What's spinal muscular atrophy?
Spinal muscular atrophy (SMA) is a genetic neuromuscular disease in which loss of a working SMN1 gene leads to the breakdown of the nerve cells that drive muscles (motor neurons). The result is progressive muscle weakness. Severity ranges widely and tracks loosely with how many copies of a backup gene, SMN2, a person carries.
| Also indexed as | OMIM:253300, MONDO:0009669 |
|---|---|
| Features mapped | 10 |
| Treatments mapped | 3 |
| Published sources | 14 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Proximal lower limb muscle weakness
Weakness is typically most pronounced in the muscles closest to the trunk, including the hips and thighs, so the proximal lower limbs are affected early in spinal muscular atrophy.
Death in childhood
Historically, the most severe (untreated) form of SMA was a leading genetic cause of death in infancy. This describes the natural history before disease-modifying treatment; outcomes have since changed (see Treatment).
Areflexia
Spinal muscular atrophy often involves areflexia, which means the usual reflexes (such as the knee-jerk reflex) are reduced or absent.
Autosomal recessive inheritance
SMA is inherited in an autosomal recessive pattern: a child develops it when they inherit a non-working SMN1 gene from both parents, who are usually unaffected carriers.
Tongue fasciculations
Fine, flickering movements of the tongue (fasciculations) can be seen in spinal muscular atrophy and reflect degeneration of the motor nerve cells supplying the tongue.
EMG: neuropathic changes
In spinal muscular atrophy, electrodiagnostic studies (electrical tests of nerve and muscle) can show denervation, the pattern seen when muscles lose their nerve supply.
Generalized hypotonia
Spinal muscular atrophy often involves generalized hypotonia, meaning low muscle tone throughout the body, so muscles feel floppy or loose.
Proximal amyotrophy
Because motor neurons are lost, the muscles they supply shrink and waste (atrophy), typically affecting muscles closer to the trunk earliest.
Respiratory insufficiency
As the muscles that support breathing weaken, respiratory insufficiency can develop in spinal muscular atrophy and is a major source of complications in the more severe forms.
Respiratory failure
As the muscles that drive breathing weaken, SMA can lead to respiratory failure. Breathing support is a central part of care, especially in the more severe forms.
How it is diagnosed
Spinal muscular atrophy, type I
Diagnosed using: SMN1 genetic testing (homozygous deletion).
“5q SMA was confirmed in one female patient (2% [95% CI 0.05-10.6]), who was found to have a homozygous deletion of exons 7-8 in the SMN1…”
Spinal muscular atrophy, type I
Diagnosed using: newborn screening.
“Prenatal carrier screening and universal newborn screening allow early identification and prompt postnatal treatment.”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
Nusinersen
Nusinersen is one of the disease-modifying therapies used in spinal muscular atrophy. It is an antisense oligonucleotide given by injection into the fluid around the spinal cord (intrathecally).
Used to help with: Spinal muscular atrophy, type I.
“The therapeutic ASO nusinersen (marketed as SpinrazaTM) targets intronic splicing silencer N1 (ISS-N1) located downstream of the predominantly skipped exon 7 of Survival Motor Neuron 2 (SMN2) gene.”
Risdiplam
Risdiplam is one of the disease-modifying therapies used in spinal muscular atrophy. It is taken by mouth and works by changing how the SMN2 gene is spliced so the body makes more SMN protein.
Used to help with: Spinal muscular atrophy, type I.
“Risdiplam is an oral disease-modifying therapy approved for the treatment of SMA in both pediatric and adult patient…”
Onasemnogene abeparvovec
Onasemnogene abeparvovec is one of the disease-modifying therapies used in spinal muscular atrophy. It is a single-dose gene replacement therapy given into a vein that delivers a working copy of the SMN1 gene using a viral vector.
Used to help with: Spinal muscular atrophy, type I.
“…eligible for intravenous onasemnogene abeparvovec…”
What changes how it shows up
The diagnosis is not the whole story. The factors and open questions below are described in the research mapped here as shaping whether, or how strongly, the condition shows up, or as points the field has not yet settled. They are not, on their own, its cause or its cure.
SMN2 copy number
The number of copies of the SMN2 gene a person has is a principal modifier of spinal muscular atrophy. A higher SMN2 copy number is generally associated with milder disease, and a lower copy number with more severe disease.
Described as modulating: Spinal muscular atrophy, type I.
“A significant inverse correlation was observed between SMN2 copy number and disease…”
autosomal recessive inheritance
Spinal muscular atrophy is inherited in an autosomal recessive way, which means a person develops it when they inherit a non-working copy of the gene from each parent.
Described as modulating: Spinal muscular atrophy, type I.
“Spinal Muscular Atrophy (SMA) is an autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in the survival motor neuron 1 gene…”
How to read the evidence labels
Where this comes from
This guide is built from 14 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.