A small number of people (about 1%) who sustain a significant head injury later develop PD. A “significant head injury” means the person was in a coma for at least an hour, was hospitalized, and may have had surgery to remove a blood clot. The clot may have increased pressure inside the skull, causing downward pressure on the substantia nigra and this resulted in the death of enough nigral neurons to cause PD. A minor head injury, however, does not result in PD.
A minor head injury is one that either doesn’t result in a loss of consciousness or results in only a brief loss of consciousness. Due to the attention that has been called to one of the greatest boxers of all time, Muhammad Ali, and his struggle with PD, many people wonder if boxing causes PD. The repeated blows to the brain that boxers endure over several years can result in a condition called dementia pugilistica.
The author has examined several boxers who suffer from dementia pugilistica, but Muhammad Ali does not have dementia pugilistica—he has typical PD. Muhammad Ali’s PD began with a resting tremor of his left hand and later developed into a slowness of his hand and a shuffling gait, all of which are typical of PD. Muhammad Ali’s PD has evolved slowly, over 22 years, unlike the evolution of dementia pugilistica, which rapidly progresses to dementia and inability to walk over the course of 5 years. By contrast, Muhammad Ali’s mind is still sharp—he is not demented—and after 22 years with PD, he is still able to walk.
Finally, the MRIs of boxers with dementia pugilistica show evidence of brain dam-age, while Muhammad Ali’s MRIs do not. In several studies, 15–40% of professional ex-boxers developed symptoms of Alzheimer’s disease. At one end of the spectrum of ex-professional boxers is George Forman, who fought longer and suffered more blows to his head than Muhammad Ali, yet shows no signs of dementia. At the other end of the spectrum are severely affected ex-boxers who suffer from dementia pugilistica and are referred to as “punch drunk.” In between are boxers with varying degrees of speech difficulty, rigidity, unsteadiness, memory loss, and inappropriate behavior.
Symptoms, when they occur, usually begin shortly after the end of a boxer’s career. On occasion they are first noticed after a hard bout. Symptoms in boxers develop an average of 16 years after they start to fight, although sometimes symptoms occur as early as 6 years into their boxing careers. Although symptoms occur in amateurs, they are more common in professionals. They occur in all weight classes, but are more common in heavyweights.
Although the mechanism by which multiple and repeated blows to the head cause brain damage is not established, it is thought that such blows result in shearing injuries and multiple small hemorrhages. As the damage accumulates, minimal symptoms merge gradually into more obvious ones.
Autopsy examination of the brains of boxers with dementia pugilistica do not reveal the changes of PD (a loss of dopamine cells in the substantia nigra with the formation of Lewy bodies), but rather a loss of cells in the frontal and temporal lobes of the brain, as well as the amygdala (the “rage center”) and the nucleus basalis. Less common are changes in the basal ganglia and the thalamus.
The microscopic changes resemble those of Alzheimer’s disease and include protein deposits around blood vessels (called amyloid plaques), and neurofibril-lary tangles (twisted fibers of a tau protein inside the neurons) in the dying brain cells. The formation of Lewy bodies is not a part of dementia pugilistica.