A cough is a voluntary or involuntary explosive expiration. After
a deep inspiration, the glottis is closed and the expiratory muscles contract,
compressing the lung and raising intrapulmonary pressure above the
atmospheric pressure. The glottis then opens, and gas is expelled
at a rapid rate. The cough reflex is initiated by the stimulation
of subepithelial mechanoreceptors in the trachea, bronchi, and interstitium.
Cough receptors may be activated by dust, chemicals, inflammation, mucus,
airway distortion, or rapid changes in airway volume. A series of
coughs that is difficult to stop is called a paroxysm and
is common in pertussis, viral infections, cystic fibrosis, gastroesophageal
reflux, and asthma. During paroxysms of coughing, central venous
blood pressure rises, cerebral venous blood flow falls, and intracranial
pressure increases. This may produce signs of cerebral hypertension
such as headache, vomiting, conjunctival hemorrhage, or blurred
vision. When foreign bodies or excess mucus is present, coughing
is essential to eliminate the obstruction or facilitate mucociliary
clearance. The high intrathoracic pressures that are created during
active expiration, however, can collapse and obstruct the unstable
airways of young infants or patients with bronchiectasis. Parents
of children with chronic cough often seek medical attention as persistent
symptoms are often uncomfortable (or even harmful), and may be associated
with chest pain or poor sleep quality. Chronic cough may also result
in pulmonary air leaks or rib fracture. The differential diagnosis
of chronic cough includes chronic sinusitis, gastroesophageal reflux,
asthma, and other chronic lung diseases including cystic fibrosis
and bronchiectasis. A diagnostic approach to chronic cough is presented
in Table 505-2.