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Fann JR, Ribe AR, Pedersen HS, Fenger-Grøn M, Christensen J, Benros ME, Vestergaard M. Long-term risk of dementia among people with traumatic brain injury in Denmark: a population-based observational cohort study. Lancet Psychiatry. 2018 May;5(5):424-431. Epub 2018 Apr 10 PubMed.
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Mayo Clinic
These studies are important contributions to the literature. The study by Drs. Gardner, Yaffe, and colleagues provides further evidence that TBI may be associated with an increased risk of parkinsonism and Parkinson’s disease. This study is consistent with the previous finding by Crane and colleagues who reported, in the largest autopsy study to date, that TBI with loss of consciousness for more than one hour was associated with PD and Lewy body pathology.
I think the associations between TBI and dementia, including Alzheimer’s disease, are less established compared to the associations between TBI and parkinsonism/Parkinson’s disease. Epidemiological studies have been somewhat inconsistent and have differed, which may in part be due to the different assessments utilized to obtain information on TBI exposure (i.e., self-report, codes, requirement of a certain time for loss of consciousness) and dementia (i.e., clinical assessments, codes).
The study by Fann and colleagues is one of the largest, if not the largest, population-based study to date assessing TBI and risk of dementia. A major strength of this study is the rigorous methodological assessments, including thorough assessments by sex, consideration of reverse causality (dementia causing/contributing to the TBI), and the consideration of non-TBI trauma. The latter is particularly important but has been assessed by few studies. Previously, Brown and colleagues from the Mayo Clinic demonstrated that after six months, individuals with TBI were not significantly associated with mortality when compared with individuals without TBI but with similar non-trauma comorbidities (Brown et al., 2014). In the Fann et al. study, a TBI was associated with a greater risk of dementia compared to a non-TBI fracture not involving the skull or spine. A potential limitation is that the matching didn’t go beyond the presence of a fracture, which is often the case in instances of TBI due to car crashes, etc. Another limitation is the use of codes for the diagnosis of dementia type. While the use of codes in such a large population-based study is clearly necessary, it is often difficult to ascertain type of dementia, and the clinical diagnosis of dementia type has evolved over time. One wonders whether some of the drivers of the association are due to a stronger association between TBI and Parkinson’s disease dementia or Lewy body dementia as opposed to Alzheimer’s disease, especially given the Crane et al. autopsy study.
References:
Brown AW, Leibson CL, Mandrekar J, Ransom JE, Malec JF. Long-term survival after traumatic brain injury: a population-based analysis controlled for nonhead trauma. J Head Trauma Rehabil. 2014 Jan-Feb;29(1):E1-8. PubMed.
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