▪ I would expect the superotemporal bleb in the left eye to be well circumscribed, cystic, and quite elevated, creating an abrupt change in surface contour at the limbus. These blebs can cause a frank dellen or, at the very least, relative thinning of the adjacent peripheral cornea. As a rule, high-domed blebs create corneal astigmatism with the steep meridian oriented with the center of the highest portion of the bleb. Typically, MMC blebs can increase in height for a few years before they begin to flatten. Very high-domed blebs can break down and leak, particularly if they are avascular. Whether they leak or not, they evolve in shape over time. As they change shape and height, the corneal astigmatism also changes.
The other thing to remember about “bleb-associated corneal astigmatism” is that it is irregular and not easily corrected with spherocylindrical lenses. The resulting anisometropia is poorly tolerated and, even monocularly, the best corrected visual acuity is worse than that achieved with pinhole. These concerns can lead to a RGP contact lens trial, as in this case. Such efforts are often not rewarded if the cylinder is very steep, off center, or both. Generally, I try to avoid rigid contact lens wear in the presence of a trabeculectomy bleb because of the increased likelihood of injury to the bleb and risk for infection. Astigmatic corneal surgery may offer short-term improvement, but if the bleb continues to change shape and size, the problem will recur.
I think that the best option for improving the corneal surface irregularity is to deal with the cause. The bleb can be surgically modified to lower its profile. In my experience, this is best done by freeing the posterior margin of the bleb that is scarred to the sclera. Additional MMC can be applied intraoperatively to the bare sclera posteriorly to discourage reformation of the posterior bleb margin scar and encourage posterior filtration. The bleb should collapse significantly if the area of filtration is extended. The cornea will remodel to result in a shallower corneal curvature in this meridian. On a few occasions, I have had to completely excise a high-domed avascular bleb when a “bleb-plasty” has not provided the desired result. In these situations, it is necessary to make a large conjunctival relaxation incision, parallel with and 8.0 to 10.0 mm posteriorly to the limbus, to allow the anterior edge of the fornix-based flap to be pulled down to the limbus. Failure to make a generous relaxation incision can lead to unacceptable upper lid ptosis.
Irregular corneal astigmatism after trabeculectomy is a vexing problem for both patient and surgeon. Surgical reduction of bleb height by increasing the area of filtration and diverting it posteriorly is most effective in improving the patient's vision. The decision to operate on a trabeculectomy bleb is made easier when a lower IOP is also needed, giving the surgeon a second reason to proceed.