• Written By: Jeffrey Freedman, MD, BCh, PhD, FRCSE, FCS

    This retrospective study used anterior segment OCT (AS-OCT) to image the internal structure of blebs in patients who had undergone Ahmed glaucoma valve (AGV) implantation, and found that bleb wall thickness was greater in failed implant surgeries compared with successful cases.

    The morphological appearance of blebs following glaucoma valve implantation, as seen on slit-lamp examination, cannot easily distinguish the difference between a successful or failed bleb. After successful standard glaucoma surgery, blebs have a distinctive appearance, characterized by a diffuse extent, a scarcity of vascularity and conjunctival microcysts.

    The authors sought to differentiate between successful and failed blebs by using AS-OCT to examine 76 patients who had undergone AGV implantation. Success was defined as IOP ≤ 21 mmHg with a maximum of two glaucoma medications.

    Roughly 60 percent of the procedures were successful. AS-OCT imaging of the bleb revealed a relatively regular surface and a homogeneous and hyperreflective bleb wall.

    Maximum and minimum bleb wall thicknesses were significantly different between the successful and failed groups. One characteristic of the failed blebs was a highly reflective bleb wall. The maximum and minimum bleb wall thicknesses were also significantly correlated with the postoperative maximum IOP and with the number of postoperative glaucoma eye drops.

    The positive correlation found between bleb wall thickness and IOP begs the question of whether the thick wall causes the increase in IOP because of poorer permeability, or the rise in IOP causes thickening of the wall as a result of cytokine secretion and increased fibrosis. It is likely a combination of both effects. The authors speculate that a thinner bleb wall allows better aqueous permeability through the bleb wall, resulting in good IOP control.

    Until there is some standard of what a successful bleb wall thickness should be, the simple measurement of bleb wall thickness cannot predict the effectiveness of the bleb. However, this study has opened up the possibility of using OCT measurements to predict the success or failure of the bleb.