Clinical Question: Does the use of impermeable bed and pillow covers reduce asthma symptoms in mite-sensitive patients?

Setting: Outpatient (primary care)

Study Design: Randomized controlled trial (double-blinded)

Synopsis: Although it makes sense that patients with asthma who also are allergic to dust mites would benefit from reducing exposure to mites and their feces, evidence that impermeable bed and pillow covers actually make a difference is limited and is derived primarily from small, poorly designed studies. In this much larger study, patients 18 to 50 years of age who were taking inhaled steroids for diagnosed asthma and using rescue inhalers at least once a day were recruited from 154 English general practices. Their sensitivity to dust mites was measured at the beginning of the study, but the results were not disclosed to the patients or their physicians.

Of 1,746 eligible patients, 1,431 started a one-month run-in phase, and 1,150 were randomized (allocation concealed) to receive impermeable or standard cotton-polyester mattress, pillow, and quilt covers. Symptoms, peak flow measurements, and use of rescue inhalers were recorded in a diary during the run-in period, and patients who were compliant (those who completed the diary for 14 of 28 days) were invited to participate in the rest of the trial. Patients were seen every three months for six months, and then were asked to reduce their dosage of inhaled cortico-steroids during the second half of the 12-month study, during which time they were more closely monitored. Mite sensitivity was documented in 366 patients from each group.

At the end of the study period, the mite-sensitive patients in the treatment and control groups did not differ significantly in peak flow measurements, symptom scores, exacerbations, days of work missed, or use of beta agonists. There also was no difference between groups in the average reduction in steroid dosage during the second half of the study, or in the percentage of patients who could stop taking inhaled steroids altogether.

This study was large enough to detect a difference of 20 L per minute in peak flow, and a 50 percent increase in the proportion of patients who discontinued inhaled steroids. Although we are not told whether the study was large enough to detect a clinically meaningful difference in symptom scores, it seems likely that it was, given the number of patients.

Bottom Line: Use of impermeable bed, pillow, and quilt covers does not improve symptoms or reduce the need for medication in adults with asthma. Because other studies have suggested some benefit in children with asthma, this intervention should not be abandoned in younger patients. (Level of Evidence: 1b)