Reduced plastic brain responses to repetitive transcranial magnetic stimulation in severe obstructive sleep apnea syndrome

Abstract

Abnormalities in cortical excitability have been proposed to underlie the pathophysiology of various neurocognitive manifestations of obstructive sleep apnea syndrome (OSAS). Transcranial magnetic stimulation (TMS) provides a noninvasive method for study and modulation of cortical excitability in the human brain, and repetitive TMS (rTMS) has been proven useful for neurophysiologic investigation in various neurologic conditions. We aimed to investigate cortical excitability in patients with OSAS during wakefulness and to determine if rTMS would change the abnormal excitability patterns. Measures of motor cortical and corticospinal excitability (resting motor threshold [RMT], motor-evoked potential [MEP] amplitude, and cortical silent period [CSP]) were taken before and after a session of 10-Hz rTMS applied to the motor cortex in 13 individuals with untreated severe OSAS (apnea-hypopnea index [AHI]>30) and 12 age- and sex-matched healthy controls (HC). OSAS subjects had a significantly higher RMT (P<.003) and a longer CSP duration (P<.002) compared to HC. No difference was observed between MEP values of OSAS subjects and HC (P>.05). In response to rTMS, the HC group had a significant increase in CSP and MEP values from baseline, which were absent in OSAS subjects. Individuals with OSAS demonstrated increased motor cortex inhibition, which did not respond to 10-Hz rTMS. As rTMS-induced changes in MEP and CSP involve a separate neurotransmitter system (N-methyl-d-aspartate [NMDA] and gamma-aminobutyric acid [GABA], respectively), these findings suggest a widespread alteration in cortical neurophysiology in severe OSAS subjects that requires clarification with further exploration.