Chronic traumatic encephalopathy (CTE) is thought to be a progressive, neurodegenerative disease, caused by repeated blunt force impacts to the head. There is not yet a proven link between concussion and CTE, but CTE has been firmly linked to activity that subjects the brain to repeated acceleration and deceleration.

First reported in 1928, this phenomena was originally referred to as Dementia Pugilistica (or more commonly as "Punch Drunk Syndrome") because it was believed to affect only boxers. The terms "traumatic encephalopathy" and "CTE" were first used in the 1960s.

CTE has captured extensive popular attention in recent years as research increases to determine its relationship to and effect on athletes.

Tau is a toxic protein that takes the form of neurofibrillary tangles (NFTs) and neuropil threads (NTs) in areas of the brain. This abnormal protein initially impairs the normal functioning of the brain and eventually kills brain cells.

CTE is only diagnosed on autopsy and is associated with a history of memory disturbances, behavioral and personality changes, mood disorders, and/or dementia. However, there is no (yet) identified clinical phenotype to allow for diagnosis (Iverson et al, 2018). Since at this point, CTE can only be diagnosed by examining the brain after death, it is limited by the number of cases discovered thus far (since autopsies are not performed on all potential CTE victims). The research is in its infancy.

A review of CTE findings highlighted the following (Maroon et al, 2013):

• 153 case reports of CTE have found that a history of mTBI was the only risk factor consistently associated with CTE.
• These are the reports of those diagnosed after autopsy;
• There is no denominator - millions have been exposed;
• It appears to be caused by brain trauma;
• In this systematic review, no relationships between CTE and age of death or abnormal ApoE allele (Maroon et al, 2015);
• In that study, suicide and the presence of premorbid dementia were not strongly associated with CTE.

“We conclude that the incidence of CTE remains unknown due to the lack of large, longitudinal studies. Furthermore, the neuropathological and clinical findings related to CTE overlap with many common neurodegenerative diseases. Our review reveals significant limitations of the current CTE case reporting and questions the widespread existence of CTE in contact sports.”

Maroon JC, Winkelman R, Bost J, Amos A, Mathyssek C, Miele V.; Chronic traumatic encephalopathy in contact sports: a systematic review of all reported pathological cases. PLoS One. 2015 Feb 11;10(2):e0117338. doi: 10.1371/journal.pone.0117338. eCollection 2015. Review. Erratum in: PLoS One. 2015;10(6):e0130507.
Iverson, G.L., Keene, C.D., Perry, G. and Castellan, R.J. (2018). The Need to Separate Chronic Traumatic Encephalopathy Neuropathology from Clinical Features. Journal of Alzheimer’s Disease 61, 17–28.