Olfactory function in patients with obstructive sleep apnea: a meta-analysis study

Purpose: This meta-analysis study was designed to analyze the olfactory function in obstructive sleep apnea patients (OSA). Methods: A comprehensive review of the English language literature regarding OSA patients and olfactory function/dysfunction was performed. The papers assessing olfactory dysfunction with Sniffin’ Sticks test were taken into consideration. Results: A total of 420 OSA patients were judged eligible for the study. The average TDI score was found to be 24.3 ± 5.6. The olfactory identification (OD), the olfactory discrimination (OD), and the olfactory threshold (OT) average values were calculated resulting 9.9 ± 2.1, 9.8 ± 1.5, and 5.3 ± 2, respectively. There were 161 healthy control subjects in this meta-analysis. The average TDI of the control group was 30.7 ± 6.0 showing a statistical difference with the group of OSA patients (p = 0.03). A linear correlation between Apnea–Hypopnea Index (AHI) increase and TDI decrease (R2 = 0.1, p = 0.05) was detected. Finally, the average values of TDI of 151 patients classified as mild–moderate OSA and 159 patients considered as severe OSA were calculated. The difference between these two groups resulted not statistically significant (p = 0.3). Conclusion: The comparison between OSA patients and healthy subjects using Sniffin’ Sticks test showed lower values of the various olfactory parameters. Although a linear correlation between AHI increase and olfactory dysfunction was observed, no statistical difference between mild–moderate and severe OSA patients in terms of the severity of olfactory dysfunction could be proved.