Spinal muscular atrophy (SMA) is a rare autosomal recessive condition caused by deletion or mutation of the SMN1 gene, leading to deficiency of survival motor neuron (SMN) protein. It causes a progressive loss of motor neurones, causing progressive muscular weakness.
Presentation
Spinal muscular atrophy affects the lower motor neurones in the spinal cord. This means there will be lower motor neurone signs:
- Fasciculations
- Reduced muscle bulk
- Reduced tone
- Reduced power
- Reduced or absent reflexes
Categories
There are four categories of spinal muscular atrophy that are numbered from most to least severe.
SMA type 1 has an onset in the first few months of life. Previously it would progress to death within 2 years. However, new genetic therapies have extended life expectancy beyond 2 years.
SMA type 2 has an onset in the first 18 months. Most never walk, but survive into adulthood.
SMA type 3 has an onset after the first year of life. Most walk without support, but subsequently lose that ability. Respiratory muscles are less affected and life expectancy is close to normal.
SMA type 4 has an onset in the 20s. Most will retain the ability to walk short distances but require a wheelchair for mobility. Everyday tasks can lead to significant fatigue. Respiratory muscles and life expectancy are not affected.
Management
New advanced therapies, such as nusinersen and risdiplam, have dramatically improved survival in patients with spinal muscular atrophy. They increase production of the SMN protein, helping motor neurones survive and function.
Physiotherapy can be helpful in maximising strength in the muscles and retaining respiratory function. Splints, braces and wheelchairs can be used to maximise function.
Respiratory support with non-invasive ventilation may be required to prevent hypoventilation and respiratory failure, particularly during sleep. Children with SMA type 1 may require a tracheostomy with mechanical ventilation, which can dramatically extend life by supporting the failing respiratory muscles.
Percutaneous endoscopic gastrostomy (PEG) feeding may be required when a weak swallow makes swallowing unsafe.
Last updated February 2026
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