Doctors have discovered a possible link between head injuries and Parkinson's disease.
The doctors backed protective head gear for cyclists and others
Researchers in the United States have found that people who suffer a head injury may be four times more likely to develop the condition compared to those who have not.
The risk appears to be even higher for those who are hospitalised after suffering a serious injury to their head. They may be eight times more likely to develop Parkinson's.
But doctors believe the risk is greatest for those who suffer severe head injuries. Their chances of contracting the disease may be as much as 11 times higher.
Generally, men and women have about a one in 75 chance of developing Parkinson's disease.
The condition affects around 120,000 people in the UK and an estimated four million worldwide.
By no means, does it mean that if you have a severe head injury that you will definitely develop Parkinson's
It is a progressive, degenerative, neurological condition, for which there is currently no cure.
Sufferers find it increasingly difficulty to move their arms and legs. They develop tremors and facial tics, and gradually become more and more immobile.
These symptoms occur as a result of the death of key cells in the brain and nervous system. Scientists do not yet know why this happens.
These latest findings are based on a study of 196 patients by doctors at the Mayo Clinic in Minnesota.
They found that the risks of developing Parkinson's increase depending on how severe the head injury is.
However, they said there was no evidence to suggest there was any link between mild head injuries and the disease.
"The risk is elevated for people with more severe head injury - longer loss of consciousness and brain bruising visible in a CT scan," said Dr James Bower, who led the study.
"We did not find mild head injury - head injury with no or only a brief loss of consciousness - to be associated with Parkinson's disease."
But he added: "I was surprised by the strength of the association."
Dr Bower said the results should be treated with caution.
"By no means does it mean that if you have a severe head injury that you will definitely develop Parkinson's."
The researchers were unable to explain why head injuries may increase the risk of developing the disease.
They suggested that injuries to the head may affect brain cells or chemicals in the brain.
Protective head gear
They said the findings suggested people who are at greater risk of suffering head injuries, such as cyclists or sports players, should wear protective head gear.
"Certainly, appropriate head gear is important in reducing the risk of serious head injury," Dr Bower said.
"Unfortunately, there is no other proven way to reduce the risk of getting Parkinson's disease once a head injury has occurred."
Dr Robert Meadowcroft, director of research at the UK's Parkinson's Disease Society, welcomed the study.
"Although this research shows that there is a strong association between head injury and Parkinson's, it should be remembered that there is no direct cause revealed by the study.
"This is clearly an important area of research and we would welcome further investigation on this subject.
"Meanwhile, the PDS endorses Dr Bower's recommendation that people should wear protective head-gear when engaging in dangerous sporting activity or cycling."
The study is published in the journal Neurology.