Incidence of infantile spinal muscular atrophy on Shikoku Island of Japan.

BACKGROUND: Spinal muscular atrophy (SMA) is an autosomal recessive disorder caused by homozygous mutations in the SMN1 gene. SMA has long been known to be the most common genetic cause of infant mortality. However, there have been no reports on the epidemiology of infantile SMA (types 1 and 2) based on genetic testing in Japan. In this study, we estimated the incidence of infantile SMA on Shikoku Island, which is a main island of Japan and consists of four prefectures: Ehime, Kagawa, Tokushima and Kochi.

METHODS: A questionnaire was sent to 91 hospitals on Shikoku Island to investigate the number of SMA infants born from 2011 to 2015. A second questionnaire was then sent to confirm the diagnoses of SMA based on clinical and genetic features.

RESULTS: Responses were received from all of the hospitals, and four patients were diagnosed with infantile SMA among 147,950 live births. We estimated the incidence of infantile SMA patients as 2.7 per 100,000 live births (95% confidence interval, 0.1-5.4). A comparison of the four prefectures indicated that the incidence of infantile SMA was significantly higher in Ehime Prefecture than in the other three prefectures; 5.6 per 100,000 live births (95% confidence interval, -0.7 to 11.9) in Ehime Prefecture and 1.1 per 100,000 live births (95% confidence interval, -1.0 to 3.1) in the other prefectures.

CONCLUSION: We estimated the incidence of infantile SMA in an isolated area of Japan. For more precise determination of the incidence of infantile SMA, further studies that include neonatal screening will be needed.