Blows to the head can be silent killers for which swift diagnosis is often the only recourse, say doctors.
Even if there is no damage to the outside of the head, the impact may inflict catastrophic damage to the brain inside, they say.
Because a shock that smashes the brain, a 2.8-pound organ with the consistency of soft jelly, against the hard protective shell of the skull, can damage nerves, brain cells and blood vessels.
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Blood clots and bruising then result, which in turn causes pressure to build up and squeeze the brain, worsening the damage and amplifying the risk of permanent handicap or death.
Doctors looking at suspected brain trauma can call on X-rays or hi-tech 3-D scanners as diagnostic tools, but treatment options are relatively few. Drugs may ease rising pressure in the brain but sometimes surgery is required.
In the case of Formula 1 star Michael Schumacher -- diagnosed immediately after his skiing accident on Sunday -- neurosurgeons first operated to tackle bleeding and bruising and then placed him in an artificial coma after a post-operative scan showed "widespread lesions" on both sides of the brain.
The lesions "are not good news", said French expert Jean-Luc Truelle, a retired professor of neurology at the Foch Hospital in the Paris suburbs. "It shows that there is bleeding across the brain's function centers."
Tried and tested, the coma reduces the patient's temperature to around 35 degrees Celsius (95 degrees Fahrenheit) to reduce swelling. By being unconscious, the brain is also switched off to sounds, light and other triggers that cause the organ to use up oxygen as it processes the stimuli.
"Right now, our goal is to reduce all external stimuli and oxygenate his brain as much as possible," said Jean-Francois Payen, head of intensive care at Grenoble University Hospital Centre, where Schumacher is being treated.
It can take up to 48 hours for symptoms of brain injury to emerge, which is why anyone who suffers even mild concussion should seek medical attention, said Truelle.
According to a 2007 Australasian study in the Journal of Clinical Neuroscience, around one in every 40 cases of head injury documented over a 10-year period at a major trauma center were of so-called "talk-and-die" syndrome.
This means the patients initially felt okay after the incident but subsequently deteriorated and died from intracranial causes.
One suspected "talk-and-die" case is that of actress Natasha Richardson, the wife of star Liam Neeson, who died after an apparently innocuous tumble on a beginners' ski slope in 2009.
Other celebrity casualties on the ski slope have been Sonny Bono, Dutch Prince Johan Friso and Kennedy clan member Michael Kennedy.
Brain injuries may also result from repeated concussive blows, rather than a single extreme incident, which is why high-impact sports such as American football, rugby and ice hockey are under close scrutiny at the moment.
Former players of American football between the ages of 30 and 49 are 20 times likelier to be diagnosed with Alzheimer's, dementia or another memory-related impairment than the general population, according to a 2009 National Football League-commissioned report.
Adam Hampshire, a neuroscientist at Imperial College London, said he was shocked to examine the frontal lobes of 13 retired American football players who took part in cognitive tests.
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There were "some of the most pronounced abnormalities in brain activity that I have ever seen," Hampshire said in October.
"It is highly likely that damage caused by blows to the head accumulates towards an executive impairment in later life."