Spinal muscular atrophy (SMA)

Overview

Spinal muscular atrophy (SMA) is a genetic condition, caused by changes in the SMN1 gene, that affects motor neurons — the nerve cells that carry signals from the brain to the muscles. As these motor neurons break down, the signal between brain and muscle is disrupted, leading to progressive muscle weakness and wasting. SMA is generally classified into five types by age of onset and severity: Type 0 (onset before birth), Type 1 (the most severe postnatal form, appearing before 6 months, historically with survival under 2 years without treatment), Type 2 (onset between 6 and 18 months), Type 3 (onset from 18 months into adulthood), and Type 4 (adult-onset, generally mild).

Symptoms

Type 1 typically presents as a very floppy baby with significant breathing and feeding difficulties. Type 2 usually allows a child to sit independently but not to walk unsupported, and carries a notable risk of scoliosis. Type 3 often allows walking to begin normally, followed by gradually progressive weakness over time. Type 4 causes gradual weakness in the arms and legs in adulthood but doesn't affect life expectancy.

Inheritance

SMA is autosomal recessive — a child needs a changed copy of the SMN1 gene from both parents to be affected; parents who carry just one changed copy are unaffected. A related gene, SMN2, can partly compensate for a faulty SMN1 gene, and the number of SMN2 copies a person has plays a significant part in how severe their SMA turns out to be.

Getting a diagnosis

Diagnosis is confirmed with a genetic blood test that checks the number of SMN1 and SMN2 gene copies present.

Management and outlook

Care is delivered through a multidisciplinary team, and several disease-modifying treatments are now available that can significantly alter the course of the condition, including nusinersen (Spinraza), risdiplam (Evrysdi) and onasemnogene abeparvovec (Zolgensma) — the right option depends on age, SMA type and individual circumstances, best discussed with the specialist team.

Physiotherapy helps maintain strength and function, and respiratory support — vaccinations, chest physiotherapy, and non-invasive ventilation where needed — helps manage breathing. Orthopaedic input addresses scoliosis, contractures and fracture risk as they arise, alongside ongoing nutritional monitoring. As with other neuromuscular conditions, careful pre-operative planning is essential before any surgery.