A guide to SMA — what it is, how it's caused, the different types, how it's diagnosed, and how treatment has transformed the outlook.
Spinal muscular atrophy (SMA) is a rare, inherited neuromuscular condition caused by a fault in a gene called SMN1 (Survival Motor Neuron 1). This gene is responsible for producing a protein called SMN, which motor neurons in the spinal cord need to survive. Without enough SMN protein, these motor neurons progressively deteriorate and die.
As motor neurons are lost, the muscles they control become progressively weaker. This is why SMA primarily affects movement — the ability to walk, sit, hold the head upright, swallow and breathe can all be impacted, depending on severity. SMA does not affect intelligence or cognitive ability.
Significant treatment advances since 2017 have dramatically changed the outlook. Today, with early treatment, many children with SMA are reaching developmental milestones that would have been impossible a decade ago.
SMA does not affect intelligence. People with SMA have the same intellectual capacity as anyone else — the condition affects motor neurons only, not thinking, memory or emotion.
Humans have a second, nearly identical gene called SMN2 that can produce a small amount of functional SMN protein — roughly 10% of what SMN1 produces. Everyone with SMA still has at least one copy of SMN2, and most people have between 1 and 4 copies. Having more copies is generally associated with a milder form of SMA. All three NHS-funded treatments work by either increasing SMN protein from the SMN2 gene, or replacing the missing SMN1 gene entirely.
SMA is classified into types based on age of symptom onset and highest motor milestone achieved without treatment. With modern treatments — especially pre-symptomatic treatment — the boundaries between types are becoming less fixed.
| Type | Also called | Onset | Milestones (untreated) | SMN2 copies (typical) |
|---|---|---|---|---|
| Type 0 | Prenatal | Before birth | Rarely survives beyond a few months | 1 |
| Type 1 | Werdnig-Hoffmann | 0–6 months | Cannot sit independently; breathing and swallowing difficulties | 1–2 |
| Type 2 | Dubowitz | 6–18 months | Can sit; cannot stand or walk unaided | 3 |
| Type 3 | Kugelberg-Welander | 18 months onwards | Can walk; many lose ability to walk in childhood or adulthood | 3–4 |
| Type 4 | Adult-onset | Adult life | Ambulatory; slow progression; often mild symptoms for years | 4+ |
These categories reflect the typical natural history without treatment. Pre-symptomatic treatment can fundamentally alter the course of the disease — a child identified via newborn screening and treated before symptoms appear may far exceed the milestones historically associated with their SMA type.
The most common form — 5q SMA — is what this site primarily addresses. There are also rarer forms including Kennedy's disease (spinal and bulbar muscular atrophy, affecting adult men), SMARD1 (SMA with respiratory distress), Distal SMA, SMA-LED and pontocerebellar hypoplasia type 1B. If your diagnosis is one of these rarer forms, your specialist neuromuscular team can advise on the specific condition and available management options.
5q SMA is inherited in an autosomal recessive pattern. A person must inherit a faulty copy of SMN1 from both parents to develop SMA. Someone who inherits just one faulty copy is a carrier — they don't have SMA but could pass it to a child. Around 1 in 40 to 1 in 50 people in the UK carry a faulty SMN1 copy without knowing it. When two carriers have a child together, each pregnancy has a 25% chance of the child having SMA, 50% chance of being a carrier, and 25% chance of having two normal copies. Carrier testing is available — ask your GP for a referral to genetic counselling.
SMA is diagnosed through a genetic blood test looking for deletions or mutations in the SMN1 gene. In Scotland, SMA has been included in the newborn bloodspot screening programme since 2021, meaning babies are tested within days of birth — before symptoms appear. A proposal to extend newborn screening to England is currently under review by the UK National Screening Committee. If you have concerns about your child's development, speak to your GP or health visitor. Adults with unexplained progressive weakness should ask for a referral to a neuromuscular specialist — Type 4 SMA is sometimes diagnosed after years of symptoms without a clear cause.
The outlook for people living with SMA has changed fundamentally since 2017. Three NHS-funded treatments now address the underlying genetic cause. For children treated pre-symptomatically, outcomes have been dramatic — babies who would previously have been severely affected by Type 1 SMA are instead walking and attending school. For people already living with SMA, these treatments can slow progression and, in some cases, lead to improvements in function. SMA remains a lifelong condition requiring ongoing specialist care, but for most people with SMA in the UK today the prognosis is substantially more positive than for previous generations.