Characterised by slowly progressive muscle weakness and atrophy of the limb muscles associated with motor neurone loss in the spinal cord

Aetiology

• The most common form of SMA is caused by a mutated or missing gene known as the survival motor neuron gene 1 (SMN1)
  • Results in the loss of anterior horn cells

Clinical features

Symptoms

• Muscle weakness and wasting
  • Proximal skeletal muscles
  • Respiratory muscles

Signs

• General clinical signs are that of lower motor neurone weakness - hypotonia, flaccid weakness, reduced/abscent tendon reflexes, normal/absent plantar reflexes, muscle fasciculation, muscle atrophy

Investigations

• Creatine kinase: normal or slightly raised
• Genetic testing
• Electrophysiology: shows diminished nerve signals
• Muscle biopsy: muscle fibre atrophy

Management

• Treatments have been developed that involve mRNA modification - correction of SM1 deficiency by altering splicing of SMN2 mRNA
  • Injection into spinal canal
  • Effective in preventing progression but do not reverse the loss of anterior horn cells - disability can be prevented by starting treatment at birth