Cough is an explosive
expiration that provides a normal protective mechanism for clearing the
tracheobronchial tree of secretions and foreign material. When excessive or
bothersome, it is also one of the most common symptoms for which medical
attention is sought. Reasons for the latter include discomfort from the cough
itself, interference with normal lifestyle, and concern for the cause of the
cough, especially fear of cancer.
Mechanism. Coughing may be initiated either voluntarily or reflexively. As a
defensive reflex it has both afferent and efferent pathways. The afferent
limb includes
receptors within the sensory distribution of the trigeminal, glossopharyngeal,
superior laryngeal, and vagus nerves. The efferent limb includes the recurrent laryngeal
nerve and the spinal nerves. The cough starts with a deep inspiration followed
by glottic closure, relaxation of the diaphragm, and muscle contraction against
a closed glottis. The resulting markedly positive intrathoracic pressure causes
narrowing of the trachea. Once the glottis opens, the large pressure differential
between the airways and the atmosphere coupled with tracheal narrowing produces
rapid flow rates through the trachea. The shearing forces that develop aid in
the elimination of mucus and foreign materials.
Etiology. Cough
can be initiated by a variety of irritant triggers either from an exogenous
source (smoke, dust, fumes, foreign bodies) or from an endogenous origin (upper
airway secretions, gastric contents). These stimuli may affect receptors in the
upper airway (especially the pharynx and larynx) or in the lower respiratory
tract, following access to the tracheobronchial tree by inhalation or by
aspiration. When cough is triggered by upper airway secretions (as with
postnasal drip) or gastric contents (as with gastroesophageal reflux), the
initiating factor may go unrecognized and the cough can be persistent.
Additionally, prolonged exposure to such irritants may initiate airway
inflammation, which can itself precipitate cough and sensitize the airway to
other irritants. Cough associated with gastroesophageal reflux is due only in part
to irritation of upper airway receptors or to aspiration of gastric contents,
as a vagally mediated reflex mechanism secondary to acid in the distal
esophagus may also contribute. Any disorder resulting in inflammation,
constriction, infiltration, or compression of airways can be associated with cough.
Inflammation commonly results from airway infections, ranging from viral or
bacterial bronchitis to bronchiectasis. In viral bronchitis, airway
inflammation sometimes persists long after resolution of the typical acute symptoms,
thereby producing a prolonged cough, lasting for weeks. Pertussis infection is
also a possible cause of persistent cough in adults; however, diagnosis is
generally made on clinical grounds. Asthma
is a common cause of cough. Although the clinical setting commonly suggests when
a cough is secondary to asthma, some patients present with cough in the absence
of wheezing or dyspnea, thus making the diagnosis more subtle (“cough variant
asthma”). A neoplasm infiltrating the airway wall, such as bronchogenic
carcinoma or a carcinoid tumor, is commonly associated with cough. Airway
infiltration with granulomas may also trigger a cough, as seen with
endobronchial sarcoidosis or tuberculosis. Compression of airways results from
extrinsic masses, including lymph nodes, mediastinal tumors, and aortic
aneurysms.
Examples of parenchymal
lung disease potentially producing cough include interstitial lung disease,
pneumonia, and lung abscess. Congestive heart failure may be associated with
cough, probably as a consequence of interstitial as well as peribronchial
edema. A nonproductive cough complicates the use of angiotensin-converting
enzyme (ACE) inhibitors in 5 to 20% of patients taking these agents. Onset is usually
within 1 week of starting the drug but can be delayed up to 6 months. Although
the mechanism is not known with certainty, it may relate to accumulation of
bradykinin or substance P, both of which are degraded by ACE.
The most common causes of
cough can be categorized according to the duration of the cough. Acute cough (<3
weeks) is most often due to upper respiratory infection (especially the common
cold, acute bacterial sinusitis, and pertussis), but more serious disorders,
such as pneumonia, pulmonary embolus, and congestive heart failure, can also present
in this fashion. Chronic cough (>3 weeks) in a smoker raises the
possibilities of chronic obstructive lung disease or bronchogenic carcinoma. In
a nonsmoker who has a normal chest radiograph and is not taking an ACE
inhibitor, the most common causes of chronic cough are postnasal drip, asthma,
and gastroesophageal reflux. Eosinophilic bronchitis in the absence of asthma
has also been recognized as a potential cause of chronic cough.
Complications. Common complications of coughing include chest and abdominal wall soreness,
urinary incontinence, and exhaustion. On occasion, paroxysms of coughing may
precipitate syncope (cough syncope), consequent to markedly positive
intrathoracic and alveolar pressures, diminished venous return, and decreased
cardiac output. Although cough fractures of the ribs may occur in otherwise
normal patients, their occurrence should at least raise the possibility of
pathologic fractures, which are seen with multiple myeloma, osteoporosis, and
osteolytic metastases.
Treatment. Definitive treatment of cough depends on determining the underlying cause
and then initiating specific therapy. Elimination of an exogenous inciting
agent (cigarette smoke, ACE inhibitors) or an endogenous trigger (postnasal
drip, gastroesophageal reflux) is usually effective when such a precipitant can
be identified. Other important management considerations are treatment of
specific respiratory tract infections, bronchodilators for potentially
reversible airflow obstruction, inhaled glucocorticoids for eosinophilic
bronchitis, chest physiotherapy and other methods to enhance clearance of
secretions in patients with bronchiectasis, and treatment of endobronchial
tumors or interstitial lung disease when such therapy is available and
appropriate. In patients with chronic, unexplained cough, an empirical approach
to treatment is often used for both diagnostic and therapeutic purposes, starting
with an antihistamine-decongestant combination or nasal ipratropium spray to
treat unrecognized postnasal drip. If ineffective, this may be followed
sequentially by treatment for asthma and for gastroesophageal reflux.
Symptomatic or
nonspecific therapy of cough should be considered when: (1) the cause of the
cough is not known or specific treatment is not possible, and (2) the cough
performs no useful function or causes marked discomfort. An irritative,
nonproductive cough may be suppressed by an antitussive agent, which increases
the latency or threshold of the cough center. Such agents include codeine (15
mg qid) or nonnarcotics such as dextromethorphan (15 mg qid). These drugs
provide symptomatic relief by interrupting prolonged, self-perpetuating paroxysms.
However, a cough productive of significant quantities of sputum should usually
not be suppressed, since retention of sputum in the tracheobronchial tree may
interfere with the distribution of ventilation, alveolar aeration, and the
ability of the lung to resist infection.
Other agents working by a
variety of mechanisms have also been used to control cough, but objective
information assessing their benefit is meager. For example, the inhaled anticholinergic
agent, ipratropium bromide (2 to 4 puffs qid), has been used with the rationale
of inhibiting the efferent limb of the cough reflex.
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