March 2020


Presentation Spotlight
How anti-glaucoma procedures affect corneal biomechanics

by Stefanie Petrou Binder, MD Contributing Writer

The EX-PRESS device inserted in the eye
Source: Aristeidis Konstantinidis, MD


A study investigated the influence of two common anti-glaucoma procedures—the EX-PRESS Glaucoma Filtration Device (Alcon) and trabeculectomy—on corneal biochemical properties, finding that both procedures significantly affected corneal hysteresis (CH) and corneal resistance factor (CRF).
“None of the indices returned to the preoperative values 1 year after surgery,” Aristeidis Konstantinidis, MD, the study’s first author, said in a presentation about the study.

What happens to CH and CRF?

The prospective study included 30 patients and took place from July 2013 to May 2016. Eighteen patients had an EX-PRESS device inserted (group 1) and 12 patients underwent trabeculectomy (group 2).
Group 1 included eight individuals with primary open angle glaucoma (POAG) and 10 patients with pseudoexfoliation glaucoma (PXG), ranging in age from 16–81 years (mean 62.4 years). The mean preoperative IOP in this group was 29.4±7.39 mm Hg, and the mean number of drops the patients required was 2.2±0.7. Before surgery, the CH was 7.31±1.13 and the CRF was 10.6±2.76.
Group 2 included five patients with POAG and seven with PXG who ranged in age from 60–78 years of age (mean 67.2 years). The mean preoperative IOP was 33.2±8.61 mm Hg and mean drops was 2.3±0.6. The CH was 7.82±2.55 and CRF was 11.11±1.99 preop.
The patients in both treatment groups were examined between 1–3 weeks preoperatively and at 1 month, 6 months, and 12 months postoperatively. All patients had a thorough ophthalmic examination at all time points and corneal biochemical properties were measured by Ocular Response Analyzer tonometer (Reichert Technologies). Measurements with a waveform score in excess of 3.5 were excluded.
CH was significantly increased at all postoperative time points compared to the baseline values, in both the EX-PRESS and trabeculectomy groups. CRF was significantly decreased at all time points postoperatively, compared to the preoperative values.
“We also compared the difference in preoperative versus postoperative values at all time points of CH between the two groups (Welch test), and we did not detect any significant differences between surgical techniques,” Dr. Konstantinidis said. “Differences in CRF between the two techniques from preoperative to postoperative values, at all time points, using the unpaired t-test, were not found to be significant.”
The study revealed that the IOP was significantly reduced at all postoperative time points for both procedures. The difference in the hypotensive effect was similar for both procedures.

Why CH is an important parameter

Corneal hysteresis is a reliable predictor of glaucoma progression. According to one review, CH was seen to be associated with optic nerve and visual field damage in glaucoma and the risk of structural and functional glaucoma progression.1 Low hysteresis was linked with more of an IOP reduction after various glaucoma therapies. Although widely accepted that central corneal thickness is a predictive factor for the risk of glaucoma progression, the review maintained that CH was more strongly associated with glaucoma presence, risk of progression, and effectiveness of glaucoma treatments than central corneal thickness.
Another study demonstrated that CH was significantly lower in chronic primary angle closure glaucoma patients and that partial recovery occurred after successful IOP-lowering therapy.2 In yet another study, it was revealed that despite a marked IOP reduction and possible weakening of ocular walls after trabeculectomy, the corneal structural tissue properties were not altered, and therefore the accuracy of IOP measurements was not changed postoperatively.3 This study concluded that it did seem likely, however, that Goldman-correlated IOP measurements were underestimated in glaucoma patients before and after surgery.
Dr. Konstantinidis concluded that the most likely reason for the increase in CH postoperatively, as observed in both of the procedures, was the reduction in IOP. The explanation for the decrease in CRF values postoperatively may relate to the fact that the cornea becomes less elastic after surgery due to the effect of remodeling of the ocular tissues in response to ocular trauma. Other possible explanations include the influence of the surgery and/or the presence of a foreign body (EX-PRESS) in the vicinity of the cornea.

About the doctor

Aristeidis Konstantinidis, MD

Department of Ophthalmology
General University Hospital of Alexandroupolis
Alexandroupolis, Greece


1. Deol M, et al. Corneal hysteresis and its relevance to glaucoma. Curr Opin Ophthalmol. 2015;26:96–102.
2. Sun L, et al. Recovery of corneal hysteresis after reduction of intraocular pressure in chronic primary angle-closure glaucoma. Am J Ophthalmol. 2009;147:1061–6.
3. Pillunat KR, et al. Corneal biochemical changes after trabeculectomy and the impact on intraocular pressure measurement. J Glaucoma. 2017;26:278–282.

Relevant disclosures

: None



How anti-glaucoma procedures affect corneal biomechanics How anti-glaucoma procedures affect corneal biomechanics
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