Using HES, ONS mortality data and registration data, we estimate that one in 16,230 babies born in England have SMA1 (around 42 babies a year). The estimated prevalence of 6.2 per 100,000 births (95% CI: 5.6-6.8) aligns with other published figures.
We identified 106 people in England who were living on 1 July 2016 with SMA1 born since 2000. This gives a population prevalence of 1.9 per million (95% CI: 1.6-2.3). Over the 10 years 2008 to 2016, the estimated birth prevalence (incidence) was 6.2 per 100,000 births (95% CI: 5.6-6.8) or approximately one per 16,350 births (Table 1).
Over the period 2009 to 2016, we estimate the infant mortality rate in patients with SMA1 to be 4.2 per 100,000 live births (95% CI: 3.7-4.8) (Table 2). We calculated infant mortality rate (IMR) for cases where SMA, SMA0 or SMA appeared anywhere on the death certificate for the years 2008 to 2017. In 2017, there appears to have been a drop the infant mortality rate in babies where SMA0, SMA1 or SMA appear on the death certificate. Nusinersen became available in the UK through the Expanded Access Programme in 2017, though access was limited. Reference: Spinal Muscular Atrophy UK. (2019) UK Access Summary: From Autumn 2016 to May 2019
Table 1. Birth prevalence (per 100,000 live births) with 95% confidence intervals for SMA0, SMA1 and SMA based on death certificate and hospital admissions data. Vital status per March 2019
Table 2. Infant mortality rate (per 100,000 live births) with 95% confidence intervals for SMA0, SMA1 and SMA based on death certificate data.
There is the possibility of misclassification of cases using both ONS mortality data and HES data. Ascertaining SMA cases from mortality data requires using both coded and free text cause of death fields. Ideally, these cases should be validated against another data source, particularly if the type of SMA is not specified. HES data did not capture all the cases that were identified using mortality data. Also, HES cases that contained a code indicating SMA1 were not all confirmed when hospital patient information was checked.
These findings demonstrate the value of ONS mortality and HES data to support rare disease registration. They also reinforce the need to apply a multi-source approach to case ascertainment and completion. By using available routinely collected data, and expanding existing and creating new NCARDRS data feeds, we will increase the accuracy and usefulness of rare disease data for healthcare planning and research.