The New England Journal of Medicine published a Perspective on Traumatic Brain Injury called "Traumatic Brain Injury – Football, Warfare, and Long-Term Effects."

In late July, the National Football League introduced a new poster to be hung in league locker rooms, warning players of possible long-term effects of concussions.  Public awareness of the pathological consequences of traumatic brain injury has been elevated not only by the recognition of the potential clinical significance of repetitive head injuries in such high-contact sports as American football and boxing, but also by the prevalence of vehicular crashes and efforts to improve passenger safety features, and by modern warfare, especially blast injuries.

The article, by Dekosky et al., N Engl J Med 2010; 363:1293-1296, Sept. 30, 2010, goes on to contrast immediate consequences of traumatic brain injury and how long they last with delayed consequences of traumatic brain injury.

Many complications of traumatic brain injury are evident immediately or soon after injury….Seemingly mild closed-head injuries (i.e., those without skull fracture) may lead to diverse and sometimes disabling symptoms, such as chronic headaches, dizziness and vertigo, difficulty concentrating, word-finding problems, depression, irritability, and impulsiveness. The duration of such symptoms varies but can be months. Post-traumatic stress disorder frequently accompanies traumatic brain injury, though the relationship is poorly understood.

However, "Causal relationships between traumatic brain injury and delayed sequelae have been less studied because of the variable latency period before overt neurologic dysfunction."  However that does not mean relationships do not exist.  We know of certain repetitive mild brain injury (boxers); pugilistic parkinsonism.

 "Neurocognitive effects of repetitive mild head injury were initially recognized in boxers, with a syndrome that was distinct from the clinical and pathological sequelae of single-incident severe traumatic brain injury." Now other contact sports and blast injuries are also known to impact the brain.  In severe cases, as soon as two hours after the injury, scientists have discovered a protein, also seen in Alzheimer’s patients, that causes cellular degeneration in the brain.  However in "mild brain injuries" the protein plaque is not evident. 

Further studies will help us understand why.  Currently precursers of the protein are seen in "mild brain injury" studies.  And, repetitive injury is replete with evidence of pugilistic parkinsonism