SMN2 copy number sets SMA severity from Type 1 to Type 4, and newborn screening finds about 300 US infants a year before symptoms appear.

Spinal muscular atrophy is an autosomal-recessive motor-neuron disease caused by biallelic loss or mutation of SMN1 in roughly 98% of cases. Severity is graded by the number of copies of the paralogous SMN2 gene, which makes a small amount of functional protein: more copies generally means a milder phenotype. US prevalence is estimated at 8,000 to 10,000 patients, with incidence near 1 in 10,000 live births. The clinical spectrum runs from Type 1, with onset before six months and no independent sitting if untreated, through Types 2 and 3 to the mildest, adult-onset Type 4.

Two features define the modern landscape. Natural history is severe at the Type 1 end: untreated, median survival is about 13.6 months, driven by rapid motor-neuron loss in the first months of life, the window where treatment matters most. Newborn screening has also transformed detection. SMA was added to the federal Recommended Uniform Screening Panel in 2018, and all 50 states now screen, identifying roughly 300 infants a year before symptoms emerge. That has created two distinct populations, pre-symptomatic and older symptomatic patients, that call for different treatment strategies.

Newborn screening has split SMA into two populations with different treatment logic.

Five questions this report answers:

Q1 - How does SMN2 copy number map to SMA type and treatment eligibility?

Q2 - How has newborn screening changed the US SMA patient population?

Q3 - What is the untreated natural history of SMA across its four types?

Q4 - What causes SMA, and how is disease severity determined genetically?

Q5 - How many pre-symptomatic SMA infants does newborn screening find each year?

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#SMA #SpinalMuscularAtrophy #NewbornScreening #RareDisease #Pediatrics #USHealthcare

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