Traumatic brain injury, a leading cause of mortality and morbidity, is divided into three grades of severity: mild, moderate, and severe, based on the Glasgow Coma Scale, the
loss of consciousness, and the development of post-traumatic
amnesia. Although
mild traumatic brain injury, including concussion and subconcussion, is by far the most common, it is also the most difficult to diagnose and the least well understood. Proper recognition, management, and treatment of acute concussion and
mild traumatic brain injury are the fundamentals of an emerging clinical discipline. It is also becoming increasingly clear that some mild
traumatic brain injuries have persistent, and sometimes progressive, long-term debilitating effects. Evidence indicates that a single
traumatic brain injury can precipitate or accelerate multiple age-related neurodegenerations, increase the risk of developing
Alzheimer's disease,
Parkinson's disease, and
motor neuron disease, and that repetitive mild
traumatic brain injuries can provoke the development of a
tauopathy,
chronic traumatic encephalopathy. Clinically,
chronic traumatic encephalopathy is associated with behavioral changes, executive dysfunction,
memory loss, and
cognitive impairments that begin insidiously and progress slowly over decades. Pathologically,
chronic traumatic encephalopathy produces
atrophy of the frontal and temporal lobes, thalamus, and hypothalamus, septal abnormalities, and abnormal deposits of hyperphosphorylated tau (τ) as neurofibrillary tangles and disordered neurites throughout the brain. The incidence and prevalence of
chronic traumatic encephalopathy and the genetic risk factors critical to its development are currently unknown.
Chronic traumatic encephalopathy frequently occurs as a sole diagnosis, but may be associated with other
neurodegenerative disorders, including
Alzheimer's disease,
Lewy body disease, and
motor neuron disease. Currently,
chronic traumatic encephalopathy can be diagnosed only at autopsy; however, promising efforts to develop imaging, spinal fluid, and peripheral blood
biomarkers are underway to diagnose and monitor the course of disease in living subjects.