Chronic Cough

Judd W. Landsberg

The cough reflex arc usually begins with stimulation of irritant receptors located primarily in the vocal cords, trachea, and airways. These sites are sensitive to mechanical, thermal, chemical, and pH disturbances commonly caused by mucus or aspiration. Afferent impulses then travel to the brain via the trigeminal, glossopharyngeal, superior laryngeal, and vagus nerves and are processed in the cough center located in the medulla. Efferent impulses are then transmitted via the vagus, phrenic, and spinal motor nerves to the glottis, diaphragm, intercostals, and abdominal muscles, culminating in a choreographed sequence of (1) inspiration, (2) glottic closure, (3) diaphragmatic relaxation, (4) forceful expiratory muscle contraction (raising intrapleural pressure up to 200 mmHg), followed by (5) sudden glottic opening, explosively releasing the large transpulmonary pressure gradient between the pleura and the airway. Expiratory volume is no greater than during a forced exhalation, but extreme narrowing of the airway caused by the pressure gradient leads to higher linear velocities (close to the speed of sound), which are generally effective in dislodging mucus and foreign materials. Patients with chronic airway obstruction produce appropriate intrathoracic pressures but generate lower linear velocities and a less effective cough because of pathologic airway collapse.

Occasional cough caused by minor irritations (e.g., aspiration of oral secretions) is normal. Cough remaining after upper or lower respiratory tract infections (typically viral, mycoplasma, chlamydia, or pertussis) may continue for 8 weeks or more. This type of postinfectious cough is the most common cause of subacute cough (lasting 3–8 weeks). An empiric course of macrolide antibiotics is usually warranted in this clinical setting, though treatment after the first 2 weeks is of unproven benefit, and the course should be self-limited. Chronic cough (lasting more than 8 weeks) can cause complications, distress, and be debilitating to the individual. Most chronic cough is attributable to upper airway cough syndrome (UACS) from postnasal drip (PND), asthma (often cough variant), or gastroesophageal reflux disease (GERD), alone or in combination.

The initial step in the evaluation of chronic cough is screening for special situations (e.g., ACE inhibitor therapy) and identifying high-risk patients, namely the immunosuppressed, smokers, and those with purulent sputum or hemoptysis. Immunosuppressed patients need an aggressive workup to exclude atypical bacterial and fungal infection usually requiring sputum culture, a noncontrast chest computed tomography (CT) scan, and a bronchoscopic evaluation if parenchymal abnormalities are discovered. Although chronic cough occurs in up to 75% of cigarette smokers, the majority do not seek treatment. Those who do should get a posteroanterior (PA) and lateral chest radiograph to screen for obvious lung cancer with most abnormalities mandating timely CT characterization and outpatient pulmonary evaluation and follow-up. Smokers must also be repeatedly counseled about smoking cessation, especially during the evaluation of potential lung cancer. Individuals with a chronic cough productive of purulent sputum with or without hemoptysis need to have the etiology established, that is, bronchiectasis, sinusitis, atypical lung infection, or chronic bronchitis (active smokers). Such individuals need cultures (for typical and atypical organisms), antibiotic therapy, and a PA and lateral chest radiograph. If parenchymal abnormalities exist, further investigation with a non-contrast chest CT scan and serologies may be appropriate to rule out an atypical lung infection requiring specific therapy (e.g., fungal or mycobacterial infection), as well as an appropriate bronchiectasis work up.

Patients on angiotensin-converting enzyme (ACE) inhibitor therapy complaining of a chronic dry cough should be switched to an angiotensin receptor blocker (ARB). Up to 15% of patients on ACE inhibitors experience an irritant-mediated cough (typically within the first 6 months of therapy) quite likely secondary to bradykinin accumulation (also broken down by the ACE).

The next step in the evaluation and management of chronic cough involves a diagnostic, therapeutic medication trial using history and physical examination to establish whether obvious symptoms of UACS, asthma, or GERD exist (e.g., PND, episodic wheeze, sour taste in the mouth). If no obvious symptoms exist, UACS can be inferred on physical exam if signs of chronic sinusitis are present, or upper airway examination demonstrates upper airway secretions, cobblestoning, and laryngeal inflammation. Ultimately, the practitioner and the patient must decide to pursue a stepwise approach in which UACS, GERD, and asthma are treated sequentially, or an aggressive approach where all three conditions are treated at once, with medications then sequentially pulled away after the cough abates. People who are fearful of medications usually prefer a stepwise approach, whereas individuals debilitated by coughing will try everything (at once) to make the cough stop. While many practitioners (as scientists) prefer a stepwise approach, many times all three diseases require maximal therapy in order for a chronic cough to resolve.

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