1. Variations in corneal biomechanical parameters and central corneal thickness during the menstrual cycle. Yakov Goldich, MD, David Zadok MD, Yaniv Barkana, MD, Isaac Avni, MD Department of Ophthalmology Assaf Harofeh Medical Center Israel The authors have no conflicts of interest and no financial interest in the article’s subject matter or methods mentioned.

2. Background The cornea of women may be influenced by hormonal changes that occur during the monthly menstrual cycle. Cyclic variations in corneal topography and corneal thickness have been previously described. 1 Corneal changes are probably driven by direct interaction of sex hormones with sex hormone receptors located in the human cornea. 2 1.Kiely, P. M., L. G. Carney, et al. (1983). "Menstrual cycle variations of corneal topography and thickness." Am J Optom Physiol Opt 60(10): 822-9. 2.Gupta, P. D., K. Johar, Sr., et al. (2005). "Sex hormone receptors in the human eye." Surv Ophthalmol 50(3): 274-84.

3. Background The Ocular Response Analyzer (ORA; Reichert Inc, Buffalo, NY, USA) can be used to assess in- vivo corneal biomechanical properties, presented by two parameters, corneal hysteresis (CH) and corneal resistance factor (CRF). These biomechanical parameters were proposed to be used in addition to corneal thickness and topography for screening for corneal ectasia before performing laser refractive procedures. 3 3. Randleman, J. B. (2006). "Post-laser in-situ keratomileusis ectasia: current understanding and future directions." Curr Opin Ophthalmol 17(4): 406-12.

4. Purpose The aim of our study was to evaluate whether there is a clinically meaningful variation in corneal biomechanical properties as measured by ORA during the menstrual cycle.

5. Methods Twenty two young healthy women aged 19.5 ± 1.5 (mean ± SD) years were prospectively recruited from among the service staff of the Assaf Harofeh Medical center. Every participant was assessed at the beginning of the menstrual cycle, then during ovulation, and at the end of the cycle. The following parameters were assessed: Corneal biomechanical parameters: corneal hysteresis and corneal resistance factor. Non-contact IOP with ORA: Goldmann correlated intraocular pressure (IOPg) and corneal compensated IOP (IOPcc). Central corneal thickness (CCT) with US pachymetry

6. Menstrual cycle From Wikipedia, the free encyclopedia

7. Results Table. Study parameters and their mean change and statistical significance during menstrual cycle. Parameter Onset of cycle Ovulation End of cycle P value Mean ± SD Onset vs Ovulatio n Ovulatio n vs End Onset vs End CCT (µm) 535 ± 40542 ± 41543 ± 42<0.0010.515<0.001 CH (mmHg) 11.1 ± 1.410.1 ± 1.111.4 ± 1.8<0.001 0.076 CRF (mmHg) 10.6 ± 1.79.8 ± 1.610.5 ± 2.00.0010.0070.673 IOPcc (mmHg) 14.3 ± 2.715.3 ± 3.913.6 ± 2.80.1920.0650.063 IOPg (mmHg) 14.6 ± 3.214.9 ± 4.213.9 ± 3.10.5400.0970.140 CCT=central corneal thickness, CH=corneal hysteresis, CRF=corneal resistance factor, IOPcc=corneal-compensated intraocular pressure; IOPg=Goldmann-correlated intraocular pressure

8. Results: CH FIGURE 1. Box and whisker plots (smallest, median and largest values with interquartile range) showing corneal hysteresis at three different time points during menstrual cycle.

9. Results: CRF FIGURE 2. Box and whisker plots (smallest, median and largest values with interquartile range) showing corneal resistance factor at three different time points during menstrual cycle.

10. Results: CCT FIGURE 3. Box and whisker plots (smallest, median and largest values with interquartile range) showing central corneal thickness at three different time points during menstrual cycle.

11. Conclusions Corneal thickness and biomechanical parameters significantly vary during menstrual cycle. The central cornea is thinnest at the beginning and significantly thicker at ovulation and at the end of the cycle. CH and CRF are statistically significantly decreased at ovulation as difference between values at the beginning and the end compared with the other two time points, with no statistically significant of the cycle. Cyclical corneal alterations may need to be considered when planning corneal refractive surgery. Correspondence: Yakov Goldich MD, E-mail: doctor.goldich@gmail.com