The effects only showed up when people began getting severe headaches hours later, said Valerie Biousse, a neurology specialist at the Lariboisiere Hospital in Paris. And she warned that the numbers of such cases, although small, was increasing as fairgrounds offer bigger rides and more people go on them.
Even healthy people should pay attention to the warning signs displayed at the entrances to modern rides, which can generate forces of more than 4g - four times that of gravity - during turns and after steep drops, she said. "When you're on a very fast rollercoaster, for your body it's like doing very hard physical exercise. Your heart rate goes up, and everything is under extra strain," said Dr Biousse.
The damage she observed in patients, who were aged between 20 and 55, ranged from small strokes to minor bleeding in the walls of head arteries. She said they must have been caused by the abrupt changes in acceleration that are typical of modern roller coasters. The effect is almost like a car crash - which can also produce invisible injuries that only become apparent days later.
A spokeswoman for Alton Towers, which operates the Oblivion ride - featuring a 60m vertical drop that generates 4.5g on riders at its end - said: "All our rides have safety warnings at the entrance. If you suffer from back, neck or heart problems or are pregnant, you shouldn't go on the ride."
But Dr Biousse told New Scientist magazine that the four patients she had examined included a dance instructor in her thirties, and that half the patients she examined had no underlying diseases that could have contributed to the injuries she observed. "In every published case, the first symptom was an unusual headache. People should see a doctor if they have a headache a little time after going on a roller coaster ride."
The Health and Safety Executive, which is responsible for licensing rollercoasters in the UK, said that there is no legal limit on the g forces that a ride can exert: "The onus is on the operator to demonstrate that risks to peoples' health and safety is kept as low as reasonably practicable. That doesn't mean zero risk," said a spokesman.
Similar problems were detected in 1992 with bungee jumping, when doctors began recording cases in which people's eyesight was affected by the force exerted on their eyeballs as the bungee comes tight, at the bottom of the jump.Reuse content