In March, journal "Molecular and Cellular Neuroscience" published three reviews in Article in Press -section related directly to this topic (Ling et al. 2015, Daneshvar et al. 2015, Gardner and Yaffe 2015).
There is major epidemiological evidence stating the association between moderate or traumatic brain injury (TBI) and neurogenerative diseases such as Alzheimers (AD) and Parkinsons disease (PD) (Gardner and Yaffe 2015).
There are several studies investigating association between one time MTBI and dementia. Schofield and co-coworkers stated "Incident Alzheimer's disease was significantly associated with head injury which occurred within the preceding 30 years" (Schofield et al. 1997). There have been many recent studies in this topic. Lee et al. stated "TBI is an independent significant risk factor of developing dementia even in the mild type" (Lee al al. 2013). Nordströn et al. stated "In the present study, we found strong associations between YOD (young onset dementia) of non-AD forms and TBIs of different severity" (Nordström et al. 2014).
All these studies are cited in recent literature review in which authors state "Taken together, these studies suggest that there is likely a small, but significant, risk of dementia following a single MTBI that is not solely due to reverse-causation or confounding. It is likely, however, that younger adults may be more resilient or may take longer to develop dementia than older adults who sustain a MTBI" (Gardner and Yaffe 2015).
CTE (Chronic traumatic encephalopathy) is a neurogenerative disease linked to exposure to repetitive MTBI (Wikipedia). CTE has been associated with mood, behavior, cognitive, and/or motor symptoms (including parkinsonism and/or motor neuron disease). CTE is officially a postmortem diagnosis. Prevalance of CTE among former professional American football players have ranged from 50% to 97%. In one study stage of CTE was associated to years of football exposure, "lending weight to a true causal association" (Gardner and Yaffe 2015).
As so there is mounting evidence of the association between (repetitive) MTBI and neurogenerative diseases but there are many epidemiological ("secular trends","reverse-causation") and study protocol related (clinical criteria for CTE, quantification of MTBI) issues which warrant further research in this topic. Authors of the recent review state "Recently, large epidemiological studies have reported that MTBI and repetitive MTBI are also significant risk factors for neurodegenerative diseases, but these associations are not yet aswell established and require further replication" (Gardner and Yaffe 2015).