Chapter 9 Cough

1 Describe the cough reflex.

Stimuli that induce cough can be mechanical (e.g., foreign body, dust), chemical (e.g., capsaicin, acetic acid), or inflammatory mediators (e.g., histamine, bradykinin, prostaglandin E₂). Such stimuli interact with cough receptors in the upper and lower airways that cause production of mediators, such as tachykinins and neurokinin A. Stimulation of the cough receptors is transmitted via the vagus nerve to the “cough center” of the brainstem (probably located in the pons). The efferent limb of the cough pathway is poorly understood, but is known to include the spinal cord from C3 to S2, spinal nerves, and the recurrent laryngeal nerve (for glottic closure). Activation of the efferent pathway causes constriction of the expiratory muscles, which produces increased airway pressure against the closed glottis. The airway also narrows slightly. When the glottis is suddenly opened, air is expelled at a high velocity, clearing the airway of secretions and foreign material.

2 How is a cough suppressed?

As with respiration, cough is largely involuntary but can be suppressed voluntarily to a certain extent (cortical modulation). Opioids that suppress cough (e.g., morphine) are believed to produce this effect by acting centrally on the cough center. Inhalation of nebulized lidocaine suppresses cough by affecting cough receptors in the airway.

3 Is the cough reflex mediated by the same pathway that produces bronchospasm?

It has been widely held that airway hypersensitivity (asthma/bronchospasm) and cough are manifestations of the same physiologic mechanism. By this thinking, persistent coughing, such as wheezing, is a common presentation of asthma. This view has led to the current trend of diagnosing children with persistent coughing as being asthmatic (“cough variant” asthma), even when these patients have no other evidence of bronchospasm (dyspnea, exercise intolerance, abnormal spirometry findings). However, recent evidence suggests that while airway hypersensitivity and cough receptor sensitivity can be triggered by the same stimuli, this probably occurs via two different pathways. For most children, it appears that persistent coughing results solely from increased sensitivity of cough receptors in the upper and lower airways and does not represent true asthma/bronchospasm.

Chang AB: Cough, cough receptors, and asthma in children. Pediatr Pulmonol 28:59–70, 1999.

4 How much coughing is normal?

One of the primary obstacles in studying cough in children has been that this symptom is difficult to reliably measure. Reproducibility of survey questions is poor, and, unlike the situation with asthma, there is no validated subjective cough scoring system. Fortunately, a new device that a child can wear like a Holter monitor provides an objective measurement of coughing. When this device was used with “normal” children who had no history of asthma and no respiratory illnesses in the previous 4 weeks, the mean cough frequency over 24 hours was 11.3 episodes (range, 1–34 episodes). Thus, the frequency of coughing episodes varies widely even among children without respiratory problems.

Munyard P, Bush A: How much coughing is normal? Arch Dis Child 74:531–534, 1996.

5 What is the most common cause of persistent cough?

While the differential diagnosis of persistent cough (>2 weeks) in children is relatively broad (Table 9-1), the most common cause is probably postviral or inflammatory cough, which has been variably called nonspecific cough, isolated cough, or cough illness. Children who develop a viral upper respiratory tract infection may continue to cough long after other viral symptoms (e.g., rhinitis, fever) have subsided. These patients have a persistent, dry cough (particularly at night), but do not have wheezing, chest tightness, dyspnea on exertion, or other symptoms of bronchospasm. This type of cough may persist for several weeks before resolving spontaneously.

Table 9-1 Differential Diagnosis of Persistent Cough by Age