A study by US scientists has shown that sufferers of allergic rhinitis, a condition that causes symptoms of having a “permanent cold”, appear to be at much greater risk of the degenerative neurological condition. The research, which explored possible links between conditions that cause inflammation and the breakdown of brain cells, found a marked increase in cell death in rhinitis sufferers.

Incidence of all types of allergy has increased significantly in recent years, with one in three people now believed to suffer from at least one condition.

About five million Britons are affected by perennial allergic rhinitis, usually triggered by indoor allergens such as dust mites, pet skin flakes and spores. It causes inflammation and irritation to the delicate linings in the nose and eyes.

A further three million people suffer from a mix of perennial and seasonal rhinitis — commonly known as hay fever — while about two million Britons suffer from hay fever alone. Hay fever showed the beginnings of a similar trend in the study but did not reach statistical significance, the scientists, based at the Mayo Clinic, said.

In the paper, published this week in the journal Neurology, they speculate that the inflammation produced by the allergy may release certain chemicals in the brain and inadvertently kill brain cells, as seen in conditions such as Parkinson’s. Between 8,000 and 10,000 Britons have Parkinson’s diagnosed each year, although experts say that in many other cases it is mistakenly identified as a different condition.

The US scientists emphasised that the study did not prove that allergies could cause Parkinson’s disease — only that it showed an association between the two conditions.

Previous studies had shown that people who regularly took some antiinflammatory drugs, such as ibuprofen, were less likely to develop Parkinson’s. These results prompted the Mayo Clinic investigators to look further into the links between diseases characterised by inflammation and Parkinson’s.

They studied 196 people who developed Parkinson’s, matched with people of similar age and gender who did not, over a 20-year period. The two groups were examined to determine if Parkinson’s sufferers had more inflammatory diseases. The scientists found that those with allergic rhinitis were 2.9 times more likely to develop the disease.

However, no such link was found with other inflammatory conditions such as lupus (connective tissue inflammation), rheumatoid arthritis, pernicious anaemia (drop in red blood cells) or vitiligo (loss of pigment-producing cells) and Parkinson’s.

The researchers suggest that this may have been due to the lower prevalence of the conditions in the general population, and hence in the study groups. No Parkinson’s link was suggested in patients with asthma.

James Bower, a neurologist at the Mayo Clinic and lead investigator, said that the link appeared to be marked and could help scientists develop medications that blocked the inflammatory process.

“The association with Parkinson’s disease is increased to almost three times that of someone who does not have allergic rhinitis. That’s actually a pretty high elevation,” he said.

“People with allergic rhinitis mount an immune response with their allergies, so they may be more likely to mount an immune response in the brain as well, which would produce inflammation. This may release certain chemicals in the brain and inadvertently kill brain cells, as we see in Parkinson’s.”

However Dr Bower added that allergy patients can do little to reduce the potential risk for Parkinson’s, which affects between 100 and 180 people per 100,000 of the UK population. There is a rising prevalence with age and a higher prevalence and incidence of the condition in males.

“I wouldn’t worry if you have allergies,” Dr Bower said. “Treat the allergy symptoms you have to alleviate them at the time. At this point, we have no good evidence that this treatment will protect you from possibly developing Parkinson’s disease later.”

Dr Bower compared the complex nature of Parkinson’s to that of heart attacks, with many factors possibly contributing to the development of both conditions. He said that the latest findings suggested that allergic rhinitis should be considered as one potential risk factors for Parkinson’s, just as hypertension, high cholesterol or smoking could be associated with a cardiac arrest.

The incidence of allergic rhinitis is increasing and it runs in families. Parkinson’s affects nerve cells in the part of the brain that controls muscle movement. Sufferers often experience trembling, muscle rigidity, difficulty walking, and problems with balance and coordination.

These symptoms generally develop after age 50, although the disease also affects a small percentage of younger people. The normal lifetime risk for men and women combined is 1.7 per cent.

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