Anyone who's watched a boxing match knows the sight of a fighter staggering after receiving a blow to the head. Neurologists call this phenomenon dementia pugilistica. Boxers call it being 'punch drunk.' Those repeated hits to the head can cause short-term disorientation, putting boxers at greater risk for long-term brain damage.

Dr. Max Albert Hietala from the Sahlgrenska University Hospital in Sweden, says doctors know that boxers suffer neurological damage, but no one's been able to explain precisely how this happens. He says the external mechanism is certainly clear:

"During a fight," Hietala explains, "the mechanical force is quite horrendous. When a punch is thrown it can be up to 630 kilograms in less than a tenth of a second. What we believe is the shearing and the damage to the neurons, can be measured by measuring the building blocks of the neurons."

Hietala compared 10 non-boxing control subjects to 14 amateur boxers - three of whom were women. Several days after they had fights, he took samples of the boxers' cerebrospinal fluid - that's a clear, protective fluid surrounding the brain and spinal cord. Hietala was looking for neurofilaments -- proteins he calls the building blocks of neurons.

"These building blocks...should be contained within the neuron structure," Hietala points out. "So if you disrupt the structure, either by force or by a stroke, the structure is broken and the building blocks -- that is, the proteins, they leak out into the cerebrospinal fluid."

Hietala found each of the boxers had elevated levels of neurofilaments in their cerebrospinal fluid after receiving blows to the head. He says that's similar to the damage seen in older patients who have small strokes. What surprised Hietala is that in Swedish amateur boxing, the fighters wear thickly padded headgear. Even so, the boxers experienced neuron damage.

Hietala presented his research at the annual meeting of the American Academy of Neurology in Boston.