Corneal Hysteresis and Intraocular Pressure Measurement Using the Reichert Ocular Response Analyzer

Yang Jiang

Objectives: To approach the accuracy and reliability of intraocular pressure (IOP) measured by Reichert Ocular Response Analyzer (ORA), to compare and analyze the relationship between central corneal thickness (CCT) and IOP value measured by ORA, Goldmann applanation tonometer (GAT) and Non Contact Tonometer (NCT).Then to study the clinical utility of the corneal hysteresis (CH) and corneal resistance factor (CRF) measured by ORA in normal , glaucoma patients and suspects, undergone kerato-refractive surgery subjects.

Method: Chose 84 healthy individuals（168eyes）for IOP measuring from November 2009 to March 2010 randomly. IOP was measured by ORA and NCT devices, then CCT was measured by A-ultrasound (A-US). Then compared the measuring values of these tonometries with each other and correlated them with CCT separately. Also, Chose 31 glaucoma and suspect（62eyes）for IOP measuring. IOP was measured by ORA and GAT devices, then CCT was measured by A-ultrasound (A-US). In addition, chose other 51 individuals (102 eyes) undergone kerato-refractive surgery to this study. IOP was measured by ORA and NCT devices, then CCT was measured by A-ultrasound (A-US). CH, CRF, IOPg and IOP corneal-compensated (IOPcc) were provided by the ORA device. Then use SPSS to analyze the data.

Results: The normal intraocular pressure values, ORA Goldmann IOP value measured analog (Goldmann-correlated IOP value, IOPg) was 17.90 ± 4.17mmHg, corneal compensated IOP (Corneal-Compensated Intraocular Pressure, IOPcc) was 18.34 ± 4.00mmHg ; NCT IOP value was 17.44 ± 3.54mmHg. IOPcc, IOPg, NCT was good correlation between intraocular pressure, the mean value between each were significantly different. CCT was 536.20 ± 29.17μm. NCT IOP measurements and CCT medium level of the correlation coefficient was 0.429; and IOPcc shows only low levels of weak correlation between CCT, the correlation coefficient was 0.258.

Significantly higher than normal male IOPg to female, IOPcc no significant difference, IOPcc, IOPg no correlation with age. Normal CH and CRF were 10.13 ± 2.11mmHg and 10.90 ± 2.31mmHg, no significant correlation with age and gender. CH and CCT has low correlation coefficient of 0.277; CRF and CCT has medium level correlation coefficient of 0.466.

Glaucoma and suspected glaucoma IOPg was 19.16 ± 4.10mmHg, IOPcc was 19.91 ± 4.44mmHg; GAT IOP value of 18.91 ± 3.75mmHg. Glaucoma and suspected glaucoma IOPg, IOPcc, GAT correlated good, the mean value of IOPcc significantly higher than IOPg and GAT IOP. There was no significant difference between GAT IOP values and IOPg . GAT IOP and CCT have medium level correlation coefficient of 0.344. In patients with glaucoma and suspected glaucoma, CH and CRF were 9.83 ± 2.13mmHg and 11.03 ± 2.11mmHg. Compared with the normal group, glaucoma and suspected glaucoma IOPg, IOPcc significantly higher than the normal group, and CH, CRF, CCT were no significant difference.

After corneal refractive surgery, LASEK patients were followed up by us. IOPg was 13.26 ± 3.52mmHg, IOPcc was 16.64 ± 3.29mmHg, NCT IOP value was 14.13 ± 3.61mmHg, CH was 8.01 ± 1.60mmHg, CRF was 7.70 ± 1.82mmHg, CCT was 447.59 ± 43.75μm; In LASIK patients, IOPg was 13.10 ± 5.29mmHg, IOPcc was 16.70 ± 5.25mmHg, NCT IOP value was 13.91 ± 4.99mmHg, CH was 7.83 ± 1.52mmHg, CRF was 7.50 ± 1.75mmHg, CCT was 447.96 ± 46.22μm. Compared with normal subjects, patients after corneal refractive surgery IOPg, IOPcc, NCT IOP, CCT, CH, CRF were lower, but IOPcc decreased no significantly than NCT IOP. Between LASEK surgery group and LASIK surgery group，the value of IOPg, IOPcc, NCT IOP, CCT, CH, CRF were no significant difference.

Conclusion: ORA IOP measurement has high repeatability and accuracy. It has good consistency with GAT, NCT and less impact of the measurement of central corneal thickness. ORA CH and CRF measurement parameters can reflect the biomechanical properties of the cornea. Normal CH, CRF values have no significant difference between men and women, no correlation to age, but low levels of correlation with CCT. In glaucoma and suspected glaucoma patients, CH, CRF values have no significant difference compared to normal controls. The CH and CRF values in patients afler corneal refractive surgery were significantly lower than the normal value. But there was no significant difference between LASEK surgery group and LASIK surgery group. ORA IOP measurements may be much closer to the real value than GAT, NCT value, especially after corneal refractive surgery.

Key words ocular response analyzer; intraocular pressure; corneal hysteresis; corneal resistance factor; glaucoma; kerato-refractive surgery.