Cortical afferent inhibition reflects cognitive impairment in obstructive sleep apnea syndrome: a TMS study

Patients with obstructive sleep apnea syndrome (OSAS) show neurocognitive impairment, but the exact mechanisms that cause cognitive dysfunctions remain unknown. The cholinergic system is known to play a key role in all attentional processes and cognitive functions. A transcranial magnetic stimulation (TMS) protocol may give direct information about the function of some cholinergic circuits in the human brain; this technique relies on short latency afferent inhibition (SAI) of the motor cortex. The objective of this exploratory study was to test the hypothesis that impaired cognitive performances in OSAS patients are associated with a dysfunction of the cholinergic system, as assessed by SAI.

We applied SAI technique in a group of 13 patients with OSAS and compared the data with those from a group of 13 age-matched healthy subjects. All the patients underwent a sleep study, an extensive neuropsychological evaluation, and TMS examination.

Mean SAI was significantly reduced in our OSAS patients when compared with controls. The neuropsychological evaluation showed impairments in most cognitive areas in the OSAS patients. SAI values were strongly correlated with the neuropsychological test scores.

These findings suggest that the cognitive deficits in OSAS may be, at least in part, secondary to alterations in cholinergic neurotransmission, presumably caused by nocturnal hypoxemia. TMS studies may shed light on the pathophysiological mechanisms of the cognitive disturbances in OSAS patients.