Dear Editor,

We thank Drs. Tsai et al. for their interest in our article and for the opportunity to clarify the data of our study. Drs. Tsai et al. correctly point out that intraocular pressures (IOP), axial lengths (AL), corneal curvatures, and refractive errors change overtime following glaucoma surgery. The purpose of our study was not to follow the changes of ocular dimensions after combined cataract operation and trabeculectomy with mitomycin C using separate incisions (combined operation) overtime, but to characterize these changes when the ocular dimensions were relatively stable.

The large majority (75%) of our patients who received combined operations had the ocular dimension measured and the IOP recorded for analysis 6 months of more after the combined operation. Eight patients (33%) had measurements obtained 12 months or more after the operation, 10 patients (42%) obtained between 6 to 11 months, and 6 patients (25%) obtained between 1 to 2 months. With the possibility that these 6 patients with a short follow-up may have unstable ocular dimensions and affect the interpretation of the results, we re-analyze our data of the 18 patients who had the data collected at 6 months or more after the combined operation. The significances of the results remain unchanged.

After a combined operation in these 18 patients, the mean axial length is significantly shortened from 24.13 (1.04) mm to 24.02 (1.06) mm (p<0.00001). The mean axial length reduction after combined operation (109 (61) µm) is still larger than the reduction after cataract operation alone (75 (38) µm), but does not remain statistical significant (p=0.07) with a smaller sample size. The axial length reduction after combined operation correlates significantly with the postoperative IOP (p=0.008). There is a mean with the rule (WTR) surgically induced corneal astigmatism of 0.42 (0.71) D by vector analysis, and a significant increase of mean keratometry reading of 0.24 (0.42) D after a combined operation. There is no significant difference between the expected and observed refractive errors.

In addition, all patients in this study received a standard regimen of topical steroid and antibiotic after the combined operation. Topical steroid was started with 4 times daily and tapered 1 drop per week for 4 weeks. Topical antibiotic was used 4 times daily for 1 week from postoperative day 1.

Previous studies on changes of axial length after trabeculectomy were based on ultrasound biometry, which requires physical contact with the eye by a transducer or a saline immersion bath. The use of a more precise non- contact method of measurement, avoiding indentation or deformation of a soft globe after trabeculectomy has clear advantage. Although induction of a mean WTR corneal astigmatism after trabeculectomy with or without using mitomycin C had been reported, studies on corneal astigmatism induction after combined operation with more recent surgical technique were few. It may not be directly applicable to compare results of ocular dimension changes in trabeculectomy alone or cataract operation alone to combined cataract operation and trabeculectomy.

In the study by Claridge et al., polar values as a balance between with the rule (WTR) and against the rule (ATR) components of any given astigmatism were calculated.[1] An increase in polar value indicates a shift towards WTR astigmatism whereas a decrease indicates a move towards ATR astigmatism. Authors described three patterns of postoperative corneal topography changes: superior corneal steepening, superior corneal flattening, and complex regional variations in corneal topography that did not fit any particular pattern. In the superior corneal steepening, the polar value of the simulated keratometry increased from preoperative value of 0.6 D to 1.0 D at 1 month and 3 months postoperatively, which was a 0.4 D WTR change. For the superior flattening group, the polar value increased from -0.3 D preoperatively to +0.1 D at 1 month and 0.0 D at 3 months postoperatively. The WTR change was 0.7 to 0.8 D.[1] The overall mean induced astigmatism reported by Kook and coauthors in patients who had trabeculectomy with mitomycin C application was larger at 1 month (+1.08 D x 90 degree) and 3 months (+1.23 D x 90 degree) after operation, and smaller at 6 months (+0.94 D x 90 degree) and 12 months (+0.65 D x 90 degree) after operation.[2] By re-analyzing our data excluding patients with less than 6 months of follow-up, we were able to characterize the changes when the ocular dimension became more stable after the operation. However, we agree with Drs Tsai at al that the intraoperative use of mitomycin C may affect wound healing and has long-lasting influences on AL and corneal curvature than operation without the use of antimetabolite, and future study with a long-term follow-up is warrant.

Sincerely Yours,

Simon K. Law, MD

Glaucoma Division
Ophthalmology Department
Jules Stein Eye Institute
University of California Los Angeles

References:

1. Claridge KG, Galbraith JK, Karmel V, Bates AK. The effect of trabeculectomy on refraction, keratometry and corneal topography. Eye. 1995;9 ( Pt 3):292-298.

2. Kook MS, Kim HB, Lee SU. Short-term effect of mitomycin-C augmented trabeculectomy on axial length and corneal astigmatism. J Cataract Refract Surg. 2001 Apr;27(4):518-523.

Dear Editor

Law and co-authors have contributed valuable data on the effect of combined cataract surgery and trabeculectomy with mitomycin C (MMC) on ocular dimensions. [1] While it is unclear from the report whether all patients' data were collected at the same interval postoperatively and when the postoperative data were collected. As we know, the change in intraocular pressure, axial length (AL), and corneal curvature after surgery could be different at varied follow-up period.

In a study by Claridge and coauthors, the induced with-the rule astigmatism after trabeculectomy was 2.63 diopters (D) at 1 month and decreased to 1.24 D by 3 months. [2] Additionally, intraoperative application of MMC in trabeculectomy may affect wound healing process by inhibition of fibroblast proliferation, and therefore has long-lasting influences on AL and corneal curvature. As reported previously by Kook and coauthors, [3] the mean AL was significantly less postoperatively and changed throughout the 12 month follow-up (-0.54mm at 1 week, -1.15mm at 1 month, and -0.9 mm at 12 months). The overall mean induced astigmatism was maximal at 3-month postoperatively (+1.23 D x 90°) and gradually decreased to +0.65 D x 90° at 12 months.

On the other hand, the use of topical steroid or non-steroid anti- inflammatory agents could affect on wound healing, and have been reported to have significant influences on ocular dimensions after operation. [4] It would also be necessary to clarify on this issue with the information of the postoperative medications. Further prospective study with a long- term follow-up period is warranted to help ascertain the change of ocular dimensions after combined cataract and glaucoma surgery with MMC is a temporary or long-term effect.

References

1. Law SK, Mansury AM, Vasudev D, et al. Effects of combined cataract surgery and trabeculectomy with mitomycin C on ocular dimensions. Br J Ophthalmol 2005;89:1021-5.

2. Claridge KG, Galbraith JK, Karmel V, et al. The effect of trabeculectomy on refraction, keratometry and corneal topography. Eye 1995;9:292¡V8.

3. Kook MS, Kim HB, Lee SU. Short-term effect of mitomycin-C augmented trabeculectomy on axial length and corneal astigmatism. J Cataract Refract Surg 2001;27:518¡V23.

4. Masket S. Comparison of the effect of topical corticosteroids and nonsteroidals on postoperative corneal astigmatism. J Cataract Refract Surg 1990;16:715-8.