Recently I had some discrepancies with senior respiratory specialists regarding some dificulties differentiating some cases of moderate/severe persistent asthma versus some COPD cases. The discrepancy was based on the usefulness of bronchial provocation as an aid in supporting one or other diagnosis. If there is a clear response to bronchial provocation in a smoker with less than 10 pack-year, then the patient should be considered asthmatic. If it is not but the obstruction at spirometry is evident, the balance favors COPD. That was my position and I tried to document myself with more background.
Excess Response at the lungs
Excess airway response is usually a clinical feature of asthma. It is called airway hyperresponsiveness (AH) (reactivity). Patients with normal spirometry (measurement of airway flows) may have AH. Other conditions associated to AH include allergic rhinitis (seasonal or perennial and intermittent or persistent), exposure to toxins or occupational substances, and respiratory infections.
Who deserves a challenge
The tests indicated for assessing the constriction of airways after the exposure to a particular stimulus are called Bronchoprovocation Tests, Bronchial Provocation Challenge Tests, Tests of Bronchial Reactivity (BT as acronym in this article). BT has a main indication: history of bronchospasm (bronchoconstriction: contraction of tiny muscles surrounding airways leading to obstruction) in a subject with normal spirometry. Then, if the patient says “I have dry cough and wheezing after this or that, but not always” and you perform spirometry with normal results, you may need to challenge with chemicals or physical triggers to evaluate if there is a hyperresponse. The doctor has now a patient with history of symptoms and normal spirometry and the logical conduct is to perform a BT which includes a new spirometry to be compared with the former.
Usefulness of BTs
The BTs are sensitive for asthma but nonspecific, which means that a positive BT supports asthma or other type of hyperresponsiveness. However a negative one just says that symptoms are not due to hyperresponsivenes.
Other applications of BTs include:
Uncovering asthma when skin tests are equivocal
Confirming occupational asthma
I would classify the methods for BT according to the type of challenge: a) Chemical, b) Biological, and c) Mechanical or Physical.
Chemical include challenges based on exposure to quantified amounts of pharmacologic non pharmacologic agents. Pharmacologic agents used with this purpose are: Methacholine, Histamine, and Carbacholine. Non-pharmacologic agents (but still chemical) include Toluene diisocyanate.
Biological agents to be used as triggers during BT may include antigens (moieties able to generate an extreme response of our defense system) like Bacillus subtilis, Pollen, Molds, House dust.
Mechanical and physical agents found during active exercise: cold air, dry air, hyperventilation (deep and rapid breathing).
In a next post I will focus on each of the BTs. The final point here is that symptoms of asthma help to say if a patient has the disease but physicians should try to find the reason instead of applying guidelines of therapy without confirming a diagnosis or finding the rationale for having the condition.