Is Asthma Overdiagnosed in Obesity?Friday, November 28, 2008
But how much of this is really asthma?
This question was addressed by Shawn Aaron and other members of the Canadian Respiratory Consortium in a study published last week in CMAJ.
Aaron and colleagues conducted a longitudinal study of 242 obese (BMI >30) individuals with physician-diagnosed asthma, identified by random dialing in 8 Canadian cities. A diagnosis of current asthma was excluded in those who did not have evidence of acute worsening of asthma symptoms, reversible airflow obstruction or bronchial hyperresponsiveness, despite being weaned off asthma medications.
Asthma was ultimately excluded in 31.8% of obese patients, of whom 65% did not need to take asthma medications or seek health care for asthma symptoms during a 6-month follow-up period.
This study shows that only 2 in 3 obese patients treated for asthma actually have asthma.
Now if anyone thinks that asthma is only overdiagnosed in obese patients, it is worth pointing out that the rate of overdiagnosis of asthma was virtually identical (28.7%) in the 254 non-obese (BMI 20-25) individuals also examined in the same study.
Thus, the really surprising finding of this study for me is that overdiagnosis of asthma is NOT more likely in obese patients than in non-obese patients. This is certainly unexpected, given the fact that exertional dyspnoe, which is indeed more common in obese individuals, can indeed mimic asthma.
The alarming piece in this finding though, as pointed out in an accompanying editorial by Matthew Stanbrook and Alan Kaplan, is that
“Asthma misdiagnosis in patients with persistent or recurrent symptoms has important and potentially serious consequences to both patients and the health care system. Symptoms assumed to be due to asthma may instead signify another medical condition that thereby goes undiagnosed and untreated. Asthma … can only be reliably distinguished with objective testing. Because asthma is a chronic disease, the consequences of misdiagnosis may extend for many years.”
The bottom line is that suspected asthma should always be confirmed with spirometry – especially in obese patients, where restriction and/or simply the increased oxygen cost of breathing can cause breathlessness, which is easily misdiagnosed as asthma.
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