Out of Breath: Diagnosing Asthma

On the first day of my internal medicine selective, I met an interesting patient. He was being followed up in our clinic for investigation of frequent episodes of intense dyspnea. As per his report, these episodes occurred most frequently in the winter and spring months, were often associated with upper respiratory tract infections and what he thought might be allergens. He had been seen in our clinic previously and was following up with the results of his Pulmonary Function Test (PFT). Since then, he had been taking daily inhaled corticosteroids with resolution of his symptoms. Interestingly, his PFT results suggested a severe restrictive pattern, without the characteristic bronchodilator response expected in asthma. Given this surprising information, I used this opportunity to review common features of asthma presentation, diagnosis, and when to consider alternative diagnoses.

Asthma is a chronic inflammatory airway disease which affects 10.8% of Canadians. It is characterized by episodic symptoms, airflow obstruction, and bronchial hyperresponsiveness on a background of underlying inflammation. A clinical presentation which includes recurrent symptoms (i.e. frequent episodes of dyspnea, chest tightness, cough or wheeze), especially with nocturnal worsening and signs of variable airway obstruction should prompt investigation for asthma. In addition, it is critical to investigate for triggers, including upper respiratory tract infections, exercise, exposure to allergens (i.e. pet dander, mold) or respiratory irritants (i.e. smoking, chemical solvents) at home or at work.

Asthma is typically diagnosed by PFT demonstrating reversible airway obstruction. Typical findings would include a forced expiratory volume (FEV1)/forced vital capacity (FVC) ratio of less than 0.75 in adults, in addition to >12% (200ml in adults) improvement in FEV1 after bronchodilator. Should these findings not be present, asthma can still be diagnosed based on peak expiratory flow (PEF) variability: an increase in PEF of  >20% (60L/min) after bronchodilator/controller therapy or diurnal variation. The final alternative criterion is a positive methacholine challenge test: if FEV1 decreases >20% after 4mg/ml inhaled methacholine.

I was also curious if an obstructive disease like asthma could ever present with a restrictive PFT, as this patient’s clinical history, presentation and response to treatment were very convincing for asthma. In one study of 413 patients diagnosed with asthma, at least 32 demonstrated restrictive impairment on their PFTs. This impairment was not pseudorestriction, which may occur secondary to air trapping, as they defined restriction as decreased total lung capacity or vital capacity without corresponding increase in functional residual capacity. The authors suggest that findings of restriction on PFTs should not preclude an asthma diagnosis or appropriate therapy. While this evidence may explain my patient’s findings, we decided that it would be important to investigate for any other causes of his restrictive PFT findings.

The differential diagnosis for restrictive lung disease is fairly broad, covering interstitial lung disease, neuromuscular disease and chest wall disorders. Covering restrictive lung disease would require another blogpost, but I thought it was important to highlight a situation where other diagnoses should be considered when investigating asthma. When considering an asthma diagnosis, it is critical to rule out other diagnoses such as COPD, heart failure, bronchiectasis, ACE inhibitor induced cough and central airway obstruction.

As a medical student pursuing a career in family medicine, this case taught me some important lessons about the differential diagnosis of episodic dyspnea which I am sure will be useful in my future practice.

-EB-

References:

Lougheed MD, Lemière C, Dell SD, Ducharme FM, FitzGerald JM, Leigh R, Licskai C, Rowe BH, Bowie D, Becker A, Boulet LP. Canadian Thoracic Society Asthma Management Continuum–2010 Consensus Summary for children six years of age and over, and adults. Canadian Respiratory Journal. 2010 Jan 1;17(1):15-24.

Miller A, Palecki A. Restrictive impairment in patients with asthma. Respiratory medicine. 2007 Feb 1;101(2):272-6.