KEYSTONE, COLO. – The diagnosis of asthma isn’t as simple as it may seem.
Asthma is a clinical syndrome with no specific diagnostic test. So, the response to therapy becomes a key element in finalizing the diagnosis, Dr. Gary R. Cott emphasized at a meeting on allergy and respiratory diseases sponsored by National Jewish Health.
"If you have features that make you think of asthma, particularly mild to moderate asthma, and you initiate therapy, you should expect a therapeutic response in 80%-85% of cases. If you don’t get a response, step back and think about whether you made the right diagnosis before you start escalating care," advised Dr. Cott, a pulmonologist and executive vice president of medical and clinical services at National Jewish Health, Denver.
How often do physicians on the front lines get the diagnosis of asthma wrong? The National Jewish experience is illuminating.
Dr. Gary R. Cott
In a series of 305 consecutive patients referred to the tertiary center with a preestablished diagnosis of asthma, all of whom were already on treatment for the disease, fully 25% didn’t have asthma at all. A mere 5% were found to have asthma only. A total of 38% had asthma plus an associated contributory respiratory condition, such as allergic rhinitis, rhinosinusitis, or aspirin sensitivity. Another 32% had asthma plus a cardiopulmonary condition that contributed to their symptoms, such as valvular dysfunction, a vascular ring, or pulmonary embolus.
"This experience has been duplicated at other specialty centers. It’s not unique to what we see," according to Dr. Cott.
He defined asthma as a syndrome characterized by increased airway responsiveness to various stimuli, along with variable obstruction of expiratory flow. It’s a physiologic definition. Four elements are essential in establishing the diagnosis: the history, physical exam, spirometry, and response to therapy. A variety of other tests are often helpful in narrowing the differential diagnosis – for example, chest imaging, blood eosinophil measurement, allergy testing, bronchial challenges, diffusing capacity of the lung for carbon monoxide, lung volumes, and elasticity. But they’re not specific for asthma.
"All that wheezes is not asthma. But most is," Dr. Cott observed. "I must say, I’m not very critical of the outside docs who send patients in and say, ‘I think they have asthma,’ and we then say, ‘No, they’ve got something else.’ Asthma is a common disorder. In Colorado, as much as 11% of the population can have asthma. It’s probably the most common thing that will cause an otherwise healthy individual to present with recurring or persistent symptoms.
"I think that sometimes leads us down a path – not always incorrect – of thinking, ‘Let’s try treating for asthma,’ " Dr. Cott noted. "The problem is that when they’re not responding, it’s time to rethink the differential carefully."
The list of disorders involving lower airways obstruction that can mimic asthma is extensive. The top two masqueraders are emphysema and chronic bronchitis. What’s more, asthma, emphysema, and chronic bronchitis aren’t mutually exclusive diseases. A given patient can have any two or even all three.
In contrast to asthma, which is defined physiologically, chronic bronchitis has a historical definition: It’s a condition involving cough with excessive sputum production for at least 3 months per year in at least 2 consecutive years. And emphysema is defined anatomically: permanent enlargement of air spaces distal to the terminal bronchiole with alveolar septae destruction.
It’s important to differentiate these conditions, because their guideline-recommended management strategies differ, as do their prognoses, Dr. Cott continued.
In addition to emphysema and chronic bronchitis, other lower airways disorders that can mimic asthma include infection, sarcoidosis, interstitial lung disease, cystic fibrosis, a tumor or foreign body, and bronchiolitis.
The list of upper airways disorders that can be mistaken for asthma includes vocal cord dysfunction, infection, laryngeal spasm, and laryngeal edema secondary to angioedema. The most useful spirometric clue to upper airways obstruction, in Dr. Cott’s view, is a ratio of the forced expiratory flow at 50% volume to forced inspiratory volume at 50% of 1 or greater.
"That’s virtually always present with upper airway or extrathoracic airways obstruction," he said.
Dr. Cott reported having no conflicts of interest.