Can Monocyte to HDL Ratio be a new Marker for Severity of Obstructive Sleep Apnea Syndrome?

Background: This study aimed to investigate the relation of serum monocyte to serum HDL cholesterol ratio (MHR) with obstructive sleep apnea syndrome (OSAS). A total of 62 patients with an apnea hypopnea index (AHI > 5) and excessive daytime sleepiness were included in this study as OSAS group. The individuals with (AHI) < 5/h were included in the study as controls. OSAS patients were compared with the control group for high density lipoprotein (HDL) levels, serum monocyte count, and monocyte to HDL ratio (MHR). Mild, moderate and severe OSAS subgroups were compared for the same parameters. Results: MHR was 11.93 ± 6.52 in the control group while it was 14.87 ± 6.98 in OSAS group, with a statistically significant difference in between them (p = 0.016). The MHR were positively correlated with AHI and the minimum oxygen saturation (p = 0.003 and p = 0.012, respectively). Conclusion: serum monocyte to serum HDL cholesterol ratio increased as OSAS severity increased. MHR is an easy and available biomarker. It can be used as a new marker for severity of OSAS.


Introduction
There is a link between Obstructive Sleep apnea syndrome (OSAS) and cardiovascular disease (CVD). Monocytes secrete prooxidant and pro-inflammatory cytokines at the site of inflammation, so, they are considered as important cells in the inflammatory process. High-density lipoprotein cholesterol (HDL-C) protects the endothelial cells against the unwanted effects of low-density lipoprotein (LDL) and to inhibit oxidation of LDL molecules [1].
Recently, monocyte to HDL Ratio [MHR] has emerged as a new cardiovascular prognostic marker. However, the association between MHR and patients with OSAS is still under investigation [2][3][4][5].
The aim of this study was to investigate correlation of MHR, with severity of OSAS.

Material and Method
A prospective study was conducted between May 2017 and July 2019. A total of 62 patients who underwent overnight polysomnography (PSG) due to complaints of excessive daytime sleepiness, snoring and witnessed apnea were included in the study. The patients with an AHI < 5 was considered as the control group while the ones with an AHI > 5 with OSA symptoms were considered as the OSAS group. Eighteen patients were a control group.
Exclusion criteria included patients with cardiovascular diseases (CVDs), hypertensive patients, chronic liver disease, chronic lung disease, malignancies, acute infection, and patients on medications that might affect the lipid profile (e.g. adrenergic blockers, steroids).
All subjects underwent a full-night Polysomnographic (PSG) study (sleep screen recorder viasys company Germany). Briefly, electroencephalogram, electrooculogram and electromyogram, thoracic and abdominal respiratory excursions, breath sounds, nasal pressure, electrocardiogram and oxygen saturation were registered Respiratory events were scored manually.
The following morning after polysomnography, fasting venous blood samples were collected after a fasting period of 12 h, and analyzed within 60 minutes after collection for LDL, HDL levels, triglyceride, cholesterol; and leukocyte, neutrophil, lymphocyte, monocyte counts. MHR was calculated by dividing the monocyte count to HDL level. Furthermore, mild, moderate, and severe OSAS subgroups were compared with the control group for these parameters.
Monocyte, HDL and Monocyte/ HDL were separately compared with minimum oxygen saturation, AHI, the time of sleep under 90 % oxygen saturation in OSAS patients.
Written consents were obtained from the patients and healthy controls and the protocol of this study was approved by the Institutional Research Board of our faculty.

Statistical Analysis
Statistical analysis was carried out using SPSS program, version 10.0. Variables with normal distribution were compared with the Student's t-test and for variables not showing normal distribution, the Mann-Whitney U test was used. Kruskal-Wallis test was used to compare the differences between the groups. Post-hoc analysis was done as pairwise comparisons. Spearman's correlation was used for relationship between AHI and MHR. Receiver operating characteristic (ROC) curve analysis was applied to determine the cutoff value for MHR use in OSAS diagnosis. A two-tailed p value of p<0.05 was considered as statistically significant.

Results
The PSG results revealed that 18 individuals did not have OSAS (control group), and 62 patients had OSAS (OSAS group). The mean age of control group was 42.4 years while it was 44.67 years in the OSAS group. The control group consisted of 12 males (66.7 %) and 6 females (33.3 %). OSAS group consisted of 40 (64.5 %) males, and 22 females (35.5 %) with no statistical difference (table1).
Volume 2(2): 2-4 The mean HDL levels were 40.12 mg/dL in OSAS group, while it was and 47.25 mg/ dL in the control group. There was a significant difference between both groups (p = 0.039) table2. The triglyceride levels were significantly different between control and OSAS groups (p<0.001) table2.
OSAS and control groups showed significant differences for mean lymphocyte (p = 0.024), while, there was no statistically significant difference as regard the mean neutrophil counts (p = 0.072) table2.
The mean monocyte count was 567.04 ± 169.16 10 3 /L in OSAS group, and 554.03 ± 156.00 10 3 /L in the control group, without any significant difference in between (p = 0.138).
MHR was 11.93 ± 6.52 in the control group while it was 14.87 ± 6.98 in OSAS group, with a statistically significant difference in between (p = 0.016). (table2) There were significant differences for monocyte counts between mild, moderate and severe OSAS subgroup (p < 0.001). There were significant differences for Monocyte/HDL between mild, moderate and severe OSAS subgroup (p < 0.001) (table 3). The MHR were positively correlated with AHI and the minimum oxygen saturation (p = 0.003 and p = 0.012, respectively).
High AHI and minimum oxygen saturation were significantly correlated with low HDL (p = 0.001 and p = 0.002, respectively) (table 4).

Discussion
Repetitive episodes of nocturnal apnea in OSAS patients result in sympathetic system activation and increase the oxidative stress, which also implicated in the pathophysiology of endothelial dysfunction [7].
It has been known that monocyte activation plays an important role in chronic inflammation, in which monocytes and differentiated macrophages can modulate inflammatory cytokines and tissue remodeling. Monocyte become adherent to the surface of the endothelial cells and extravasate into the damaged tissue, thereby, induce the production of several cytokines-including interleukin (IL)-1, IL-6 (potent inflammatory cytokines) and tumor necrosis factor-a. Thereafter, monocytes differentiate into the macrophages that ingest the oxidized LDL cholesterol and form dangerous foamy cells [8][9][10].
Contrarily, HDL molecules have an anti-inflammatory effect. It inhibits extravasation of the monocytes. It counteracts the monocyte activation and promotes efflux of oxidized cholesterol from the macrophages. The reduced HDL levels promote monocyte activation, and increase the risk for inflammation [11].
The relationship between OSAS and MHR was first investigated by Atan et al. in 2017, and they concluded that MHR could be a predictive marker in OSAS [12]. In the currentt study, the value of MHR of the patients with OSAS was statistically highly significant than that of the control group. This coincides with the study of Tamaki et al. who demonstrated that monocyte counts were higher in patients with OSAS than in the control group [13]. In our study, monocyte count increases as the severity of OSAS increases. In a recent multicentre study by Inonu Koseoglu et al., it was determined that MHR, as a systemic inflammation marker, could predict cardiovascular disease in OSAS patients [14]. In the same study, a relationship was found between MHR and OSAS severity.
Our results coincides with the results of Atan et in 2017 who found also that MHR significantly increased as AHI increased, and minimum oxygen concentration decreased [12]. There was a positive correlation between MHR and the severity of OSAS.
Conclusion: MHR increased as OSAS severity increased. MHR is an easy and available biomarker and can be used as a new predictor for severity of OSAS.
Limitations of this study should be mentioned. First, few number of patients were included in this study. Second, we did not study MHR changes before and after CPAP treatment. Thus, a further prospective clinical trial with CPAP treatment is needed in the future.