Health reporter, BBC News
Natasha Richardson fell on a beginners' slope while skiing on Monday
The news that 45-year old British actress Natasha Richardson has died from head injuries after a seemingly minor skiing accident is shockingly tragic.
Experts say this sort of head injury is uncommon but doctors are taught to be very vigilant of patients who initially seem fine, but can deteriorate quickly.
Martin Shalley, consultant in emergency medicine at Birmingham Heartlands Hospital, said it is the location of the injury that can be important.
Hitting the head on the side just above the ear can be particularly dangerous as there is an artery protected by only a very thin layer of skull.
If that artery is damaged, even if there is no injury to the brain itself, continued bleeding leads to a build up of pressure which ultimately causes the brain to stop functioning.
It is important to remember there are patients who deteriorate despite everyone's best efforts
Mr Ian Swann, accident and emergency specialist
"With that injury, you're absolutely normal initially but over a period of time you become more drowsy - there's a change but it's very subtle and difficult to detect.
"It's a very uncommon injury and these very minor things are very difficult to spot, if the person presents at just the wrong time there will be no abnormal signs."
The treatment, once detected, is to get rid of the clot and tie off the vessel but that has to be done before the pressure has caused irreversible damage to the brain itself.
"After a critical point, recovery is very rare," said Mr Shalley.
"Doctors are always taught to be very aware of these patients who are talking and seem fine."
He added the key signs to look for after a head injury are a change in the level of consciousness, vomiting and a headache that does not go away with normal painkillers.
Mr Ian Swann, a retired emergency doctor in Glasgow and an expert on head injury, said there can be many reasons why a head injury might be missed - including if the patient has other injuries or if they have been drinking.
Also, a patient might have a concussion but not seem very seriously hurt but then the situation can deteriorate very quickly.
He says even in a busy emergency department it is fairly rare that a person can walk and talk and then suddenly be in a very serious condition.
"This kind of injury is most likely to be caused by a bleed in the brain but there can be swelling occurring over a period of even a few days.
"You need to talk to the patient and find out what they remember and a patient who doesn't remember the accident needs to seek help.
He added: "It's always a tragedy but sporting accidents are notable because you have fit people who probably don't have anything wrong with them before the injury and don't have significant primary damage to the brain.
"It is important to remember there are patients who deteriorate despite everyone's best efforts - it isn't always the situation that the damage can be salvaged."
SIGNS TO LOOK FOR
Any change in consciousness
Mr Chris Chandler, a neurosurgeon at King's College Hospital in London, agrees that such a severe injury from a minor fall is unusual but even a fall from standing is a six-foot fall as far as the head is concerned and can produce a tremendous impact.
"After severe trauma where there is a collection of blood building up inside the head causing trauma and threatening brain function and brain integrity there is a window of opportunity of two or three or four hours during which emergency surgery can be life-saving and in fact can protect the patient from any long-lasting brain damage.
"Beyond about four hours the odds of permanent brain damage and fatal brain injury increase dramatically."
Peter McCabe, chief executive of the brain injury association, Headway, said anyone with even what might seem a minor head injury should seek medical advice.
"Please get it checked out - it might seem inconsequential but under the surface there could be all sorts of things going on, there could be a blood clot, swelling, so even if it does seem simple get yourself assessed."