Concussion is a clinical term that may be defined as loss of consciousness after an injury to the head without macroscopic damage to nervous tissue. Recovery is usually prompt after less severe injuries, but long periods of unconsciousness or death may occur, and amnesia for events before and after the accident is common. Concussion implies that contusions and lacerations of the brain have not accounted for the symptoms, but the diagnosis must be presumptive except in occasional fatal cases studied at autopsy. Information about the pathophysiology of concussion therefore is derived largely from experiments with animals.
How an injury to the head causes unconsciousness is not entirely clear. Transmitted pressure gradients and rotational forces distort the brainstem, but bilateral injury to the cerebral hemispheres may be equally important. Changes in the brainstem probably account for the apnea and bradycardia that immediately follow the blow as well as for the unsteadiness and giddiness with change in position of which patients often complain for a time.
Amnesia, forgetfulness, irritability, fatigue, and impaired memory are attributed to neuronal damage in the cerebrum, although long persistence of these vague symptoms after cerebral trauma may be hard to evaluate, particularly if a lawsuit is unsettled. Microscopic changes in the brain after a concussive blow are observed in the neurons and glia within a few hours. Some cells may recover, but. others do not, and thus permanent damage to the nervous system occurs. Psychological tests and neurologic examination may not reveal evidence for the loss of a relatively few neurons, but the effects of repeated small injuries are cumulative as the dull, querulous of the punchdrunk fighter testifies.