Can head injury cause Parkinson's disease?
There is some evidence suggesting a positive correlation between traumatic head injury and risk of developing Parkinson's disease. One meta-analysis which pooled data from 22 separate studies revealed that individuals who had experienced previous head injury had a 57% higher risk of Parkinson's disease compared to those who had not experienced head trauma (reflected by a pooled odds-ratio of 1.57) [1]. Another individual study reported that head injuries which were accompanied by amnesia or loss of consciousness correlated with a significantly higher risk of Parkinson's disease incidence (odds-ratio = 3.8) [2], although the precision of the methods used in this study is questionable.
It is crucial to recognise that such studies reveal population-level associations and cannot reveal direct causation. Some researchers suggest that the observed association between head injury and Parkinson's disease could actually be caused by injuries that occur near the time of Parkinson's disease diagnosis, implying reverse causality (i.e., PD-related issues with balance and motor control may lead to head injury, rather than vice versa) [3, 4]. Thus, while there is some evidence of an association between head trauma and PD, the causal relationship between the two remains unclear.