A written informed consent was obtained from each subject. The study protocol was approved by the local Ethics Committee. The study was conducted in accordance with the principles of the Declaration of Helsinki.
Patients who were newly diagnosed with OSAS at the Sleep and Sleep Disorders Laboratory of the Neurology Clinic at Kocaeli Derince Education and Research Hospital between 01.2012 and 12.2014 were included in the study. Patients aged between 18 and 60 years were evaluated. Patients with neurological diseases, mental disorders, severe psychiatric diseases, previous cancer, and other sleep disorders including central sleep apnea syndrome, periodic limb movement disorder, narcolepsy, and restless legs syndrome were excluded from the study. Patients were questioned for hypertension (HT), diabetes mellitus (DM), heart disease, and chronic obstructive pulmonary disease (COPD). The WCST and Stroop tests were performed in all patients by an experienced psychologist and cranial MRIs were obtained. A control group consisted of 15 age- and education status-matched healthy individuals.
The level of education of the patients were divided in three levels, as education duration of five years, between 8 and 12 years, and ≥13 years, defined as low, intermediate, and high education status, respectively.
The control group also received PSG, WCST, and Stroop tests, and cranial MRI were performed. Body mass index (BMI) was also calculated in all patients and controls. A full night PSG was performed in all patients and individuals of the control group at the sleep laboratory. The PSG included electroencephalography, electrooculography, chin and leg electromyography, electrocardiography, snoring, thermistor, nasal pressure transducer, finger pulse oximeter, thoracic and abdominal respiratory movements, and body position. Scoring was performed according to the 2010 American Academy of Sleep Medicine criteria. Patients with an Apnea/Hypopnea Index (AHI) of equal to or more than 5/h were accepted to have OSAS. An AHI value equal to or greater than 5/h and <15/h was defined as mild; an AHI value equal to or greater than 15/h and less than 30/h was defined as moderate, and an AHI value more than 30/h was defined as severe OSAS.
Magnetic resonance imaging
Magnetic resonance imaging examinations of the patient and control groups were performed using the same MRI equipment (1,5 Tesla, Intera Master, Philips Medical Systems, USA) and a standard head spiral. In addition to the conventional MRI examination, A 2-mm section was obtained with inversion recovery (IR) placed at the coronal plane with the TSE method (TR/TE/TI: 2250/10/400 msn, NSA: 2, TSE factor 15). An IRTSE examination was used for volumetric measurements. All images were transferred to a CD and the measurements were performed on another computer by the same radiologist who was blind to the both patient and control groups.
The right and left sides were evaluated separately in the prefrontal cortex. Sections from the anterior frontal lobe to the anterior genu of the corpus callosum were evaluated. The gray matter area was calculated using mathematical subtraction following the measurement of the area of the total PFC (white matter and gray matter) and the area of white matter (Fig. 1). These areas were multiplied by the thickness of the cross section, and volumetric data (raw data) was, then, obtained (Rademacher et al. 1992).
Since the magnitude of the head is variable in each individual, corrected volumes of those structures were calculated according to the formula used in the study by Watson et al. (1997) and Insausti et al. (1998).
$$ {\text{Corrected}}\;{\text{volume}} = \left[ {\left( {{\text{Mean}}\;{\text{intracranial}}\;{\text{area}}\;{\text{of}}\;{\text{all}}\;{\text{cases/the}}\;{\text{intracranial}}\;{\text{area}}\;{\text{of}}\;{\text{the}}\;{\text{case}}} \right) \times {\text{raw}}\;{\text{data}}\;{\text{volume}}\;{\text{of}}\;{\text{the}}\;{\text{case}}} \right] $$
For normalization, the section in which anterior commissure was present at the coronal IR sequence in the MRI examination of the patient and control groups was defined and the intracranial area of this level was measured. Intracranial area of all cases was calculated and divided into the intracranial area of the case. The calculation of this value with the raw data volume which was measured manually yielded the calculation of separate corrected volume values in each patient (Fig. 1).
Neuropsychological examination
One of the main tests used in measuring the executive functions is the WCST (Karakaş and Karakaş 2000). Primarily, the perseveration and abstract scrutinizing; concept formation; defining property; attention; working memory; mental flexibility; problem solving; category creation; and changing categories are among the specifications which WCST is able to measure. The WCST, which is used as a frontal lobe test, is lateralized to the right hemisphere (Karakaş and Karakaş 2000) and has a distribution in the right frontal lobe including the dorsolateral PFC (Karakaş 2006). Normative data of the WCST were collected in the context of the Neuropsychological Test for Cognitive Potentials Battery. Two decks consisting of four stimulating cards and 64 test cards are included in the WCST test. The patient chooses a card from above the first deck and he/she should find the stimulating that card matches (matching is expected to be made according to the color, shape, or amount for two cycles) with the card he/she picks. While the instructions are given, no hints should be given to the patient about how and according to what the cards should be matched. Feedback should be given to the patient after each match on whether the matching is correct or not, and the patient should be expected to find the matching rule. The test is complete, when the patient completes six categories or when he/she uses all 128 cards. Scoring is performed according to a total of 13 categories, which are total number of answers, total number of erroneous answers, total number of correct answers, number of completed categories, perseverative responses, perseverative errors, non-perseverative errors, percentage of perseverative errors, number of trials used to find the first category, percentage of cognitive level response, points for failure to continue setup, point for learning how to learn, and points for failure to learn to learn (Karakaş 2006). In this study, the WCST-2 (total number of errors), WCST-4 (number of completed categories), WCST-6 (total number of perseverative errors), and WCST-10 (number of cognitive level responses) were included. When the patient insists on using previous principles and does not change the principle (perseveration), although the behavioral principle has been changed and he/she has received warning directions on this subject by the tester, it implies impairment in performance (Karakaş and Karakaş 2000).
In addition, we used the Stroop test, which is utilized to measure attention, in addition to executive functions. The Stroop test used in Turkey is prepared using the combination of original Stroop test and Victoria form of the Stroop test in the context of Neuropsychological Test for Cognitive Potentials Battery. Its reliability and validity tests were performed and normative data were collected. The form created as the Stroop test was called Basic Sciences Research Group (BSRG) Form (Karakaş 2006). The Stroop test measures the speed of data processing and attentive skills, mainly cognitive setup and responsive skills under destructive effects (Karakaş 2006).
There are four cards used in the Stroop test. In the first card, there are words of color names printed with black ink. The second card involves the same words of color names printed in different colors (e.g., the word ‘red’ printed with green ink), in the third card, there are colored circles and in the fourth card, there are neutral words (Turkish words; kadar, zayıf, ise, orta) printed in different colors.
During the test, the patient is asked to read the words of the color names printed in black in the first section (first card), to read the words of color names printed in different color, but not to tell the color of ink in the second section (second card), tell the names of the colors of colored circles in the third section (third card), tell the printed color of the word, but not to read the word (neutral Turkish words; kadar, zayıf, ise, orta) in the fourth section (fourth card). Finally, in the fifth section, the patient is asked to tell the printed the colors of the word, but not to read the word and tell red, but not green for the word ‘green’ printed in red-(second card again).
Impairment in performance is defined as lengthening of the duration of telling the colors with inability to resist the accustomed response (reading) or as telling the wrong color. Patients with a low resistance to interference and who are unable to cope with distractors read the article instead of telling the color. Spontaneous corrections and errors and time are recorded during scoring. Duration of reading the words printed in colored letters was subtracted from the duration of the section in which the patient told the colors, instead of reading the words. If this difference of duration is high and a high number of errors and spontaneous corrections is present, it indicates that the attention of the individual can be easily distracted and that the individual has difficulty in suppressing an inappropriate response tendency (Oktem 2006). The Stroop test performance is particularly associated with the left frontal lobe (Karakaş 2006). It is also quite sensitive to the frontal function disorders (Oktem 2006).
Furthermore, the patients were also evaluated using the ESS. It was used in the measurement of daily sleepiness condition of the patients (Karakoç et al. 2007; Izci et al. 2008). Patients with an ESS score >10 were considered to have excessive daytime sleepiness.
Statistical analysis
The NCSS (Number Cruncher Statistical System) 2007 (Kaysville, Utah, USA) software was used for statistical analysis. Descriptive data were expressed in mean, standard deviation, median, frequency, percentage, and minimum and maximum values. The Mann–Whitney U test was used to compare two groups without normal distribution, while the Kruskal–Wallis test was used to compare the quantitative variables of three groups. Qualitative variables were also compared using the Pearson’s Chi square and Fisher’s exact tests. The Spearman’s correlation analysis was used to analyze correlations between the variables. p values of <0.01 and <0.05 were considered statistically significant.
