A 47-year-old male with primary open-angle glaucoma underwent trabeculectomy with mitomycin C in his right eye in May 2006. His preoperative BCVAs were 20/25 OD (-5.00 D) and 20/20 OS (-3.50 D). An examination of the optic nerve in his right eye showed significant narrowing of the inferior neuroretinal rim with a corresponding loss of the retinal nerve fiber layer (RNFL; Figure 1A, B) and a corresponding superior arcuate scotoma (Figure 1C). Suture lysis was performed in the early postoperative period, and the patient developed hypotony with a shallow anterior chamber. The trabeculectomy was revised in July 2006 via the placement of additional sutures in the scleral flap. His IOP subsequently remained between 6 and 8 mm Hg OD.
Figure 1. An optic disc photograph taken of the patient's right eye demonstrated a marked loss of the RNFL in the inferotemporal region associated with thinning of the neuroretinal rim (left). A Stratus OCT (Carl Zeiss Meditec, Inc., Dublin, CA) Fast RNFL thickness analysis of the eye showed marked flattening of the RNFL's double-hump profile in the inferior sector (center). Standard automated perimetry detected a superior arcuate defect in the eye (right).
The patient developed a visually significant cataract in his right eye and underwent uncomplicated, temporal, clear corneal phacoemulsification with the placement of a posterior chamber IOL in January 2007. Three months postoperatively, there was marked astigmatism (+1.75 D -6.00 @ 180º), and he had a BCVA of 20/30. Given the high cylinder, the patient could not tolerate a spectacle prescription, and his UCVA was 20/200. The conjunctival bleb was diffuse and elevated, and there was no corneal overhang.
Comments on Management
SM: Given the patient's intolerance of spectacles, the options are limited. Although refractive surgery could correct the astigmatism, it might not be appropriate, because the degree of astigmatism after trabeculectomy might change over time and thus require additional corrective procedures. A contact lens could also correct the patient's cylinder, but it might compromise the integrity of the avascular bleb and, perhaps, increase the likelihood of an infection.
RNW: How much astigmatism was present before the cataract surgery? Might measures have been taken to correct it during cataract surgery?
SM: The astigmatism was manifest before cataract surgery but to a lesser degree.
RNW: Although a small, temporal, clear corneal incision can induce a small amount of with-the-rule astigmatism,1 it seems that the astigmatism actually worsened after the procedure. Performing limbal relaxing incisions or implanting a toric IOL might correct the cylinder. Due to the instability of astigmatism after trabeculectomy, however, the results of these techniques would be unpredictable.
What are the hypotheses to explain the astigmatism induced by trabeculectomy?
SM: Changes in axial length and induced with-the-rule astigmatism occur after trabeculectomy with or without mitomycin C, but they tend to regress to preoperative values within 12 months of surgery.2,3 Induced astigmatism has been attributed to tight sutures.4,5 Excessive cautery is another possible factor.6 It has also been suggested that the internal sclerostomy causes the corneal edge of the trabeculectomy to sink,7 thus inducing with-the-rule astigmatism. These factors may act alone or together with the healing process in promoting changes in the vertical meridian of the cornea.8
FAM: What was the corneal thickness of this eye? Are eyes with thin corneas more likely to develop astigmatism following trabeculectomy?
SM: The corneal thicknesses were 500 µm OD and 483 µm OS. It is therefore possible that a thin cornea in this case contributed to the increase in cylinder following glaucoma surgery. No studies investigating this hypothesis, however, have yet been reported.
GV: Does the amount of induced astigmatism depend on the glaucoma filtering procedure performed and the technique used?
SM: Small-flap trabeculectomy may produce lesser changes in corneal curvature that resolve sooner than large-flap techniques.9 Also, nonpenetrating trabecular surgeries induce less astigmatism than trabeculectomy.10
The patient was referred for the fitting of an RGP contact lens. He returned with a small-diameter (9.2 mm) lens yielding a visual acuity of 20/20 OD and a soft contact lens producing a visual acuity of 20/20 OS. A slit-lamp examination of the patient's right eye showed the contact lens sliding over the superior limbus and anterior aspect of the bleb with blinking and demonstrated its settlement over the central cornea between blinks without contact with the bleb.
Comments on Contact Lenses
FAM: A retrospective case-controlled study of the risk factors for blebitis and bleb-related endophthalmitis following trabeculectomy found a fourfold higher chance of infection in patients using contact lenses compared with those not wearing them.11 The small number of patients using a contact lens in the study, however, did not allow for a definite conclusion about the risk of bleb-related infection in contact lens wearers.
RNW: Would continuous treatment with a topical antibiotic be indicated for this particular patient?
SM: Prolonged antibiotic treatment may not be advisable due to the chances of inducing resistance over time. In fact, the prolonged use of these drugs is a demonstrated risk factor for bleb-related infections.11
CONCLUSION
Induced astigmatism is a relatively common finding after a glaucoma filtering procedure but may sometimes be overlooked. The astigmatism is usually with the rule. It tends to decrease over time and to regress significantly within 1 year of the surgical procedure. The therapeutic options for persistent astigmatism should take into account the presumed instability of the induced astigmatism over time in order to provide the patient with the best functional results. If a contact lens fitting becomes necessary, careful monitoring of the conjunctival bleb is advisable in order to detect the occurrence of erosions or bleb-related infections.
Section editors Felipe A. Medeiros, MD, PhD, and Robert N. Weinreb, MD, are glaucoma specialists at the Hamilton Glaucoma Center, University of California, San Diego. Dr. Medeiros is Assistant Professor, and Dr. Weinreb is Distinguished Professor of Ophthalmology and Director. They acknowledged no financial interest in the product or company mentioned herein. Dr. Medeiros may be reached at fmedeiros@eyecenter.ucsd.edu, and Dr. Weinreb may be reached at weinreb@eyecenter.ucsd.edu.
Sameh Mosaed, MD, is Assistant Professor for the Department of Ophthalmology at the University of California, Irvine. She acknowledged no financial interest in the product or company mentioned herein. Dr. Mosaed may be reached at smosaed@uci.edu.
Gianmarco Vizzeri, MD, is a fellow at the Hamilton Glaucoma Center, University of California, San Diego. He acknowledged no financial interest in the product or company mentioned herein. Dr. Vizzeri may be reached at mvizzeri@glaucoma.ucsd.edu.
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