Washington, Jan 30 : A research carried out by multi-centres to investigate severe asthma has found that there is a key physiological difference between severe and non-severe forms of the disease.
The finding could help to explain why those with severe asthma do not respond well to treatment.
The study from the Severe Asthma Research Program (SARP) found that those suffering from severe asthma showed signs of "air trapping" in the lungs, a condition that prevents a person from exhaling fully.
It was revealed in the study that those who suffered from severe asthma were more likely to have an airway obstruction even after getting maximal treatment. The results suggested that those who suffered from severe asthma had a different form of the disease.
"SARP was formed to look for an underlying cause of severe asthma, because it is not responding to treatment," said Ronald Sorkness, a physiologist at the University of Wisconsin in Madison.
Sorkness is also the lead author of the study, "Lung Function in Adults with Stable but Severe asthma: Air Trapping and Incomplete Reversal of Obstruction with Bronchodilation." Understanding the pathophysiology of severe asthma and improving its treatment is the goal of SARP.
The study compared lung function measurements from 287 people with severe asthma and 382 people with mild and moderate (non-severe) forms of the disease.
There is still much to be known about asthma, for example, it is still not clear why more women than men suffer from the illness, and the opposite happening among children.
If severe asthma is different from milder forms of the disease, and those differences can be identified, it might suggest new treatments.
On the other hand, if severe asthma is not different, only a more extreme version of the same disease, then using current treatments more aggressively might work.
Sorkness and his fellow SARP researchers examined four aspects of lung function by using lung function data collected at 10 research centres that were part of SARP:
(1). Airflow limitation: When exhaling a volume of air, a longer period of time is required. This is probably related to the narrowing of the airway, which is a trademark of asthmatics, whether their condition is mild, moderate or severe.
(2). Air trapping: When there is an inability to exhale completely, like most people who exhale about 70% of their lung volume. Air gets trapped when exhaling of air is considerably less than normal, and it is related to extreme narrowing and complete closure of airways during exhalation.
(3). Reversibility: In cases where the asthma is not severe, it is reversible with a bronchodilator treatment.
(4). Hyperresponsiveness: The last finding was that, an irritant like smoke can cause muscles in the airways to contract and close. People who are suffering from asthma are much more sensitive, that is, hyperresponsive to these irritants.
According to the study, even though airflow limitation was common among asthma sufferers, regardless of whether they had mild, moderate or severe forms of the disease, it was found that air trapping is a trait seen in those who were suffering from severe asthma, and not in those with non-severe asthma.
It was observed that, as airflow limitation became more pronounced, there was more air trapping in the severe group, but not among those who suffered moderate or mild asthma.
"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.
The researchers came to the conclusion that it could be that airflow limitation occurs in the larger airways of the lungs, and that air trapping occurred in the small airways that branch to the outer portions of the lung.
They also found that those who suffered from severe asthma showed incomplete reversibility with bronchodilator treatment. That is, the severe group was more likely to have airway obstruction even after maximal treatment.
When measuring them on the level of hyperresponsiveness, there was not much difference between severe asthmatics and non-severe ones.
However, the subjects with the most severe asthma were not included in the airway challenge portion of the study for fear of setting off a serious attack.
"Air trapping and non-reversibility were most important factors in defining the severe asthma group," Sorkness said.