Chronic cough or cough that lasts more than 8 weeks, once a chest x-ray and spirometry are confirmed normal, is caused by an alteration in a section of the route between peripheral receptors, mainly in the upper and lower airway and oesophagus, spinal cord and the cough centre in the brain stem involving the cortex. These mechanisms of cough have their homology in the circuit of chronic pain, and on that basis, should expand future research of chronic cough. Clinically chronic cough is easy to diagnose by an excessive response or hypertussia to low-intensity stimuli or banal stimuli, which we now call hypersensitivity cough syndrome, quantified by a positive reflex cough with capsaicin or citric acid. However, hypersensitivity cough syndrome can be impossible to quantify in the laboratory when the hyper-responsiveness originates in the central nervous system. This is normally caused by excessive peripheral input or convergence of stimuli from different sources. Once central hypersensitivity is acquired, peripheral input is not as important for activation of the cough.
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