Could reverse causality or selective mortality explain associations between leg length, skull circumference and dementia? A South Indian cohort study.

In cross-sectional studies, skull circumference and leg length are often inversely associated with dementia prevalence (Prince et al., in press). Skull circumference and leg length are thought to remain stable across the adult life course, but the associations might yet be explained by reverse causality. Weight loss in dementia could lead to loss of subcutaneous scalp fat. Osteoporosis mainly affects trunk proportions (loss of disc space, vertebral fractures and kyphosis), but limited knee extension could also lead to apparent reductions in leg length. No previous studies have assessed changes in these measurements over time in older people with and without dementia. Only two cohort studies have examined the effect of skull circumference (Borenstein et al., 2001) and knee height (Huang et al., 2008) on incident dementia. Even in cohort studies differential mortality can lead to bias. Accordingly, in a population-based cohort study of older people in Chennai, India we aimed to answer five questions For reverse causality: