There were 3,065 pairs of twins all of whom had lived together while growing up and both of whom had been on active duty during the Vietnam War. The results showed that those who suffered from the most symptoms of PTSD had a 2.3 times higher chance to have asthma as those who had fewer symptoms.
The twins were both identical - twins that have all the same genetic material- and fraternal - twins that have just half the same. If the connection was genetic, then the results between the two types of twins would have been very different, but there was no marked difference from one group or the other. The association was there even when they adjusted for variables such as smoking, obesity and socioeconomic status, which are all associated with both conditions.
"If there had been a strong genetic component to the link between asthma and PTSD, the results between these two types of twins would have been different, but we didn't find substantial differences between the two," said lead researcher Renee D. Goodwin, PhD, assistant professor of Epidemiology at the Mailman School of Public Health.
Several other studies have found a relationship between asthma and other anxiety disorders, Dr. Goodwin noted, yet this study is the first to investigate the link between asthma and PTSD. This new research also confirmed previous findings that linked asthma with a higher risk of depression.
"The reason (s) for the association between asthma and mental disorders is not known. Asthma could increase the risk of anxiety disorders, or anxiety disorders might cause asthma, or there could be common risk factors for both asthma and anxiety disorders. Our study found the association between asthma and PTSD does not appear to be primarily due to a common genetic predisposition," she said.
The researchers found the association between asthma and PTSD existed even after they took into account factors such as cigarette smoking, obesity and socioeconomic status, all of which are associated with both anxiety disorders and asthma. "It is conceivable that traumatic stress, which has been associated with compromised immune functioning, leads to increased vulnerability to immune-system-related diseases, including asthma," Dr. Goodwin said.
"Alternatively, it may be that having asthma places adults at increased risk for PTSD as it increases thelikelihood that they will be exposed to a traumatic situation because they have a life-threatening chronic medical condition," she added.
The findings suggest that a person with asthma who experiences a traumatic event may benefit from seeking professional help, because they could be more vulnerable to developing post-traumatic stress disorder, she said.
The research is published in the first issue for November 2007 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.