• Indirect estimation of the prevalence of spinal muscular atrophy Type I, II, and III in Iran
  • Yasamin Ghaffari,1,*
    1. Faculty of life sciences and biotechnology, Shahid Beheshti University


  • Introduction: Spinal muscular atrophy linked to chromosome 5q (SMA) is a recessive, progressive, neuromuscular disorder caused by deletion or mutation in SMN1 gene.In the absence of a functional SMN1 gene, the body relies on its homolog gene called SMN2 to produce SMN protein .The IMDA published a report that estimates of SMA genotype prevalence at birth consistently range from 0.8-1.2 per 10,000 live births. Among infants born with an SMA genotype, it is reported that 66% will develop SMA Type I, 24% will develop Type II, and 10% will develop Type III, respectively.Since SMA is a proportionately rare condition, studies of its prevalence and incidence are challenging.
  • Methods: Indirect estimation using a life table method was employed to estimated the age and type-specific prevalence of SMA.The goal of this article was to estimate the total number of people living with symptomatic SMA types I, II and III in Iran.Current US age-specific population projections for the year 2018 were obtained from the IMDA.Prevalence estimates were calculated separately by SMA type to account for the differing rate of birth prevalence and estimated survival for each type. This method relied on the following 5 types of information.(1)Birth prevalence(We assumed that each baby born alive with an SMA genotype would eventually be recognized as having an SMA phenotype.)(2)Disease onset: the estimated age of reported symptom onset and diagnosis (as a proxy for disease onset) from the literature for SMA Types I and II was <1 year. For SMA Type III, the earliest age at onset was in the second year, implying cases were not observed at birth or 1 year of age and thus no onset during this interval.(3) SMA survival from birth: for SMA Types I and II, we used available survival estimates from the literature for studies in which survival was reported. We note that a limited body of literature was available and reflected.(4) Age-specific survival estimates for the Iranian population by 1-year age groups were taken from the most recent available life table estimates published in the National Vital Statistics Report (5) Number of persons in the population by age: age-specific large variation in standards of care and survival
  • Results: survivalThe estimates of SMA genotype prevalence at birth reported in the literature are consistently in a range from 1per 10000 live births globally. Survival probabilities across the 2 studied ranged from 42-92% at 1 year and26–84% at 2 years.Some studies reported survival probabilities at the following other time points: 26–72% at 4 years and 8–20% at 10 years(most type3). It is unknown how many patients with SMA in Iran receive respiratory support but as we study in 100 patient 82 percent recieve this treatment.The survival of patients with SMA Type II was 100% at 1, 2, and 4 years of age. Beyond 4 years of age, the survival was 82% at 10 years and was unchanged at 15 years of age when the study terminated.For SMA Type III, the literature reports a normal life expectancy.
  • Conclusion: Estimates of the number of people living with SMA in Iran in the published literature were previously unavailable. In the absence of a survey or other means to directly estimate prevalence in the Iranian population, we used an indirect method. By utilizing available published estimates of genotype prevalence at birth, age of disease onset, and subsequent survival, we were able to estimate the SMA population for each year of age and subsequently for the entire population.
  • Keywords: Incidence; Prevalence; Spinal muscular atrophy