Keywords:
Cough reflex, Neurological disorders, Cerebral disorders, cerebellar disorder, Vagal Neuropathy, Parkinsonism.
Introduction:
Cough is a forced exhalation effort against a closed glottis, which suddenly opens, expelling secretions and foreign bodies from the respiratory tract, making a characteristic sound. Cough is one of the most common complaints that prompt patients to seek medical attention. It is one of the most important protective mechanisms of the airways, indicating the presence of a potential or actual airway lesion and helping to clear secretions and foreign bodies from the airways (1). There are 3 main types of cough: reflex cough (type I), voluntary cough (type II), and evoked cough (type III), which is preceded by a coughing stimulus. Both reflex cough and voluntary cough trigger a similar mechanism of motor cough behavior. Cough is a reflex mediated predominantly by control centers in the respiratory areas of the brainstem, modulated by the cerebral cortex. Cough production proceeds through three coordinated phases: Inspiratory, compressive, and expiratory phases. It begins with contraction of the inspiratory muscles (drawing air into the lungs), closure of the glottis (creating subglottic pressure), and abduction of the vocal folds with forced expiration (forcing the glottis to open) with expulsion of secretions. However, the cough reflex is under voluntary control of higher neurological centers such as the cerebral cortex, which plays an important role in both the initiation and inhibition of coughing (2). The reflex has afferent sensory nerve fibers (mainly branches of the vagus nerve) that conduct afferent impulses diffusely to the spinal cord to reach the upper brainstem and pons. Other parts of the brain are associated with the proper function of the cough center in the medulla, such as the pontine respiratory group, lateral tegmental field, and deep cerebellar nuclei, which play a role in the pattern of cough formation and regulation. The efferent fibers conduct signals from the cough center to the diaphragm, abdominal wall, and muscles via the vagus, phrenic, and spinal motor nerves (3). Since the cough reflex is a reflex, it can be affected or influenced by various neurological disorders [Table 1]. Both reflex and volitional cough can be tested in various neurological and otolaryngological disorders. Different methods can test the sensitivity and efficiency of the cough reflex. Sensitivity can be assessed by the concentration or duration at which the cough can be evoked when exposed to variable concentrations and/or durations of nebulized aerosols of a cough-inducing substance (such as citric acid, L-tartaric acid, or capsaicin). However, there is considerable variability in the methods used in performing the test (4,5,6). To assess the efficiency and strength of the cough, a group of Japanese scientists developed a device to measure cough strength while testing the cough reflex. They connected an electronic spirometer to an ultrasonic nebulizer through a special tube with a double lumen. The spirometer measures the peak cough flow of the provoked involuntary cough (7).
Neurological Conditions Associated with Increased Cough Reflex Sensitivity: