Variations in the central corneal thickness during the menst
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Refractive procedures have evolved over the years, from radial keratotomy to the modern-day laser-assisted surgeries. The corneal thickness is an important consideration for these surgeries. In women, fluctuating hormone levels due to menstruation, pregnancy, and menopause can influence the corneal thickness. The menstrual cycle and its effects have been a subject of discussion for many researchers. Varying women's hormone levels cause various cyclical changes in the reproductive system, the most significant of which are ovulation and endometrial changes. With time and research, it has been elucidated that these hormones have receptors beyond the reproductive system, influencing different tissues, organs, and biochemical processes.

Gonadal hormone receptors have been observed in human ocular tissues such as the cornea, iris, ciliary body, lens, conjunctiva, and lacrimal and Meibomian gland. Physiological changes in the hormone milieu, oral contraceptive use, or hormonal replacement therapy can influence the management of glaucoma and dry eyes and are important in contact lens users as well. Noninvasive methods to monitor these physiological events such as identifying the time of ovulation include basal body temperature monitoring and urinary luteinizing hormone (LH) levels. In a multicentric trial, Leiva et al. found that the urinary LH values of 25–30 IU/L was the best predictive values for ovulation within 24 h. Previous studies have shown that these hormones can increase the central corneal thickness (CCT) by 5.6% on days 15 and 16, increase in the intraocular pressure (IOP) during the menstrual phase, and influence tear film production and stability during the menstrual cycle. Along with corneal topography and tomography, the CCT or pachymetry is an important consideration for planning the ablation zone in refractive surgery. A literature review on PubMed did not reveal any studies in the Indian population on the variations in CCT during the menstrual cycle, which, therefore, is the aim of study.

A prospective observational clinical study at a tertiary care center between December 2015 and December 2018. One hundred and twenty sixty women between 18 and 45 years were included. The CCT was measured using an ultrasound pachymeter at three specific timelines of the menstrual cycle: at the beginning (1st to 3rd day), during ovulation time (14th to 16th day), and at the end of the cycle (28th to 33rd day). Phases of the cycle were confirmed by the urine luteinizing hormone level. Results: The mean CCT of both eyes was 541.76 ± 4.21 m, 559.21 ± 4.50 m, and 544.52 ± 8.06 m at the beginning, mid, and end of the cycle, respectively. The mean CCT of the right eye was 541.68 ± 4.15 m, 559.08 ± 4.50 m, and 544.44 ± 8.06 m and of the left eye was 541.84 ± 4.27m, 559.35 ± 4.50m, and 544.61 ± 8.06 m at the beginning, mid, and end of cycle, respectively.

Based on findings of the CCT being significantly higher during ovulation, menstrual history seems to be an important consideration during refractive surgery workup, for contact lens fitting and glaucoma evaluation in women. One limitation of our study is that the stages of the menstrual cycle and follicular maturation were not verified by an abdominal ultrasound since we relied on the history and urine LH levels. Since the LH levels during all the phases of the menstrual cycle were not measured, a correlation between CCT and LH levels was not possible. Studies with a larger number of subjects taking into consideration the above factors may further confirm findings.