Review of estimates of birth incidence and population prevalence over time and between countries of the rare neurodevelopmental condition Prader-Willi syndrome

Epidemiological data such as birth incidence or population prevalence for rare conditions is difficult to obtain because of the large sample size required in order to obtain a valid estimate (ie based on a reasonable number of cases) and the difficulty of either identifying all cases or meeting the cost and logistics of screening the whole general population in that large sample. This article illustrates these difficulties in the case of the rare neurodevelopmental condition Prader-Willi syndrome (PWS). Before reviewing attempts that have been made over the years to estimate birth incidence and/or population prevalence in this condition, this review discusses aspects of PWS, clear definitions of the measures themselves and aspects of general populations that are pertinent to estimates of birth incidence and population prevalence. Although most published studies on PWS quote a prevalence (meaning not always clear) estimate, very few quote the original study on which it is based. Our review found very few (12) original studies. These are described in some detail and where relevant methodological details that render the findings suspect are discussed. Finally, we answer the question of the existence of global estimates for birth incidence and population prevalence of PWS, showing that they will vary from one population to another at a given time and from one time period to another for a given population. However, for very similar populations at a given time, the values may lie within a very close range.