MANCHESTER, UK—A strategy of allergen avoidance is often recommended to help reduce symptoms of asthma or allergies. While this approach seems logical, the clinical effectiveness of such tactics is arguable. Recently, two randomized, double-blind, placebo-controlled studies—one in asthma patients, the other in patients with allergic rhinitis—evaluated the use of allergen-impermeable bedding covers. After one year, neither study demonstrated a reduction in symptoms, despite a reduction in allergen exposure.[1,2]

ASTHMA STUDY IN THE UK

Ashley Woodcock, MD, and colleagues from the University of Manchester recruited 1,122 adults with asthma who were randomly assigned to receive allergen-impermeable mattress covers or regular polyester-cotton mattress covers. The mattress covers were put on the beds by research nurses and left in place for one year. At baseline, levels of dust mite on mattresses were measured. A 10% random sample of participants was visited at six and 12 months for dust sample collection.

Before randomization, participants’ sensitivity to house dust mite allergen was measured using serum immunoglobulin E (IgE) levels to Dermatophagoides pteronyssinus 1 (Der p1).

In the first six months of the study (phase I), patients continued their usual inhaled corticosteroid therapy and were evaluated by their physicians at three and six months. Diary cards and peak expiratory flow (PEF) rates were completed for the final four weeks of this phase.

During phase II, patients were asked to enter a program of controlled corticosteroid reduction. Those who agreed had their doses reduced by 25% to 50% each month. Reduction in inhaled corticosteroid dose continued until the medication had been discontinued or asthma control deteriorated.

The primary outcome for phase I was morning PEF rate. For phase II, the primary outcome was the percentage of patients who were able to discontinue inhaled corticosteroids.

BOTH GROUPS IMPROVED

Baseline mattress dust mite allergen levels were similar in the two groups. At six months, the treatment group had lower levels of dust mite allergen per gram of mattress dust than the control group (geometric mean, 0.58 µg/g vs 1.71 µg/g). This difference was no longer apparent at 12 months, however.

At the end of phase I, PEF rates had improved significantly in both groups. No differences between the two groups were seen. An analysis of a subgroup of patients with high sensitivity to dust mite allergen and high baseline levels of mattress dust mite allergen also showed no significant difference between groups at the end of six months.

At the end of phase II, 14% of all participating patients had stopped using inhaled corticosteroids completely; the overall mean reduction in dose was nearly 50%. Again, there was no significant between-group difference in any outcome.

Although allergen-impermeable mattress covers are recommended, this study failed to show their clinical efficacy in managing asthma symptoms. “These results,” said Dr. Woodcock, a Professor of Respiratory Medicine at the University of Manchester, “were disappointing but not unexpected.”

Why did the control group improve as much as the intervention group did? The authors speculated that even in the control group, the knowledge that the study was centered on allergen avoidance might have motivated them to clean more, vacuum more, and generally change their behavior in ways that had a clinical effect on asthma symptoms.

Roy Gerth van Wijk, MD, PhD, who led the second study, noted that extensive environmental interventions can have an effect. “As clinicians we have all had the experience of patients reporting that extensive environmental control [eg, removing wall-to-wall carpets, keeping pets out of the house] improves symptoms,” said Dr. Gerth van Wijk, Head of the Department of Allergology at Erasmus Medical Center in Rotterdam. However, complex interventions may not be practical or acceptable for many people, he admitted.

THE NETHERLANDS EXPERIENCE

Dr. Gerth van Wijk and colleagues conducted a trial to determine the effects of bedding covers on symptoms of allergic rhinitis.2 Two hundred thirty-two patients with allergic rhinitis were included, and some also had allergic asthma or atopic dermatitis.

Skin tests and levels of dust mite–specific IgE were used to determine sensitization. An additional diagnostic criterion was a positive result on a nasal allergen challenge. At baseline, dust samples were collected, and then patients received either allergen-impermeable bedding covers or control bedding covers. For 14 days prior to randomization, patients recorded their daily symptoms. The study’s primary end point was the score on a rhinitis-specific visual analog scale, which measured symptom severity on a scale of 0 (no symptoms) to 100 (very severe symptoms). Secondary end points included daily symptoms score, score on nasal allergen challenge, and levels of Der p1 and Dermatophagoides farinae (Der f1) from the patients’ mattresses.

At one year, the levels of Der p1 and Der f1 were significantly lower in the treatment group. Both groups experienced a significant decrease in symptoms according to the visual analog scale, but neither this decrease nor changes in the nasal allergen challenge and the daily symptoms scores differed between groups.

THE BOTTOM LINE

Both groups of investigators stressed that the results do not mean that allergen avoidance is ineffective or that it should not be recommended for those with dust-mite allergy. However, the studies show that an allergen avoidance strategy using dust mite–impermeable bedding covers is probably not an effective measure when used alone. Dr. Woodcock said that the use of allergen-proof bedding covers was “probably too little too late. I believe that the lungs are damaged in early childhood.”

“It is possible that allergic disease develops to a point of no return, and beyond that point allergen avoidance has no effect,” Dr. Gerth van Wijk agreed. “There are studies in children with asthma showing an effect from bedding encasings,” he said. “I cannot preclude a beneficial effect in younger children.” Dr. Woodcock also suggested that such intervention studies be performed in children.

As for using mattress covers as a single intervention in adults with asthma or allergic rhinitis, both investigators agreed there is no benefit. “I think and expect that bedding covers are recommended only in the framework of a more extensive allergen avoidance program,” said Dr. Gerth van Wijk. He added that although bedding covers are expensive, they are very easy to use and may be routinely recommended for this reason. “Our studies do not justify such a simple approach,” he stated.

Commenting on the findings, Thomas A.E. Platts-Mills, MD, PhD, wrote in an accompanying editorial, “If minor reductions in exposure to allergen are not sufficient, we need either to persuade families to change their living conditions on a large scale or to identify the real cause of the increase in the prevalence of asthma.”[3]

—Gale Jurasek

References
1. Woodcock A, Forster L, Matthews E, et al. Control of exposure to mite allergen and allergen-impermeable bed covers for adults with asthma. N Engl J Med. 2003;349:225-236.
2. Terreehorst I, Hak E, Oosting AJ, et al. Evaluation of impermeable covers for bedding in patients with allergic rhinitis. N Engl J Med. 2003;349:237-246.
3. Platts-Mills TAE. Allergen avoidance in the treatment of asthma and allergic rhinitis. N Engl J Med. 2003;349:207-208.

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