OBJECTIVE: To synthesize the best available evidence on the risk of Parkinson's disease (PD) after mild traumatic brain injury (MTBI).
DATA SOURCES: MEDLINE and other databases were searched (1990-2012) with terms including "craniocerebral trauma" and "parkinsonian disorders." Reference lists of eligible articles and relevant systematic reviews and meta-analyses were also searched.
STUDY SELECTION: Controlled clinical trials, cohort studies, and case-control studies were selected according to predefined criteria. Studies had to have a minimum of 30 concussion cases.
DATA EXTRACTION: Eligible studies were critically appraised using a modification of the Scottish Intercollegiate Guidelines Network criteria. Two reviewers independently reviewed and extracted data from accepted studies into evidence tables.
DATA SYNTHESIS: Evidence was synthesized qualitatively according to modified Scottish Intercollegiate Guidelines Network criteria. Sixty-five studies were eligible and reviewed, and 5 of these with a low risk of bias were accepted as scientifically admissible and form the basis of our findings. Among these admissible studies, the definitions of MTBI were highly heterogeneous. One study found a significant positive association between MTBI and PD (odds ratio, 1.5; 95% confidence interval, 1.4-1.7). The estimated odds ratio decreased with increasing latency between MTBI and PD diagnosis, which suggests reverse causality. The other 4 studies did not find a significant association.
CONCLUSIONS: The best available evidence argues against an important causal association between MTBI and PD. There are few high-quality studies on this topic. Prospective studies of long duration would address the limitations of recall of head injury and the possibility of reverse causation.