Peripheral Neuropathy:
Since cough is a defensive reflex, it could be affected by diseases targeting peripheral nerves. Consequently, vagotomy or anaesthesia-induced vagus block abolishes cough (41). Hereditary Sensory Autonomic Neuropathies (HSAN) are rare hereditary peripheral neuropathies characterized by loss of large myelinated and unmyelinated fibers resulting in decreased pain sensation. Congenital insensitivity to pain with anhidrosis (CIPA) is HSAN type-IV; it occurs because of a mutation in the gene encoding neurotrophic tyrosine kinase receptor type I (42). Both pain and cough can be elicited experimentally by stimulation of nociceptive C fibers as well as by faster conducting Aδ fibers. Consequently, CIPA may affect both pain and cough (43).
Diabetic autonomic neuropathy is one of the most common complications of diabetes mellitus (DM). The vagus nerve is one of the first nerves to be DM damaged. Several studies showed a significant increase in cough threshold when the cough reflex was impaired. Ciljakova et al. found a strong negative correlation between cough reflex sensitivity and heart rate variability as an indicator of diabetic autonomic neuropathy (44). Downregulation of the cough reflex may begin very early in the pathogenesis of diabetes. Varechova et al. found decreased sensitivity of the cough reflex in children with type I DM with subclinical autonomic neuropathy. Testing these children for a decreased cough reflex may reflect the presence of autonomic dysfunction and its effects on breathing and general health (45). The sensitivity of the cough reflex could also decrease with age, during sleep, in the presence of cranial nerve conduction changes due to vitamin B12 and folate deficiency, and in the presence of inhibition of dopamine receptors by antipsychotic medications (34). Phrenic nerve paralysis or injury is associated with a decreased cough reflex (46).
How can cough help to diagnose Neurologic disorders?