Cholinergic neurotransmission and olfactory function in obstructive sleep apnea syndrome: a TMS study

Odor identification and discrimination are reduced in subjects with obstructive sleep apnea syndrome (OSAS), but the pathophysiology of the olfactory dysfunction in OSAS is still poorly understood. Experimental evidence suggests that olfactory impairment could be related to central cholinergic dysfunction. Short latency afferent inhibition (SAI) is a paired-pulse transcranial magnetic stimulation (TMS) protocol that gives the opportunity to test an inhibitory cholinergic circuit in the human cerebral motor cortex. The objective of the study was to assess the cholinergic function, as measured by SAI, in OSAS patients with olfactory impairment.

We applied SAI technique in 20 patients with OSAS and in 20 healthy control subjects; SAI values were correlated with the Sniffin' Sticks olfactory test results.

SAI was reduced in OSAS patients when compared with control subjects. We also found a strong negative correlation between olfactory parameters and SAI.

These findings suggest that cholinergic dysfunction is a robust determinant of hyposmia also in OSAS patients. Reduced SAI values and presence of olfactory impairment might indicate an increased risk of cognitive decline in patients with OSAS.