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?