June 2008

 

OPHTHALMOLOGY NEWS

 

False high pressure readings


by Rich Daly EyeWorld Contributing Editor

 

 

Patient eyes three days post-op after undergoing DSEK with peripheral scraping

Source: Mark A. Terry, M.D.

Study findings indicate that ophthalmologists should not disregard high IOP readings in DSEK eyes as inaccurate

The thicker corneas that result from DSEK do not necessarily provide inaccurately high intraocular pressure readings, according to recent research.

A first-time study of the effect of increased corneal thickness after Descemet’s stripping endothelial keratoplasty (DSEK) on intraocular pressure (IOP) measurement did not find false high readings, which are generally expected from thick corneas. The finding led the study authors to recommend that ophthalmologists who find high IOP readings with Goldmann applanation tonometry (GAT) in post-DSEK eyes not to disregard them.

“We believe that elevated IOP GAT readings following DSEK should raise the suspicion of a truly elevated IOP,” the authors wrote in the study.

The prospective cross-sectional study evaluated 50 eyes of 38 patients with successful DSEK at least three months prior to testing. The IOPs were measured with GAT, pneumatonometry, and dynamic contour tonometry (DCT) in an unmasked randomized sequence, and central corneal thickness (CCT) was measured by ultrasonic pachymetry.

The study, published in the March issue of the American Journal of Ophthalmology, found that the IOP measurements from pneumatonometry and DCT were significantly higher than GAT, while the difference between pneumatonometry and DCT readings were not statistically significant.

The study did not find significant correlations between IOP and corneal thickness among these patients.

Farnaz Memarzadeh, M.D., assistant professor of ophthalmology, Doheny Eye Center, Los Angeles, said she was surprised by the central finding of the study. “What I think this paper highlights is the importance of corneal biomechanics other than pachymetry and their effect on the readings that we obtain with GAT,” Dr. Memarzadeh said. “The importance of factors such as viscosity, rigidity and elasticity, hydration, and other factors—some of which we don’t fully understand—may indeed far outweigh the effect of central corneal thickness in what we are measuring with GAT.”

Any of those factors can be altered by any type of corneal incisional surgery, such as RK, LASIK, or DSEK.

Kenneth M. Goins, M.D., professor of ophthalmology, University of Iowa Hospitals, Iowa City, said he was not surprised that an artificially high GAT reading was not found in DSEK patients.

“The final IOP depends upon the diagnosis and the duration of corneal edema pre-operatively prior to DSAEK,” Dr. Goins wrote in comments to EyeWorld. “Patients with Fuchs’ Dystrophy and those with prolonged corneal edema may actually lose mechanical strength or have more elasticity, as seen with keratoconus.”

Dr. Memarzadeh cautioned that the study findings do not warrant concluding that DSEK corneas behave like normal or thin normal corneas because the sample size is too small to draw that conclusion and because eyes that were only three months post-op were included in the study.

It is quite possible for eyes that have undergone corneal endothelial transplant to have edema for up to about six months afterward, she said. Although the investigators excluded eyes with “clinical” evidence of edema, corneas may have some degree of swelling and edema that is not clinically evident by slit lamp examination. Any amount of edema may alter the biomechanical properties of the cornea. Dr. Goins agreed with the authors’ hypothesis that the partial-thickness graft did not influence the measurements biomechanically because the graft was only attached centrally and not at the limbus. The low dose use of steroids by most post-DSEK patients did not appear to significantly affect the findings, Dr. Goins said.

Although the findings were significant and warrant further study, he noted that the small size of the study limited any conclusions drawn from it to generalizations. One area in particular that would need more research was the finding of no significant correlation between IOP and corneal thickness. Among the further research that is needed, he said, is an examination of the change in IOP and incidence of glaucoma after DSAEK.

Dr. Goins pointed out that research from the deep lamellar endothelial keratoplasty (DLEK) database maintained by the University of Iowa Hospitals has shown that DLEK results in a “moderate risk” of developing ocular hypertension or glaucoma and worsening of pre-existing disease.

“The risk appears lower than that published for penetrating keratoplasty,” Dr. Goins said about previous research.

Editors’ note: Drs. Memarzadeh and Goins have no financial interests related to their comments.

Contact information
Goins: 319-356-2861, kenneth-goins@uiowa.edu
Memarzadeh: 323-442-6415, FMemarzadeh@doheny.org

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