

Prenatal diagnosis
If the genetic defect in a family affected by SMA has been identified, prenatal screening is possible via amniocentesis after 15 weeks of pregnancy or chorionic villus sampling between 10 and 13 weeks 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 invididuals affected by all forms of SMA and raises awareness of the condition. They have outreach workers who can visit newly diagnosed families at home. It funds and supports research into the causes, treatment and management of SMA. They can provide multi-sensory toy packs for babies diagnosed with Type I SMA.

Group details last reviewed June 2022.

As spinal muscular atrophy is a neuromuscular disease advice and information about the condition is also available from Muscular Dystrophy UK (see entry Congenital Muscular Dystrophy).

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