Chronic traumatic encephalopathy (CTE) is characterized as a progressive, neurodegenerative disease caused by repetitive mild traumatic brain injury (1). CTE is different from other neurological diseases due to differences in prominent tauopathy patterns (3,4). These differences are noted by a histopathological analysis postmortem. Since diagnosis of CTE can only be made postmortem and effects are only starting to show now due to changes in lifestyle, there are not very many reported cases. However, most reported cases of CTE have been of athletes that compete in high contact sports. Football and boxing athletes have yielded the population with the highest number of subjects since the athletes suffer frequent trauma to the head (2).
CTE has varied pathology, and is commonly of two aspects: gross neuropathology and microscopic neuropathology. The pathology depends on the stage and progression of CTE. Grossly, while there may be cases that show enlargement of the lateral ventricles, the brain appears to be intact in mild CTE. More advanced CTE includes further enlargement of the lateral and third ventricles, as well as septal fenestrations with associated cavum septum pellucidum, atrophy in the frontal and temporal lobes, mammillary body atrophy, and thinning of the hypothalamic floor and corpus callosum (34). In severe CTE, there is atrophy of the hippocampus and amygdala, which may result in decreased brain mass. However, gross pathology is not definitive enough to determine CTE; microscopic pathology provides the necessary support for diagnosis since there are characteristics that make it distinguishable (1). The presence of neurofibrillary tangles (NFTs) formed in areas by aggregates of hyperphosphorylated tau protein indicate CTE (30). CTE is therefore one of the many neurodegenerative diseases that includes progressive tauopathy (36). The distribution of tau aggression in CTE, as well as the nonuniform regional distribution of NFTs, is distinctive (37,30). Furthermore, tau aggregates in CTE is of much higher density than other neurodegenerative diseases.
Contact sports that yield head trauma have been associated with neurological diseases since early modern times, but the relationship between CTE and contact sports has only recently gained exposure (6). This is due to more cases of athletes suffering from CTE and media influences, such as Espn’s 30 for 30 films and Concussion, a movie starring Will Smith as a doctor with ground-breaking research.
CTE stems from mild trauma brain injuries (mTBIs), which occur extremely frequently throughout many sports. In fact, sports account for over 20% of TBI cases in America (11), and most likely the actual number is even higher since many cases go unreported at all levels. Also, there is a high association between the cases and boxing, hockey, and rugby, and especially football (19,20). On average, high school football players sustain over 24 blows to the head that exceed 14 grams in force per game (21). Also, similar data shows that the same is the case for hockey and boxing (21, 22).
Research has not proven that there is a relationship between head trauma and CTE since most data regarding CTE can only be diagnosed postmortem. Studies done on professional boxers suggest that mTBIs leading to CTE has at least 17% frequency (7). Furthermore, previous research has found a significant association between an mTBI event and a degeneracy found linked with CTE, chronic cognitive impairment. Also, studies conducted on over 3,400 National Football League (NFL) players showed that death from a neurodegenerative disease is three times as likely than the general population, and that CTE may be the primary or secondary cause of these deaths (28). As football and boxing athletes have the greatest incidence of CTE, other sports contribute to the disease, as stated earlier. However, case reports indicate that athletes from unique sports show differences in rapidity and severity of CTE (29), with football and boxing being the most aggressive. Most of the athletes from these sports had been diagnosed with a plethora of neurodegenerative symptoms: pseudobulbar signs (dysarthria and dysphagia), motor and language disturbances, and a Parkinsonian-like syndrome (5). Furthermore, these clinical manifestations can be separated into two areas: one affecting mood/behavior and the one affecting primarily cognitive function (32). From a study at the Boston University brain bank, those who had behavior and mood dysfunction had much earlier onset of symptoms than those with cognitive impairment (32). These symptoms can largely be attributed into one of four pathological stages: Stage I is characterized by headache, reduced attention span, and loss of concentration; Stage II includes additional short term memory loss, depression, and impulsive behavior; Stage III adds clinical presentation of further cognitive impairment; Stage IV involves language dysfunction and increased aggression (1). However, even though these symptoms are linked with CTE, they are very varied and take a long time to diagnose. This makes it even more difficult to determine CTE presence in living athletes (30).
Furthermore, diagnosis of CTE is made even more difficult due the overlap with other neurodegenerative diseases. These include Alzheimer’s Disease, Parkinson’s Disease, frontotemporal and vascular dementia, and chronic postconcussive syndrome. The main differences between these diseases and CTE are that the average age at death is much younger (32), and that those who suffer CTE have a history of repeated TBIs, while those who suffer from other diseases only suffer a few TBIs (1). However, since contact sports are synonymous with repeated TBIs, there is a connection between contact sports and CTE, and not just other neurodegenerative diseases.