OBJECTIVE: It is known that obstructive sleep apnea syndrome affects many systems due to hypoxemia and hypercarbia. We aimed to demonstrate with the utilization of well-standardized questionnaire tools and electrophysiological tests that cognitive impairment, depression, autonomic dysfunction, and metabolic syndrome may occur in association with obstructive sleep apnea syndrome.METHODS: The electrophysiological examination protocol of autonomic nervous system functions was performed with sympathetic skin response and R-R Interval. Patients were administered Epworth Sleepiness Scale, Pittsburgh Sleep Quality Index, Montreal Cognitive Assessment, and Hamilton Depression Rating Scale by physicians in face-to-face interviews.RESULTS: This study included 148 participants, consisting of 73 patients and 75 controls. There was a statistically significant difference between the patient group and control group with regard to sympathetic skin response, R-R Interval, post-hyperventilation R-R Interval, and R-R Interval variation (p<0.001). A statistically significant difference was observed between the patient group and control group in terms of median Epworth Sleepiness Scale, Pittsburgh Sleep Quality Index, and Montreal Cognitive Assessment scores. It was observed that the control group achieved significantly better scores than the patient group in delayed recall (p<0.001) and language (p<0.05) categories.CONCLUSION: Obstructive sleep apnea syndrome patients should be screened for diseases, especially in the cardiovascular system, that cause serious morbidity and impair functionality such as dementia and depression. We believe that many comorbid diseases encountered in obstructive sleep apnea syndrome patients can be prevented with early diagnosis and continuous positive airway pressure treatment.KEYWORDS: Sleep apnea, obstructive. Cognitive dysfunction. Autonomic nervous system diseases.