Spinal muscular atrophy (SMA)

Spinal muscular atrophy (SMA) is a rare genetic disease that mostly affects children. 

SMA leads to an early loss of motor neurons – these are the cells in the brain and spinal cord that send commands from the brain to the muscles. Motor neurons are important for the health of muscle, so people with SMA will experience muscle weakness. 

SMA is caused by a gene mutation in our DNA. The gene affected is known as the Survival Motor Neuron 1 or SMN1. The mutation means the body doesn’t produce enough of a protein called SMN, which is very important for our development, especially when we are babies. 

A similar gene (SMN2) produces some backup protein, but this isn’t enough for keeping the motor neuron alive. Depending on the ‘copy number’ of the SMN2 gene, children can be, more or less severely, affected by the condition. 

A copy number is the number of times a gene appears in your DNA. Most people have two copies of most genes – one from each parent – but some people have more or fewer copies.

You can read more about SMA: 

• What is SMA?
• Spinal muscular atrophy (SMA) - NHS

SMA is usually divided into five main types, based on when symptoms start and the severity of the condition.  

The amount of copy numbers will vary between type. 

Please note that this is a general overview of what is typically seen, and life expectancy is discussed below. 

Type 0:

• Symptoms start before birth
• The rarest and most severe type of SMA
• Severe muscle weakness and breathing difficulties
• Babies with type 0 usually have a very short life expectancy.

Type 1: 

• Symptoms start at around 0–6 months.
• A severe form of SMA.
• Causes muscle weakness and breathing difficulties.
• Babies with type 1 usually have a short life expectancy.

Type 2:

• Symptoms start at around 6–18 months.
• Children can usually sit, but may not be able to stand or walk independently.
• Generally survive to adolescence or adulthood.

Type 3: 

• Symptoms usually start after 18 months, but sometimes not seen until adolescence. 
• Able to walk and stand unaided. Running or climbing may be difficult. 
• Usually normal life expectancy. 

Type 4: 

• Symptoms start in adulthood, often after age 21.
• Normal motor development. Mild muscle weakness. 
• Normal life expectancy. 

We do not currently have a cure for SMA. However, there are three disease modifying treatments (DMTs) that greatly improve the life of people with SMA.

Spinraza (Biogen)

This was the first DMT to be approved for SMA. It was approved in 2019 for use in all types of SMA (1 to 3) under a managed access agreement and in 2026 received full approval. Spinraza treatment could be possible for a baby diagnosed with the most severe and rare SMA Type 0. Parents and clinicians would discuss the possible risks and benefits and make a joint decision about this.

Zolgensma (Novartis)

This was approved in 2021 and was commissioned for SMA type 1 with up to 3 SMN2 copies.

Evrysdi (Roche)

This was also approved in 2021, for all types of SMA (1 to 3). under a managed access agreement and in 2026 received full approval.

Treatments

All these treatments aim to increase the SMN protein production, to help improve motor function and survival. These treatments are also available for pre-symptomatic patients. 

Some of these treatments are only available under managed access programmes. 

Every child is different, and decisions about treatment can be complex. Treatment also usually needs to be accompanied by respiratory, nutritional, and orthopaedic support, as well as regular hospital appointments as part of the standard of care. Your child’s neuromuscular team will discuss the treatment options and management with you. 

Whilst it is known that early treatment provides the most favourable outcome, there is still little known about long term prognosis since the introductions of the DMTs. This is because DMTs were only recently introduced a few years ago, so we are still waiting to see long term outcomes. Clinicians and physiotherapists are working together to input clinical data on DMTs into the SMA REACH UK database, so we will have more knowledge on long term outcomes moving forward. 

When you attend your neuromuscular appointment, we will ask for your permission to store your child’s clinical data on this database, to contribute to this research. Data is stored anonymously, but it will help us to build more knowledge on SMA and treatment options in the future. 

Great Ormond Street Hospital (GOSH) is the coordinating centre of SMA REACH UK.

SMA REACH UK collects information about people with all types of SMA and keeps it in a secure online database. This includes details about their diagnosis and how their SMA is managed. With people’s consent, we collect this information during routine clinic appointments to help us learn more about SMA.

Find out more about SMA REACH UK.

SMA is treated by the Dubowitz Neuromuscular Centre at Great Ormond Street Hospital. 

Find out more information about the Dubowitz Neuromuscular Centre.