Cutting Edge Glaucoma - Issue 2

6 • CUTTING EDGE - GLAUCOMA in eyes with GDI [2–4]. Sa and Kee studied the effect of temporal clear corneal phacoemulsifica- tion on IOP in 13 eyes with prior Ahmed glaucoma valve insertion [4]. An increase in antiglau- coma medication at the final follow-up was reported in their study. Also, 1 month after cataract surgery, the mean IOP increased by 4 mmHg or more in six patients (46%), one of whom had an IOP spike (10 mmHg increase). The authors concluded that phacoemulsification may adversely affect the capsule bleb of the Ahmed glaucoma valve. Another study reported outcomes in 23 eyes with functioning Ahmed glaucoma valves which later underwent phacoemulsification [2]. The authors found that four eyes (17%) developed an IOP spike (>10 mmHg increase) on the first postoperative day. They also found that the average (± SD) IOP increased from 14.5 (± 3.9) pre- operatively to 19.2 (± 6.3) mmHg on the first postoperative day, although the mean IOP declined to the same level as preoperatively at 1 month. In a retrospective analysis of nine eyes with func- tioning Baerveldt implants subsequent to clear corneal phacoemulsification, the authors found no significant change in mean IOP after phacoemulsification [3]. However, IOP increased in two eyes (9%), while one eye (4%) required repeat glaucoma surgery. According to the present study, the hooked tube technique may reduce the risk of poor IOP control after cataract surgery in eyes with GDI. In contrast to results in the control group and in other studies [2, 4], none in the treatment group developed an IOP spike during the follow-up visits and the IOP remained within 4 mmHg of the preoperative level throughout the follow- up. Similar to other studies [2, 4], in the control group two eyes had IOP spikes at 1 month and at 3 month after cataract surgery. Also, the number of postoperative antiglaucoma medications increased in the control group but not in the treatment group. The capsule of the GDI represents the outflow area for aqueous humor. Resistance to fluid flow through the capsule wall determined the postoperative IOP achieved with GDI. One reason why cataract surgery may adversely affect filtration through the bleb capsule wall might be the flow of lens debris through the GDI tube into the bleb cavity during surgery. In theory, this accu- mulation of debris can cause an inflammatory reaction and/or mechanical obstruction of the capsule wall, which may lead to impaired filtration of aqueous humor through the bleb capsule. However, one can speculate that any inflammatory reaction caused by the debris is short-lived because it appears not to alter long-term IOP control after GDI surgery. Also, the cataract type could influence to the amount of the accumulated debris in the bleb cavity. In case of a hard cata- ract, very dense nuclei often break into many tiny particles that can get trapped more easily into the bleb cavity. This can be one explanation for the late IOP spikes in two eyes of the control group. Another explanation for postoperative IOP spikes, especially during the first postoperative days, might be the viscoelastic material used during surgery. This material can become trapped in the GDI and obstruct it, as reported by Ressiniotis and Down [5]. They presented a case of raised IOP (31–58 mmHg despite maximum antiglaucoma treatment) 1 month after penetrating keratoplasty (PKP). In this particular patient, the viscoelastic material was evacuated about 1 month after the PKP procedure from the GDI bleb cavity through a stab incision made in the GDI capsule wall. The authors concluded that the raised IOP was due to accumulation of the viscoelastic material in the bleb cavity over the GDI plate. Degradation of the viscoelastic material retained inside the bleb can be very slow, possibly due to low aqueous humor production [5].