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Newborn screening programs have revised prevalence estimates of spinal muscular atrophy (SMA), with data showing fewer cases than expected in the U.S.

Based on population-level data from 30 states' screening programs, approximately one in every 14,694 babies born from 2018 through 2022 had SMA, reported Lisa Belter, MPH, of the patient advocacy organization Cure SMA in Elk Grove Village, Illinois, and colleagues.

Prior studies had estimated a rate of one in 10,000, the group noted in a research letter in .

Those earlier estimates relied on "a variety of mechanisms including carrier screening, small studies performed before genetic testing was available, and patient registries," whereas this is the first to specifically utilize newborn screening programs, noted Amanda Barone Pritchard, MD, of the Mott Children's Hospital at the University of Michigan in Ann Arbor, who was not involved in the study.

Widespread newborn screening for SMA has been in place across the U.S. since 2018, the paper noted, and all 50 states now screen newborns for the condition. Early identification of SMA through screening has been shown to improve motor development by allowing treatment to begin sooner. It also catches milder cases where symptoms may not develop until later in life, Pritchard added.

"As newborn screening is performed on nearly all infants and involves genetic testing of the SMN1 gene, this will be an effective way to determine how many individuals are born with this condition," Pritchard told MedPage Today.

Better prevalence data can be used "to anticipate healthcare resource use for SMA and to plan future research in SMA care and treatment," Belter's group wrote.

Belter and colleagues noted that in 2017, the American College of Obstetricians and Gynecologists joined the American College of Medical Genetics and Genomics in recommending carrier screening for SMA to all people who were considering their reproduction options.

"This change in care practice informs reproductive choices and may be a factor in the current birth prevalence," they wrote.

The research team from Cure SMA looked at data on SMA newborn screening between January 27, 2018, and December 31, 2022. Data on newborn screening were collected from 41 of the 48 state public health labs doing screening at the time. The study included a total of 6,244,825 infants, of whom 425 had a confirmed SMA diagnosis.

National birth prevalence of SMA was calculated by dividing the number of babies with SMA identified by screening with a confirmed SMA diagnosis divided by the total number of infants screened in each state. The researchers did not include false-positive screening results.

New York, Pennsylvania, and Florida had the most positive test results (43, 41, and 40 cases, respectively), although they also screened a larger number of infants than most states. The prevalence of SMA was lower than the national average in some states. For instance, although New York had the most confirmed cases of SMA, its prevalence was only 1 in 20,310.

While both copies of the SMN1 gene are typically missing in the neurodegenerative disease, having more copies of the SMN2 gene to produce a small amount of functional SMA protein makes for milder disease. Twenty-one states also provided a SMN2 copy number for infants with confirmed SMA. Of those 240 infants, 49% had two copies, 33% had three copies, 13% had four or more copies, and just 5% had one copy.

In terms of limitations, the authors noted that the data were collected at different time points following statewide adoption of SMA newborn screening; thus, the results were actually minimum birth prevalence rates. For the same reason, the authors couldn't report the proportion of births covered by their analysis and birth prevalence calculation was limited to states that reported the number of infants screened.

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