Pulmonary Conditions & Diseases: Cough
What is a Cough?
A cough is initiated by chemical and pressure receptors that line the upper respiratory tract (trachea and the main-stem bronchi) and lower respiratory tract (the lungs), the diaphragm, the esophagus, the stomach, and portions of the tissue that line the surface of the heart. These receptors exist as a mechanism for protecting the lungs from pathogens (harmful microorganisms). When these receptors become stimulated by changes in temperature (cold or heat), changes in acidity, chemical triggers (inhaled gases/volatile compounds), or other triggers (such as dust or pollen), the cough reflex begins and triggers an impulse to a portion of the brain that generates a return signal that travels to nerves which supply the signal to the respiratory muscles to induce a cough.
Duration of Cough
A cough is not a diagnosis in itself but a symptom of another underlying cause. Coughs are initially classified based on their duration. An acute cough lasts less than 3 weeks, a subacute cough lasts between 3-8 weeks, and a chronic cough lasts >8 weeks.
Timing is a key component in evaluating a cough because it helps to determine the appropriate steps in evaluation to determine the root cause of the cough.
Acute Coughs
As described above, an acute cough lasts less than 3 weeks. The majority of acute coughs are post-infectious (following a viral or bacterial respiratory infection), allergy-mediated (from exposure to respiratory allergens), asthma exacerbation or swallowing difficulties (aspirating fluid into the trachea). These coughs resolve within this 3-week window.
Subacute Cough
As described above, a sub-acute cough lasts between 3-8 weeks. The most common causes of sub-acute coughs are post-infectious (as stated earlier, this is following a viral or bacterial respiratory infection), asthma or COPD exacerbation, gastroesophageal reflux, post-nasal drip, allergies, bronchitis, bronchiectasis, pulmonary embolism, heart failure or tuberculosis in a region where tuberculosis is common.
Chronic Cough
As described above, a chronic cough lasts greater than 8 weeks. The most common causes of chronic cough include asthma or COPD, post-nasal drip, allergies, gastroesophageal reflux, upper airway cough syndrome, ACE inhibitor class of medications, swallowing abnormalities, chronic bronchitis, bronchiectasis, interstitial lung disease, tuberculosis or malignancy.
Evaluation
Once the duration/timing of a cough has been established, further evaluation of a cough includes a detailed history to understand a patient’s triggers, underlying medical conditions, family history of lung diseases, allergy history, and recent illnesses. Depending on the outcome of a fully-elicited medical history, the following appropriate diagnostic and treatment steps would commence.
In situations where reflux, post-nasal drip, or allergies are suspected, a trial of medication therapy is often initiated because this can serve as both an initial therapeutic and diagnostic approach. The patient will follow up closely with the provider to see if the therapy improves the cough. Baseline testing usually occurs at the initial evaluation as well and includes pulmonary function testing (PFTs) which can illustrate underlying obstructive lung diseases such as asthma, COPD, or restrictive lung diseases. If any of these diseases are demonstrated on PFTs, then the patient is started with appropriate medical treatment for those diseases.
If a possible infection is suspected, then microbiologic testing is often performed to look for infectious markers. If warranted, a bronchoscopy (endoscopic evaluation of the lungs) can also be performed during which biopsies of lung tissue can be obtained for further diagnosis.
If the patient’s history indicates other chronic medical conditions, such as heart failure or kidney failure, a chest x-ray can help to determine if the patient has fluid backing up into the lungs (pulmonary edema) which can result in a cough. The resolution would include medical treatment to remove the fluid out of the lungs.
If blood clots in the lungs (pulmonary emboli) are suspected, a blood test can be performed, and a CT angiogram of the lungs can be done to evaluate for possible blood clots in the lungs (pulmonary emboli).
Treatment
As illustrated above, since there are multiple causes of cough, the treatment will always depend on the underlying cause of the cough. Once that is determined, then treatment (as discussed above) will begin, and regular follow-up with the provider is essential in managing the underlying process that caused the cough.