The authors conducted this study to determine the incidence of and risk factors for corneal graft failure among eyes that had undergone Ahmed valve placement prior to penetrating keratoplasty performed as a separate procedure.
They retrospectively reviewed records of 77 patients (77 eyes) who underwent penetrating keratoplasty between 1993 and 2004 in an eye with a previously implanted Ahmed valve and were followed for at least six months.
They found that 40 eyes (52 percent) required increased numbers of glaucoma medications following PK and 10 eyes (13 percent) required additional glaucoma drainage devices. Graft failure at one, two and three years was 42.4 percent, 57.1 percent and 59.1 percent, respectively. Prior penetrating keratoplasty (P = 0.006) and stromal vessels (P = 0.0005) were associated with increased risk of graft failure. Use of glaucoma medications (P = 0.009) and evidence of lower intraocular pressures (IOP) excluding hypotony during follow-up (P = 0.010) were associated with reduced risk of graft failure. Endothelial rejection episodes were observed in 13 eyes (17 percent) but were not a risk factor for graft failure.
The authors conclude that long-term survival of corneal grafts is poor in eyes with Ahmed valves, and the majority of graft failures are associated with progressive loss of endothelial function without observed immunologic rejection. Despite the presence of an Ahmed valve, escalation in glaucoma therapy often follows penetrating keratoplasty, and graft failure may be related to poor IOP control.
This study confirms the observations of other studies that graft survival associated with the use of glaucoma implants remains a difficult and challenging problem. However, the failure rate of the corneal transplants included in this study appears to be slightly lower than those reported in the literature when implants were placed before, after or together with the corneal transplants.
There was a greater need for post-transplant use of glaucoma medications over time in this study due to an increase in IOP, indicating a progressive lack of control of IOP by the valve. Although other studies imply that lack of IOP control results in a higher incidence of graft failure, the authors did not find that this factor related to graft survival in this study. Their main explanations for graft failure were previous graft failures and the presence of stromal neovascularization.
They indicate that their study documents the high rate of graft failure when the glaucoma implant is placed in the anterior chamber but do not think this is due to tube interference with the endothelium. I have found that very often the cornea shows opacification in the area of the tube, even though the tube is not close to the endothelium. This occurs particularly in nonvalved implants when aqueous from the bleb is bathing the post-surface of the cornea. The bleb is known to manufacture pro-inflammatory cytokines, and the constant bathing by these cytokines may be responsible for inflammation and damage to the endothelium. The slightly higher incidence of graft survival reported in this study was achieved using a valved implant, which lacks backflow to the endothelium, and may support this theory.