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: