The purpose of this thesis was to investigate the effect of corneal swelling on the accuracy of Goldmann tonometry estimates of intraocular pressure (IOP). In the first experiment, central corneal thickness (CCT, ultrasonic pachymetry), IOP (Goldmann tonometry) and corneal curvature (keratometry) was measured in one eye of 25 subjects every two hours for 24 hours, except for 8 hours overnight (no measurements taken), and for the first two hours after awakening (measurement frequency 20 minutes). CCT (+20.1??10.9 pm) and IOP (+3.1??2.4 mmHg) peaked on eye opening, and then decreased at a similar rate (r=0.967, p<0.001) for the next two hours. Corneal swelling may have influenced the accuracy of Goldmann IOP measurements during this time. In the second and third studies, the CCT, IOP and corneal curvature were measured in both eyes of two groups of 25 subjects before and after the induction of corneal swelling, resulting from two hours of monocular closed eye contact lens wear. The increase in IOP was correlated to the increase in CCT at a rate of 0.33 to 0.48 mmHg per 10 pm, which signified an overestimation error in Goldmann IOP measurement. However, the change in IOP could not be accounted for solely by the change in CCT. In the fourth study, CCT, IOP and corneal curvature were used in conjunction with the Orssengo-Pye algorithm to determine the range of Young's modulus in the normal population, which was 0.29??0.06 MPa. Physiological variations in Young's modulus had a similar effect on Goldmann tonometry to CCT. In the fifth study, the data collected for studies 2 and 3 was used to calculate the Young's modulus changes associated with corneal swelling, again with the assistance of the Orssengo-Pye algorithm. No systematic change in Young's modulus was recorded after contact lens wear, but the model suggested that corneal biomechanical changes were responsible for the remainder of the change in IOP. All experimental results were combined to develop a model to calculate the diurnal variation of Goldmann IOP errors. The likely error in IOP due to overnight corneal swelling was 0.6 to 1.4 mmHg, which may explain as much as 45% (1.4 mmHg) of the 3.1 mmHg diurnal variation of IOP. In summary, small amounts of corneal swelling were shown to have a clinically significant impact on the accuracy of Goldmann tonometry. This may interfere with the measurement of the diurnal variation of IOP, particularly if measurements are taken prior to the resolution of overnight corneal swelling.
| Identifer | oai:union.ndltd.org:ADTP/212630 |
| Date | January 2006 |
| Creators | Hamilton, Kirsten, School of Optometry & vVsion Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Optometry and vVsion Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Kirsten Hamilton, http://unsworks.unsw.edu.au/copyright |
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