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