Two-hit hypothesis?
The researchers compared time from diagnosis to the development of significant motor impairment, such as the need for assistance with walking, and cognitive impairment, such as having a score of less than 43 on the Penn Daily Activities Questionnaire.
They also examined the role of more severe head injuries. In the head injury group, over half (54%) had had a severe head injury, including 543 who had lost consciousness and others who had suffered a fracture or had had a seizure.
Results showed that the adjusted hazard ratio for developing motor impairment among those with a head injury, compared with those who had not had a head injury was 1.24 (95% confidence interval, 1.01-1.53; P = .037). For severe injuries, the aHR for motor impairment was 1.44 (95% CI, 1.13-1.83; P = .003).
For cognitive impairment, the aHR for those with versus without head injuries was 1.45 (95% CI, 1.14-1.86; P = .003); and for severe injuries, the aHR was 1.49 (95% CI, 1.11-2.0; P = .008).
Aside from severity, the researchers did not examine subgroups. However, Dr. Brown reported that his team would like to stratify results by sex and other variables in the future.
He noted that various mechanisms may explain why Parkinson’s disease progression is faster for patients who have a history of head injury, compared with others. Chronic inflammation due to the injury and “co-pathology” might play some role, he said. He noted that head injuries are associated with cognitive impairment in other conditions, including Alzheimer’s disease.
There is also the “two hit” hypothesis, Dr. Brown said. “A head injury could cause such broad damage that once people develop Parkinson’s disease, it’s harder for them to compensate.”
Dr. Brown also noted there might have been a “higher magnitude” of a difference between groups had the study captured participants with more severe symptoms.
‘Provocative’ findings
Michael S. Okun, MD, medical advisor at the Parkinson’s Foundation and professor and director at the Norman Fixel Institute for Neurological Diseases, University of Florida, Gainesville, said the new data are “provocative.”
“The idea that a head injury may be important in predicting how quickly and how severely deficits will manifest could be important to the treating clinician,” said Dr. Okun, who was not involved with the research.
He noted that the results suggest clinicians should elicit more information from patients about head trauma. “They should be seeking more than a binary ‘yes or no’ answer to head injury when questioning patients,” he added.
Dr. Okun reiterated that head injury is a “known and important risk factor” not only for Parkinson’s disease but also for other neurodegenerative diseases. “It’s important to counsel patients about the association,” he said.
The study was supported by the Michael J. Fox Foundation. Dr. Brown reports having received grant support from the Michael J. Fox Foundation. Dr. Okun has reported no relevant financial relationships.
A version of this article first appeared on Medscape.com.