The Relationship between CPAP Usage and Corneal Thickness

Citation: Gelir E, Budak MT, Ardc S ( The Relationship between CPAP Usage and Corneal Thickness Ethem Gelir 0 Murat Timur Budak 0 Sadik Ardc 0 Karen L. Gamble, University of Alabama at Birmingham, United States of America 0 1 Physiology Department, Hacettepe University Medical School , Ankara, Turkey, 2 Sleep Laboratory , Pulmonary Medicine Department, SGK Ankara Education Hospital , Ankara , Turkey The purpose of this study was to determine whether there is a correlation between CPAP usage and corneal thickness in patients with sleep disordered breathing. Full-night polysomnography (PSG) recordings were collected. Ten patients had undergone PSG recordings with continuous positive airway pressure (CPAP), and seven patients had undergone PSG recordings without CPAP. We measured corneal thickness by ultrasonic pachymeter before sleep and ten minutes after waking. We also measured visual acuity with a routine ophthalmologic eye chart before and after sleep. We asked patients to fill out a post-sleep questionnaire to get their subjective opinions. In the without-CPAP group, corneal thickness increased significantly during sleep in both eyes (left, p = 0.0025; right, p,0.0001). In the with-CPAP group, corneal thickness did not increase significantly (p.0.05 for both left and right cornea). There was no significant difference in visual acuity tests (p.0.05 for both left and right eye) between the two groups. According to our results, there is a significant increase in corneal thickness in the without-CPAP group. Our data show that a low percentage of Rapid Eye Movement (REM) sleep may cause an increase in corneal thickness, which can indicate poor corneal oxygenation. In fact, many sleepdisordered breathing (SDB) patients have low REM. Since a contact lens may cause low corneal oxygenation, SDB patients with contact lenses should be monitored carefully for their corneal thickness. - SDB is the most common sleep disorder associated with excessive sleepiness. SDB is characterized by episodes of sleep apnea (cessation of breathing over 10 s or more) or hypopnea (significant reduction of breathing), oxygen desaturations, and frequent arousals [1]. The most common form of SDB is obstructive sleep apnea (OSA) and is associated with airway collapse as the cause of breathing cessation or reduction. The standard, first-line treatment for OSA is continuous positive airway pressure (CPAP) [2]. CPAP is fan-generated air pressure delivered via a nasal mask and titrated to offset negative intrathoracic pressures produced during inhalation. As such, CPAP acts as a pneumatic splint to maintain airway patency. It has been shown to be very effective in most patients leading to improved daytime alertness, cognitive function, and quality of life [35]. Since the discovery of rapid eye movement (REM) sleep in 1953, it has been established that REM sleep is homeostatically regulated. Selective REM sleep deprivation produces compensatory increases in REM on subsequent sleep opportunities [6]. This phenomenon is commonly called REM rebound. REM rebound occurs regardless of whether the original REM suppression was instrumental [7] (i.e., waking subjects up when they entered REM sleep), pharmacologic [8] (e.g., amitryptaline or fluoxetine), or disease related [9] (e.g., sleep-related breathing disorders). Subsequently, researchers have found that in adults, REM occupies 2025% of total sleep time and many physiologic changes are associated with REM sleep, including atonia [10], poikilothermia [11], nocturnal penile tumescence [12,13], middle ear muscle activity [14], and increased cerebral blood flow [15]. Recently, corneal thickening was added to the list of physiologic properties affected by REM sleep. The hypothesis was advanced by Maurice (1998) who proposed that eye movements in REM sleep help corneal oxygenation. According to Maurice, thermal circulation of the aqueous humor is needed for adequate corneal respiration. This circulation is suppressed when the lids are closed, and REM is required to stir the anterior chamber and thus prevent corneal anoxia during sleep [16]. Corneal thickness measurements give valuable information about the physiological status of the cornea [1719]. Healthy human corneal thickness is around 500 microns [20]. However the thickness can change under some circumstances, such as hypoxia and hypercapnia. It has also been shown that corneal thickness significantly affects intraocular pressure measurement, and may itself be a risk factor for developing glaucoma [21,22]. Early studies showed that the normal human cornea would swell by 7% every hour in an oxygen-free environment [23]. Diurnal variation of central corneal thickness (CCT) has also been described, with swelling overnight; this swelling resolved by early afternoon, suggesting it was caused by the lid closure creating hypoxia [24]. Corneal swelling caused by hypoxia is a well-known phenomenon, especially in relation to contact lens wear [25]. Long-term use of contact lenses was shown to alter the following conditions in the cornea: epithelial oxygen uptake, epithelial thickness, stromal thickness, and corneal endothelial morphology [26,27]. OSA causes decreased REM sleep percentage and CPAP usage can usually reverse this decrease. If Maurices hypothesis is right, then decreased REM percentage should jeopardize corneal oxygenation. So far, no study has elucidated the relationship between the CPAP usage and the corneal thickness. Thus, the purpose of this study was to determine whether a correlation exists between CPAP use and corneal thickness in patients with OSA. Materials and Methods Ethics statement We obtained Institutional Review Board approval from Baylor College of Medicine (Houston,TX) for the procedures of the study. Our study has been carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans. Subjects In this study, patients underwent standardized sleep center clinical procedures. Men and women, admitted to the Sleep Laboratory at Baylor College of Medicine, Michael E. DeBakey Veterans Affairs Medical Center, for overnight polysomnography, were eligible for this study. Subjects who met the following inclusion criteria were selected for this study: patients must be diagnosed with OSA, be 2165 years of age (inclusive), and provide written informed consent. We excluded patients with eye (e.g., glaucoma) or neurological (e.g., periodic leg movement) diseases. Twenty patients participated in the study, subjects were randomly assigned to one of two groups (10 subjects in each group). Patients who had used CPAP were called the withCPAP group, and patients who had not used CPAP were called the without-CPAP group. However three subjects in the withCPAP group were excluded because they did not tolerate the CPAP treatment. All subjects (1 woman and 16 men) were CPAP nave. The with-CPAP group subjects underwent full-nigh (...truncated)