Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by mutations in the survival motor neuron 1 (SMN1) gene, resulting in SMN protein deficiency. This leads to muscle atrophy and a progressive decline in motor abilities due to muscle weakness. Secondary complications include scoliosis, joint contractures, fatigability, and progressive respiratory decline.

There are four main types of SMA: 

• SMA Type 1: Symptoms usually begin between 0 and 6 months – Never achieve sitting
• SMA Type 2: Symptoms usually appear between 7 and 18 months of age – Achieve sitting but never walk
• SMA Type 3: Achieve walking. There are two types of SMA Type 3
• SMA Type 3a: Symptoms usually begin between 18 months and 3 years of age.
• SMA Type 3b: Symptoms will usually begin after 3 years of age.
• SMA Type 4: Symptoms begin in adulthood.

However, they are all on a continuum, which is evolving with new therapeutic options. Current functional status classification can therefore be useful:

• Non-sitter – Unable to sit without arm support for 3 seconds – Score 0 or 1 on Item 1 of HFMSE or RHS
• Sitter – Able to sit for at least 3 (or 10) seconds unsupported (without arm support)- Score 2 on Item 1 of HFMSE or RHS
• Walker – Able to *walk independently for 5 steps without arm support (this includes walking aids or hands on legs)

*Definition of a walker varies from country to country, trial to trial so check with your local guidelines