As we noted in Chapters 1 and 3, few older people with PWS have been reported in the literature and in our population-based cohort no one was identified older than 46 years of age at the standardisation date, the turn of the twenty-first century. We estimated a birth incidence for PWS of 1:22 000 and a population prevalence of no less than 1:52 000, with a mortality rate of about three per cent per year. Some support for the interpretation of our findings, of a more or less constant mortality rate across ages, has come from a report on sudden death of people with PWS in Australia, which found mortality among all age groups (see Chapter 4). Previous reports indicated that the most common cause of death was cor pulmonale possibly attributable to severe obesity. Also the neonatal hypotonia is thought to contribute to the presence of strabismus, scoliosis and respiratory problems in children, and the obesity to non-insulin dependent diabetes mellitus (NIDDM) and heart conditions in older people.

This chapter describes the prevalence rates of physical morbidity in the population-based cohort of children and adults with the syndrome, and we argue that some are a direct and others a secondary consequence of the syndrome.