The airways of asthmatic patients are said to be hyperreactive or hypersensitive. This means that they react to several stimuli with bronchoconstriction, that is, the airways are narrowed and cause difficult breathing.
These triggering stimuli are very varied: infections, cigarette smoke, exercise, cold weather, allergens, stress, chemical compounds… The airways of non-asthmatic persons do not react with bronchoconstriction when exposed to those stimuli.
The hyperreactivity test is used to check if the airways of a patient are hyperreactive. It is used for the diagnosis or follow-up of asthma, to assess severity or response to treatment.
First the patient performs a pulmonary function test, such as spirometry or another. Then the patient inhales an amount of a chemical compound, such as methacholine (or others like adenosine or mannitol), and the pulmonary function test is repeated. Bronchial hyperreactivity can also be studied inhaling cold air or making exercise instead of using compounds. All these tests are sometimes named as unspecific. When an allergen is inhaled, the tests are said to be specific.
If there is no significant decrease in lung function, the patient inhales successive increasing amounts of the compound followed by functional tests, until there is a decrease or until the maximal amount is reached.
When there is a decrease, the test is considered positive, the test is stopped and the patient is given a bronchodilator for relief. If there is no decrease, the test is considered negative.
The test takes 30-60 minutes. The patient has to perform several manoeuvres of forced breathing, which can be tiring. The risk of the test is the appearance of bronchoconstriction, asthma symptoms of wheezing and difficult breathing. The test starts with low amounts of the compound; thus if symptoms appear they are mild but, occasionally, in very sensitive patients, severe symptoms may appear.
The medication used for asthma may change the results of the test. If you are scheduled for a hyperreactivity test you should ask your doctor if you must withdraw the medication. The doctor might want to know the result of the test with no modification by the medication, but might also want to know if the medication is protecting you.