according to research presented online as part of the 2020 American Academy of Neurology Science Highlights.
Dr. Ethan G. Brown
Neurologists have identified various phenotypes among patients with Parkinson’s disease; however, the factors that determine these phenotypes, which may include genetic and environmental variables, are poorly understood. Ethan G. Brown, MD, assistant professor of neurology at the University of California, San Francisco, and colleagues hypothesized that head injury, which is a risk factor for Parkinson’s disease, would be associated with a more severe phenotype.
“Head injury is a risk factor for other conditions that involve cognitive impairment,” said Dr. Brown. “The mechanisms of how head injury contributes to neurodegenerative disease are not clear, but may be related to the initiation of an inflammatory cascade that can have a long-term, chronic effect. We hypothesized that these long-term sequelae may contribute to symptoms in Parkinson’s disease.”
An analysis of data from two cohorts
The researchers examined the relationship between head injury and clinical features by analyzing data for two cohorts of patients with Parkinson’s disease. Through an online survey, the investigators elicited information about head injury and other exposures from participants in the Parkinson’s Progression Markers Initiative (PPMI) and the Fox Insight (FI) study. Dr. Brown and colleagues determined disease phenotypes for participants in PPMI using baseline Movement Disorder Society-Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) score and 5-year change in Montreal Cognitive Assessment score. For participants in FI, the researchers determined phenotypes using baseline self-reported MDS-UPDRS-II score and self-reported cognitive impairment. They used parametric and nonparametric tests as appropriate and adjusted the results for age, sex, and smoking history.
In all, 267 participants with Parkinson’s disease in PPMI and 25,308 in FI submitted information about head injury. In the PPMI cohort, head injury before Parkinson’s disease diagnosis was associated with greater nonmotor symptom burden at enrollment. The mean MDS-UPDRS-I score was 7.73 among participants with any injury, compared with 6.19 among participants with no injury. Similarly, the mean MDS-UPDRS-I score was 8.29 among participants with severe head injury, compared with 6.19 among participants with no injury. Motor symptoms were worse among participants with severe injury (MDS-UPDRS-II score, 8.35). Among 110 participants who were followed for 5 years, patients who reported severe head injury before diagnosis had a decline in cognitive function. The mean change in Montreal Cognitive Assessment score was –0.60 for patients with severe head injury and 0.76 in those with no head injury.
“The improvement from baseline in the participants with Parkinson’s disease but without head injury was small and not statistically significant,” said Dr. Brown. The increase could have resulted from practice effect, although it is not certain, he added. “We are continuing to evaluate other, more sensitive tests of cognitive impairment to try to understand these results more completely in this population.”
In the FI cohort, participants who reported a prior head injury had more motor symptoms (MDS-UPDRS-II, 14.4), compared with those without head injury (MDS-UPDRS-II, 12.1). Also, the risk of self-reported cognitive impairment was elevated in participants who reported head injury (odds ratio, 1.58).
“The results most affected by the self-reported nature of [the] FI [data] are the cognitive impairment results,” said Dr. Brown. “Subjective cognitive impairment ... is very different from objective cognitive impairment, which could be measured through in-person testing in the PPMI cohort. Many factors may contribute to noticing cognitive decline, some of which can be measured and controlled for, but some cannot. There may be a correlation between subjective cognitive decline and true cognitive impairment, but this has not been fully studied in Parkinson’s disease.”
The search for the underlying mechanism
Clarifying whether the relationship between head injury and Parkinson’s disease phenotype is causal or whether falling is an early indication of worse symptoms will require more longitudinal data. “We would like to further characterize the differences between people with Parkinson’s disease with and without a history of head injury,” said Dr. Brown. “More detailed understanding of these phenotypic differences could point to an underlying mechanism, or whether or not other comorbid conditions are involved. We would also like to understand whether genetics plays a role.”
The PPMI and FI studies are funded by the Michael J. Fox Foundation. Dr. Brown has received compensation from HiOscar, NEJM Knowledge Plus, and Rune Labs and has received research support from Gateway Institute for Brain Research.
SOURCE: Brown EG et al. AAN 2020, Abstract S17.002.