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Study Clarifies Prevalence and Incidence of SMA in Japan


An epidemiological investigation of spinal muscular atrophy (SMA) in Japan provided clarity on the genetic condition’s prevalence and incidence in the country and highlighted the importance of newborn screening for SMA

Research published in Brain & Development aims to provide an understanding of the actual number of patients with spinal muscular atrophy (SMA) Japan, based on data from 1005 medical facilities nationwide. Previous surveys regarding the rare disease were limited to Shikoku, one of the 5 major islands of Japan, despite the epidemiology of the condition becoming clearer in other countries in recent years.

SMA is an inherited motor neuron disease that causes muscle atrophy and progressive weakness due to degenerating spinal cord anterior horn cells. Genetic testing can identify the condition, which is classified into 5 levels based on age of symptom onset and motor milestones. This spectrum is collectively referred to as 5q-SMA, and in recent years there has been an increase in treatment options.

“There is an urgent need to analyze the prevalence and incidence of 5q-SMA, which is a rare disease, and its clinical features to properly introduce new treatments without missing the timing of treatment,” the authors wrote, adding that one goal of the survey is clinical implementation of new treatments. “Considering the importance of therapeutic intervention, including neonatal screening, in the early neonatal period, the incidence was calculated in addition to the prevalence.”

Questionnaires were sent out to 1936 medical facilities, and responses were received from 1005. The window for responses was between January 30, 2018, and March 31, 2018. All types of clinical SMA in patients, from newborns to adults, were included.

After confirming that each facility had patients with SMA, the researchers requested information on the following:

  • Number of patients by SMA type
  • Prevalence between January 1, 2017, and December 31, 2017
  • Incidence between January 1, 2015, and December 31, 2017
  • Number of participants of genetic testing on SMA
  • Number of the patients who had received nusinersen (a treatment the authors note has only been on the market since 2017)
  • Number of patients who wished to receive nusinersen treatment
  • Number the patients who wanted to receive a novel treatment.

Secondary surveys collected data on the clinical features of each patient’s case.

In all, 24.5% (n = 246) of the facilities confirmed they had patients with SMA, with an average of 2.67 patients with SMA per facility.

The actual number of patients with SMA as of December 2017 was 658 across the facilities, excluding duplicate cases between locations. Of those patients, 523 (79.5%) were confirmed as 5q-SMA with genetic testing. In regard to types, there was 1 patient with type 0 SMA, 187 patients with type 1, 262 patients with type 2, 143 with type 3, and 49 with type 4. Sixteen were 16 listed as unknown/unanswered.

Based on these data, the estimated overall number of patients with SMA in Japan was 1478 (95% CI, 1122-1834) with a prevalence of 1.17 (95% CI, 0.89-1.45) per 100,000 individuals. The estimated incidence was 0.51 (95% CI, 0.32-0.71) per 10,000 live births. Notably, 363 cases (82.7%) occurred at ages younger than 2 years and 88 (20.0%) occurred at 2 months or younger.

“As infantile onset accounts for most cases of SMA,” the authors concluded. “Newborn screening and subsequent treatment are important to save lives.”


Ito M, Yamauchi A, Urano M, et al. Epidemiological investigation of spinal muscular atrophy in Japan. Brain Dev. Published online August 24, 2021. doi:10.1016/j.braindev.2021.08.002

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