Changes In Intraocular Structure And Outflow Facility Coefficient In Response To Increased Intraocular Pressure

PURPOSE.Intraocular pressure(IOP)is determined by the dynamic balance between the production and discharge of aqueous humor.The difficulty of aqueous humor passes through the traditional outflow channels is usually expressed by the outflow facility coefficient(C).The purpose of this study was to explore the relationship between intraocular pressure,the structure of the traditional outflow channel,and the outflow facility coefficient.METHODS.A total of 27 healthy volunteers were included in this study.A noninvasive oculo-pression tonometry(OPT)was used to press the eyeballs along the center of the eye from the inferior-temporal eyelid of the subject to maintain a constant pressure(0.15 Newton),quickly increase the intraocular pressure from the basic value to about 50 mm Hg,keep the pressure level unchanged,and record the value of the intraocular pressure(once every 10 seconds for a total of 4 minutes).After the subject rested for 30 minutes,the above-mentioned pressing process was repeated,and the EDI OCT images of the anterior segment of the nasal side of the test eye was collected before the pressing,immediately after the pressing,and 1min,2min,3min,and 4min after the pressing.The meridional diameter,coronal diameter,and cross-sectional area of Schlemm’s canal(SCAR)and trabecular meshwork thickness at various time points were measured with Image J software.Draw the curve of intraocular pressure over time,calculate the outflow facility coefficient,use SPSS software to perform One-way repeatedmeasures analysis of variance,and linear regression analysis to compare the difference between the outflow facility coefficient,the thickness of the trabecular meshwork and the cross-sectional area of Schlemm’s canal at different timeperiods,and the correlation between intraocular pressure,the morphological parameters of traditional outflow channels and the outflow facility coefficient was compared.Then explore the relationship between intraocular pressure,the outflow facility coefficient,and the structural changes of traditional outflow channels.RESULTS.Excluding two volunteers with unclear images,the data of the last 25 volunteers were included and analyzed,including 14 females and 11 males,with an average age of 25.04 ± 1.27 years,and the average axial length of the left eyeball was 24.24 ± 0.80 mm.The average basal intraocular pressure was 16.8 ± 2.5 mm Hg.After pressing the eyeball,the intraocular pressure increased obviously and reached a peak,with an average of 38.6 ± 11.1 mm Hg.At this time,the average value of the outflow facility coefficient was 0.020 ± 0.017 ul/min/mm Hg.When the intraocular pressure was 40 and 20 mm Hg,the average values of the outflow facility coefficient were 0.018 ± 0.0071 and 0.058 ± 0.0146 ul/min/ mm Hg,respectively.Both the basal IOP value and the changes in IOP at 0-4min were significantly negatively related to the outflow facility coefficient(R2 = 0.945).With the increase of intraocular pressure,the cross-sectional area of Schlemm ’s canal decreased significantly(5440.0 ± 3140.82 vs.3947.6 ± 2246.88 um2,p <0.05),and the average area decreased by 27%.The cross-sectional area of the Schlemm’s canal had returned to the basic state 4 minutes later,and there was no significant difference between them(5375.7 ± 2662.71 vs.5440.0 ± 3140.82 um2,p> 0.05).Both the basic SCAR value and the changes of SCAR value at 0-4min were significantly correlated with intraocular pressure(R2= 0.9944,P <0.001).There was no significant difference in trabecular meshwork thickness before and after intraocular pressure elevation(134.7 ± 32.80 vs.132.5 ± 39.19 um,p> 0.05).CONCLUSIONS.Using a non-invasive oculo-pression tonometry,the morphological changes of the Schlemm’s canal that depend on the increase in intraocular pressure can significantly affect the resistance of aqueous outflow.