About 30% of Parkinson's patients experience a sudden feeling of being "stuck in wet cement" which leaves them unable to walk and which no amount of willpower can overcome. This condition is known as an on-off episode.

Scientists have found simply shining a red dot laser can help Parkinson's patients overcome the problem, and start walking again.

Shining the laser at a point in front of the patient appears to somehow help the brain to overcome whatever block it is that causes the patient's movements to freeze in the first place.

The technique has been tested by Dr Timothy Counihan, a neurologist at the University of Rochester in New York.

Dr Counihan told a meeting of the American Academy of Neurology in Toronto that a patient had originally stumbled on the idea.

Patient developed laser

Michael J Fox suffers from Parkinson's Disease

Stan Clark, of Old Forge, New York, found that the only way to get moving again when he was assailed by the problem was to think about stepping over a pattern on the floor or a crack in the pavement.

He tried home-made remedies, including etching a line in his glasses and shining a flashlight beam on the floor.

Then he tried the laser. "I looked at the laser spot, and my feet just seemed to go to it," he said.

Clark told his doctors, Dr Counihan and Dr Lin Zhang, about his discovery. They gave laser pointers to six other patients.

Three patients improved an average of 30%, while one patient found her freezing condition became worse, perhaps because she was concentrating so much on using the device.

Two other patients had no freezing episodes during which to try the pointer.

Brain cell destruction

Parkinson's, which affects up to 1% of all people over the age of 60, is caused by the destruction of brain cells that produce dopamine, an important signalling chemical involved in movement.

Symptoms start with tremors, but the disease grows progressively worse, eventually paralysing and killing its victims.

About 30% of Parkinson's patients suffer "sudden transient freezing" that can leave them paralysed for as long as several minutes.

Dr Counihan said: "For these patients, it's like their feet are stuck in wet cement, and currently there's no way to treat it."

They try home-made solutions, from tape on the floor to flipping their canes around so they can use the curved top as a guide to walk toward.

Dr Counihan said: "Using such cues is a way of tricking your brain to tell your body, 'Step over there'.

"A laser pointer is an easy way to give yourself a visual cue. It is a very practical, inexpensive device that patients can carry with them. It seems to get them moving again."

Professor Adrian Williams, chairman of the medical advisory panel of the Parkinson's Disease Society, said: "Various measures are used, including metronomes and physiotherapy techniques to give patients a visual cue to overcome the freezing episode.

"This sounds like another technique to provide that visual cue."