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Postop corneal edema linked to lower IOP measurement

Cornea. 2011;30(9):962-967.

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Corneal edema after phacoemulsification was associated with low Goldmann applanation tonometry IOP readings, in contrast to earlier data showing that corneal edema from contact lens wear under closed lids increased IOP readings, according to a study.

"The clinical implication of our main result is that [Goldmann applanation tonometry] IOP measurements in the postoperative period tend to be artificially low in persons with corneal edema, potentially leading to clinically significant undertreatment of vulnerable individuals," the study authors said.

The prospective study included 136 patients (mean age: 62.5 years) who underwent phacoemulsification and IOL implantation. The Ocular Response Analyzer (ORA, Reichert) was used to assess Goldmann applanation tonometry (GAT) IOP, corneal hysteresis and corneal resistance factor preoperatively and 1 day postoperatively. Central corneal thickness and radius of corneal curvature were also evaluated.

Study results showed that on postop day 1, mean central corneal thickness increased 10.3%, from 534 µm to 589 µm. The increase was statistically significant (P < .001). GAT IOP and corneal-corrected IOP increased significantly (P < .001 for both). Corneal hysteresis diminished appreciably (P < .001). Results showed an insignificant change in corneal resistance factor.

Higher corneal resistance factor (P < .001), lower corneal hysteresis (P < .001), and steeper radius of corneal curvature (P < .001) were associated with a higher GAT IOP, while greater corneal edema was associated with lower GAT IOP (P = .01).

"The reasons for these apparently contradictory results are not clear," the study authors said. "It is possible that the apparent differences in GAT IOP measurements on the edematous cornea, resulting from short-term contact lens wear versus phacoemulsification cataract surgery, may result from differences in the distribution of fluid in the cornea because of these different [biomechanical] mechanisms."