Cough reflex threshold in diabetes mellitus with and without autonomic neuropathy.

The cough reflex, a vagus-mediated respiratory protective reflex, was investigated in diabetics and healthy controls by establishing a cough reflex threshold to increasing concentrations of citric acid aerosol. Diabetics with autonomic neuropathy (n = 20), whether overtly symptomatic (n = 5) or evident only on certain non-invasive cardiovascular autonomic function tests, were found to have significantly (p < 0.05 by chi 2 test) raised cough reflex threshold (range 1 to > 100%, median value 50%) in contrast to either the non-neuropathic diabetics (n = 20) or normal controls (n = 20; range 1-10%, median value 2%). This difference suggests vagal (parasympathetic) denervation of the respiratory tract, with possible serious consequences, hitherto little appreciated.