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Diagnosis of Asthma: Review & Update
Asthma is a poorly understood disease characterized by chronic inflammation of the airways (bronchi and bronchioles). This inflammation causes periodic constriction of the airways in people with asthma, with shortness of breath, wheezing, and coughing that is often worse at night or early in the morning.
Asthma is thought to affect more than 300 million people of all ages worldwide. Developed countries have higher asthma rates, with Australia topping out the list at 27%. In childhood, asthma affects more boys than girls, but more boys "outgrow" their asthma, so more women than men are affected by asthma as adults.
No one knows why some people get asthma and others don't. Genetic predisposition is partly responsible for the development of asthma in children (asthma runs in families). Viral infections and exposure to airborne allergens (dust mites, dog and cat dander) may increase asthma risk. However, the children regularly exposed to the highest concentration and spectrum of allergens and microorganisms -- those who live on farms -- have lower rates of asthma, supporting the "hygiene hypothesis" that cleaner environments in the developed world are responsible for increasing rates of childhood asthma. Among adults, smoking, obesity, inhalational exposures at work, and numerous other risk factors are weakly associated with developing asthma.
Most people with asthma have mild asthma. Even without any treatment, people with mild asthma have infrequent symptoms of mild severity, often defined as:
- Going days without any symptoms;
- Waking up at night with asthma symptoms less than once a week;
- Rare asthma exacerbations / flares requiring oral steroids (prednisone, Solu-Medrol, etc.);
- Normal lung function in between asthma exacerbations (completely reversible airflow obstruction).
The true number of people with mild asthma is unknown, because many just tolerate their mild asthma symptoms without seeking treatment. As a result, although mild, their asthma is often uncontrolled.
This definition of "mild" is debated because it does not include the severity of symptoms when they do occur. For example, does a person who has no asthma symptoms for a year while taking no treatment, but then has a sudden, severe exacerbation requiring intubation have "mild" asthma? (By the above definition, yes; you see what the critics mean.)
How to Diagnose Asthma: Symptoms + Airflow Limitation
For a conclusive diagnosis of asthma, both of the following must be present:
- Symptoms typical of asthma including shortness of breath, cough, and/or wheezing;
- Objective evidence of partially or wholly reversible airflow limitation.
Airflow limitation can be demonstrated by wheezing on auscultation or spirometry. Reversibility is shown by the relief of airflow obstruction, with improvement in any spirometry measurement after administration of short-acting beta agonist (albuterol / salbutamol):
- Increase in FEV1 ≥ 12% from pre-bronchodilator baseline
- Increase in predicted FEV1 by ≥ 10 percentage points
- Improvement in peak expiratory flow by 60 liters / min or ≥ 20% over baseline
Alternatively, peak expiratory flow can be measured at home and reversibility demonstrated if measurements vary by >10%. This criterion is "softer," as it relies on unsupervised patient technique and reporting, and peak flow measurements are much less reproducible than spirometry. Some experts endorse a 20% variability in peak flow as a better (i.e., more specific) cutoff for diagnosing asthma.
Diagnosing Asthma When Spirometry Is Normal
Because asthma symptoms come and go, it's common for people with mild asthma to have normal spirometry and peak flow measurements during an office visit. Various approaches are appropriate to diagnose asthma in people with apparently normal lung function:
- Advise the patient to return as a "walk-in" to a medical clinic when experiencing symptoms, for repeat spirometry;
- Provide a peak flow meter, teach the patient on its proper use, and instruct them to check and record morning and evening values, including some before and after use of short-acting beta agonist.
- Perform bronchoprovocation challenge with methacholine, mannitol, or exercise. A negative methacholine challenge test in someone with normal lung function effectively rules out asthma, but a positive result (a 20% decrease in FEV1 at a methacholine concentration of 8mg/mL or less) does not prove asthma (i.e., it is sensitive but not specific), because many people without asthma will develop bronchoconstriction with methacholine exposure. Methacholine testing is cumbersome and costly, and is not commonly performed.
Because of the ease of prescribing medications, and the inconvenience of confirming airflow limitation using the above techniques, asthma is most often diagnosed "clinically," i.e., without evidence of objective airflow limitation. Short-acting beta agonists and inhaled corticosteroids are prescribed empirically as a trial of therapy; if the patient's symptoms improve, the presumptive diagnosis of asthma is made. Expert bodies like the NAEPP recommend that whenever possible, these clinical diagnoses of asthma should be validated with objective data.
Other Tests for Asthma Diagnosis
Chest X-rays are almost always normal in people with asthma and are not recommended as routine for people with mild asthma. Some experts advise checking a chest X-ray in anyone over 40 with moderate-to-severe asthma, to identify other potential diagnoses like heart failure. This approach has not been evaluated for effectiveness.
Exhaled nitric oxide shows promise in diagnosing asthma, but is not widely available and its role as an established part of the workup for asthma and other respiratory illnesses has yet to be defined.
Blood tests for IgE, eosinophil counts, and skin allergy testing may add to the understanding of a person's allergic burden and its potential contribution to their asthma, but these tests do not have an established role in diagnosing asthma.
Is It Really Asthma, Or Something Else? Alternative Asthma Diagnoses
All that wheezes is not asthma. Many conditions besides asthma can present with shortness of breath, wheezing, cough, and/or airflow limitation. While asthma is probably more common than all of its mimics combined, it's important to consider alternative diagnoses before (and after) diagnosing a patient with asthma:
|Differential Diagnosis of Asthma*|
|Vocal cord dysfunction||Congestive heart failure||Foreign body aspiration|
|COPD||Rhinosinusitis||Gastroesophageal reflux disease|
|Post-viral coughing||Eosinophilic bronchitis||Pertussis|
|ACE-inhibitor cough||Chronic bronchitis||Pulmonary embolism|
|Tracheal stenosis||Laryngeal stenosis||Bronchiolitis obliterans|
|Postnasal drip||Tracheomalacia||Mediastinal mass|
|Carcinoid tumor||Goiter (huge)||Vascular rings / aneurysms|
* not a complete list
Elisabeth H. Bel. Mild Asthma. N Engl J Med 2013; 369:549-557.