Heterogeneity in male to female risk for Parkinson's disease

We read with interest the recent meta-analysis of seven studies that reported that the age-adjusted incidence of Parkinson’s disease was 1.5 times greater in men than in women.1 However, this meta-analysis excluded several informative studies (such as those with <50 patients and those restricted to older cohorts) and did not explore heterogeneity in the male to female (M:F) ratios. We were also unable to replicate some of the data used in the meta-analysis. For example, we believe it misquoted some crude incidence rates as being age-adjusted (eg, the studies from Ferrara and Olmstead County). In the process of updating our previous systematic review of studies on the incidence of Parkinson’s disease,2 we performed a new meta-analysis of age-adjusted M:F incidence ratios for Parkinson’s disease and attempted to identify the causes of heterogeneity. Additional studies published between January 2002 and April 2005 were identified using the same search strategy and inclusion criteria as those in the previous review.2 Where possible, the age-standardised M:F incidence ratio for each study was calculated …