Antimetabolites, especially mitomycin C (MMC), increase the incidence of late bleb-related endophthalmitis in trabeculectomy. This is related to a higher incidence of avascular, thin, cystic, translucent blebs, which may be caused by a toxic effect on conjunctival tissue. An MMC dose–response study was carried out focusing on bleb morphology and function.
Patients and Methods
In a retrospective, comparative case series study, 2 successive groups of patients with complicated glaucoma were compared 2 years after a special, minimally invasive, filtering procedure (intrastromal holmium laser keratostomy). Preoperative local subconjunctival injections of a fixed MMC dose (4 μg) were used in group A, and lower MMC doses, calculated individually (1 or 2 μg), were used in group B. Bleb vascularity and morphology were evaluated by masked grading of photomicrographs. Bleb function was evaluated by intraocular pressure (IOP).
Total bleb avascularity occurred in 63% of the blebs in group A and 0% in group B (P<0.01). In eyes with IOP ≤20 mmHg without medical treatment, the mean IOP was significantly lower in group 1 (8 vs 15 mm Hg, P<0.002). A translucent cystic bleb without conjunctival stroma was observed in only 1 eye in group A. An optimal spongy stromal bleb was observed in all other eyes (96%) despite the different MMC doses. The numbers of complications in the 2 groups were nearly equal.
Bleb avascularity after 4 μg MMC could be avoided by the use of 1 or 2 μg MMC on the basis of preoperative prognosticators for failure, but at the expense of some of the IOP-lowering effect. This indicates that the therapeutic index (clinical safety margin) of MMC seems to be narrow. An MMC dose–response relation was not observed for the thin, cystic, and translucent bleb. The low incidence of this bleb (4%) indicates that the operative technique, apart from the vascularity, may be the most essential determinant of bleb morphology.