July 17, 2003
By Lisa Ellis
InteliHealth Staff Writer
It's both common sense and common medical practice that when you're allergic to something, it helps to stay away from it as much as possible.
If you're allergic to the common household dust mite, studies show you can reduce exposure by encasing your mattress, pillow and blanket in coverings that don't let the microscopic mites escape (impermeable coverings).
But two new studies found that reducing exposure by using these coverings did not improve symptoms for people with dust-mite allergies.
Doctors widely recommend impermeable bedding covers as one preventive measure to help patients with dust-mite allergy. Many of these patients also have asthma.
Several recent studies, however, have failed to document any benefit from using impermeable covers, says Aidan A. Long, M.D., clinical director of allergies at Massachusetts General Hospital. With the two newest studies, "it seems that the drumbeat is getting a bit louder," he says.
But Dr. Long says the new studies will not change what he recommends to patients. "There are limitations of these studies. I don't view them as proof positive that these measures are a waste of time."
The two new studies were published in the July 17, 2003, issue of the New England Journal of Medicine.
Both confirm previous research that shows impermeable coverings reduce exposure to dust mites, at least for a while. But participants who used the coverings for six months to a year showed no more change in symptoms than other people who were given ordinary bed coverings.
"We have to take that information and figure out how to weigh it against what we routinely see in clinical practice," says Dr. Long, an assistant professor of medicine at Harvard Medical School.
"There are clearly groups of patients for whom these measures have a dramatic effect," he said. "It's very common for us to hear patients come back and say that it's made all the difference in the world."
The larger of the two studies involved 1,122 adults with asthma, two-thirds of whom also were allergic to dust mites.
Researchers recruited participants at six medical centers in the United Kingdom. They were randomly divided into two equal groups.
One group received impermeable mattress, pillow and quilt covers that were fitted by a nurse. The other group received ordinary covers.
In the first six months, participants kept up their normal medical regime. For the rest of the year, most people entered a program to help them reduce their use of inhaled corticosteroid medicines.
After the first six months, the concentration of dust mites in mattress dust was much lower in the group with impermeable bedding covers, the published study states. Levels rebounded by the end of the year, however.
At the six-month point, both groups had improved significantly in their morning peak-flow rate, one way to measure breathing. But there was no significant difference in peak-flow rate between the groups with impermeable and ordinary mattress covers.
Results were similar for people who had dust-mite allergy as well as asthma.
After 12 months, the average participant was able to reduce the use of inhaled medicine by about 17 percent. The type of bed covering made no difference in these results.
One limitation with the study, Dr. Long said, is that it did not focus on a group most likely to benefit from efforts to reduce dust mites in the environment.
Although two-thirds of the participants were sensitive to dust mites, "we don't know whether dust mites are a dominant contributor to their asthma," he said.
Asthma is a complex disease that can be triggered by many factors, Dr. Long noted. Many patients in the study had other possible airway irritants in their lives. For example, more than half had dogs or cats, and one-quarter were smokers.
"I think you have to identify the patients for whom dust-mite allergy is a significant problem. It's perhaps not surprising that [impermeable covers] didn't help in a mixed and unselected population," Dr. Long said.
In addition, he pointed out that the study was designed to test whether the use of impermeable covers alone, without other environmental-control measures, can improve symptoms.
The researchers said they did this because a more comprehensive program of environmental control "is neither practical nor acceptable for the majority of patients with asthma."
Dr. Long said patients in his practice who have dust-mite allergy are urged to use impermeable covers for bedding, wash bedding often in hot water, avoid feather pillows, reduce clutter in the bedroom and steam-clean or remove carpets.
If a pet may be causing the symptoms, the pet should be removed, he said. "We'll never say, 'Leave the cat on the bed but cover the mattress.'"
The smaller study, conducted at six medical centers in the Netherlands, enrolled 279 patients with dust-mite allergy. This is a form of allergic rhinitis.
As in the asthma study, participants were randomly assigned to receive either impermeable or ordinary bedding covers.
In this case, however, both groups were encouraged to follow other environmental-control measures. They were asked to wash bedding weekly and to clean, heat and ventilate their homes on a schedule. All of these measures were intended to further reduce dust-mite exposure.
After 12 months, the level of dust mites in mattresses was much lower in the group with impermeable coverings, the researchers write. But there was no difference between the groups in several different measurements of allergy symptoms.
The Dutch study was smaller but better designed than the English one because it focused on patients with dust-mite allergy, and they used multiple measures to combat exposure, Dr. Long said. In addition, patients who had a dog or cat in the home were excluded.
"One would hope that rhinitis, which is more directly related to allergen exposure, would have improved more [in this study], and it didn't," Dr. Long said. "This is a worrisome study."
More research is needed, Dr. Long said.
Rather than trying to find one simple program that will work for everyone, future researchers should focus on figuring out which patients are most likely to benefit from environmental controls such as covering bedding, Dr. Long said.
It's also important to determine how much you have to reduce dust mites in the air – not on the mattress – before symptoms improve, he said. Then, he said, researchers should figure out how to achieve that low level.
Meanwhile, Dr. Long said, he'll continue to recommend impermeable covers and other measures for patients who clearly have a serious reaction to dust mites.
"The problem is that this study is going to be picked up and interpreted as showing that these strategies are useless," he said. "In unselected groups of patients, that seems to be the case, but there are clearly patients that these strategies do benefit."
