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Not All Asthma Created Equal

Distinct Differences Between Severe, Mild Asthma Could Lead to Targeted Treatments
By
WebMD Health News
Reviewed by Louise Chang, MD

Jan. 30, 2008 -- There are key differences between severe asthma and milder forms of the disease. And those differences could have implications for the management of the most at-risk and hard-to-treat patients.

In most cases, asthma can remain well controlled with medication. But 5% to 10% of asthma patients suffer from a severe form of the disease that does not respond well to treatment.

These patients have a higher risk of asthma-related hospitalization and death, but it has not been clear if there are unique features about severe asthma that cause patients to be sicker.

"If severe asthma is just the extreme of garden-variety asthma, then we would probably treat it the same way, but more aggressively," says researcher Ronald Sorkness, PhD, of the University of Wisconsin at Madison. "But if there are different underlying disease processes, treatment may be different."

"Air-Trapping" More Common

Using grants from the National Institutes of Health, the Severe Asthma Research Program (SARP) was formed in 2001 in an attempt to answer the question.

In a newly reported study, pooled data from 10 SARP centers were analyzed to look for physiological differences between patients with severe forms of asthma and milder forms of the disease.

Researchers compared lung function measurements of 287 people with severe asthma to 382 patients with mild to moderate asthma.

They found that the severe asthma patients were much more likely to show signs of "air-trapping," in which exhaled air is not fully expelled from the lungs.

Sorkness, who led the study team, tells WebMD that the finding points to greater involvement of the small airways than what is seen in less severe cases of asthma.

The study appears in the latest online edition of the Journal of Applied Physiology.

"This is an indication that there is something different about this population with severe asthma, and it gives us the impetus to look more closely at what is going on, with the hope of finding unique ways to treat these patients," he says.

Severe Asthma Unique

Another difference between the two groups: Patients with severe asthma were more likely to have continued airway obstruction even after aggressive treatment.

There was no significant difference between patients with severe and milder asthma, however, in the length of time required to exhale a volume of air -- a measure known as air-flow limitation.

Airflow limitation was common among asthma sufferers, regardless of whether they had mild, moderate, or severe forms of the disease.

But there was little air trapping seen in patients with mild to moderate disease, even when air flow limitations occurred. Conversely, patients with severe disease had some air trapping even in the absence of airflow limitation.

"That tells us that something different is going on in people classified as having severe asthma, either physiologically or in the airways that are affected," Sorkness said. "It's likely that airflow limitation occurs in the larger airways of the lungs, while air trapping is occurring in the small airways that branch to the outer portions of the lung."

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