Spinal muscular atrophy (SMA) is diagnosed primarily through a genetic test that looks for a missing or altered SMN1 gene. About 95 to 98% of people with SMA have zero copies of this gene, making the test highly reliable. In many cases today, the diagnosis happens before symptoms even appear, thanks to newborn screening programs now active across the United States.
Newborn Screening Catches SMA Early
Between 2018 and 2024, every U.S. state adopted SMA screening as part of its standard newborn panel. The test uses a DNA amplification technique to check whether a baby is missing a critical section of the SMN1 gene (exon 7), which accounts for roughly 95% of SMA cases. In most states, this test runs alongside screening for severe combined immunodeficiency (SCID), so no extra blood draw is needed.
Early detection matters enormously for SMA. Treatments work best when started before motor neurons are lost, and newborn screening has compressed the time between birth and diagnosis from months or years to days or weeks. Babies identified through screening can begin treatment in a window where it has the greatest impact on their ability to sit, stand, and breathe independently.
Genetic Testing Is the Gold Standard
Whether SMA is flagged through newborn screening or suspected based on symptoms, the definitive diagnosis comes from a genetic test. The standard method is called MLPA (multiplex ligation-dependent probe amplification), which counts how many copies of the SMN1 gene a person has. A result showing zero copies of SMN1 confirms the diagnosis in 95 to 98% of cases.
The remaining 2 to 5% of people with SMA have one copy of SMN1 plus a smaller mutation within that copy. These cases require a more detailed analysis called gene sequencing to find the specific change. If a doctor suspects SMA but the initial deletion test comes back with one or two copies of SMN1, sequencing is the logical next step.
SMN2 Copy Number and Severity
Once SMA is confirmed, the lab also counts copies of a related gene called SMN2. This gene produces a small amount of the same protein that SMN1 would normally make, and having more copies generally means a milder form of the disease. The correlation is strong but not absolute:
- 1 copy of SMN2: Usually associated with the most severe, congenital form of SMA.
- 2 copies: Over 90% of these newborns develop type I SMA, the most common severe form, though rare exceptions with milder disease exist.
- 3 copies: About 60% develop type II (intermediate severity), 35% develop the milder type III, and roughly 5% still develop severe type I.
- 4 or more copies: Generally linked to milder forms, though some individuals still develop type I or II disease.
Because of these exceptions, SMN2 copy number helps guide treatment urgency and set expectations, but it cannot predict exactly how severe a child’s SMA will be. Doctors use it as one piece of the picture alongside clinical findings.
Clinical Signs That Raise Suspicion
Before widespread newborn screening, and in places where it isn’t available, SMA is typically suspected based on physical exam findings. The signs vary by age and severity, but a few are distinctive enough to point a doctor toward SMA specifically.
Tongue fasciculations, visible involuntary twitching of the tongue, are considered a hallmark of SMA and are present in about 56% of patients. Unlike most other neuromuscular diseases of infancy, tongue fasciculations strongly suggest SMA. Absent reflexes are another consistent finding. Loss of the knee-jerk reflex is characteristic of type II SMA, and absent reflexes throughout the body are essentially universal across all types.
In infants with the severe type I form, the picture is often striking. Newborns appear floppy and move their limbs very little. Their hips tend to rest in a “frog-leg” position, flexed and turned outward. They have a bell-shaped torso, where the chest appears narrow and collapsed while the belly protrudes, creating a distinctive breathing pattern where the abdomen rises while the chest sinks with each breath. Feeding difficulties, a weak cry, and trouble swallowing are common. These babies cannot lift their heads or sit without support.
In milder forms, the signs can be subtler. Children with type II or III SMA may show upper-limb tremors, progressive scoliosis (which eventually affects nearly all patients), and calf muscles that appear unusually large. That last feature, called pseudohypertrophy, can initially lead doctors to consider muscular dystrophy instead. The combination of enlarged calves with absent reflexes and tongue fasciculations helps distinguish SMA from conditions like Duchenne muscular dystrophy.
Nerve and Muscle Testing
Electromyography (EMG) and nerve conduction studies are sometimes used when the diagnosis is uncertain or when genetic results are pending. These tests involve placing small electrodes on or into muscles to measure their electrical activity and how well nerves communicate with them.
In SMA, EMG typically shows a pattern consistent with motor neuron loss rather than muscle disease. The electrical signals from individual motor units are large, reflecting the fact that surviving nerve cells have taken over control of muscle fibers abandoned by dying neurons. At the same time, signs of ongoing nerve damage appear: spontaneous electrical flickers (fibrillation potentials) and repetitive discharges that indicate muscles are losing their nerve supply. These findings can help distinguish SMA from other conditions that cause weakness, particularly in older children and adults where the presentation overlaps with amyotrophic lateral sclerosis (ALS).
Blood tests for creatine kinase (CK), an enzyme released by damaged muscle, play a limited supporting role. CK levels in SMA reflect the degree of muscle denervation and tend to be higher in type III than in types I or II. However, CK levels alone cannot diagnose SMA, and they overlap significantly with other neuromuscular conditions. Their main value is in tracking disease progression over time rather than making an initial diagnosis.
How the Diagnostic Process Typically Unfolds
For most newborns in the U.S., the process starts with the routine heel-prick blood test done in the first days of life. If the screen flags a possible SMN1 deletion, the family is contacted and a confirmatory genetic test is ordered. This confirmatory test also counts SMN2 copies to help gauge likely severity. Results typically come back within one to two weeks, and treatment discussions begin immediately.
For children or adults who were not screened at birth, the path is different. A parent or doctor notices weakness, delayed motor milestones, or difficulty with movement. A physical exam reveals some combination of reduced reflexes, tongue fasciculations, or characteristic posture. The doctor orders genetic testing for SMN1 deletion, and if the result confirms zero copies, the diagnosis is made. If one copy is found, sequencing follows to look for a point mutation on that remaining copy. EMG and nerve conduction studies may fill in the picture while genetic results are pending, especially if other diagnoses are being considered.
The overall trend is toward earlier and faster diagnosis. With newborn screening now standard across the country, most children with SMA in the U.S. are identified before they show symptoms. That shift has fundamentally changed the disease’s trajectory, since the treatments available today are most effective when started in the presymptomatic window.