Changes in axial length following trabeculectomy and glaucoma drainage device surgery

AIM: This study examines the changes in axial length (AL) after trabeculectomy and glaucoma drainage device (GDD) surgery and enabled an equation to be derived allowing prediction of AL change after filtering surgery. METHODS: This was a prospective, interventional case series from the Glaucoma Service of the Doheny Eye Institute. Patient population: One eye of 39 patients undergoing trabeculectomy and 22 undergoing Baerveldt tube shunt implantation for uncontrolled glaucoma. INTERVENTION: These patients had AL measurements by non-contact, partial coherence interferometry preoperatively, at 1 week, 1 month, and >3 months after surgery. MAIN OUTCOME MEASURES: Axial length and intraocular pressure were compared at preoperative and postoperative visits. Postoperative intraocular pressure (IOP) was categorised as hypotonous (0-4 mm Hg), low (5-9), normal (10-17), and high (18 or more). RESULTS: There was a statistically significant reduction in IOP after 3 months of -12.8 (SD 1.5) mm Hg following trabeculectomy (p<0.001), and -10.7 (1.9) mm Hg after GDD (p<0.001). There was a statistically significant reduction in AL, which was similar after trabeculectomy and GDD at all time points (p<0.001), of -0.15 (0.03) and -0.21 (0.04) mm (1 week), -0.18 (0.02) and -0.10 (0.02) mm (1 month), and -0.16 (0.03) and -0.15 (0.03) mm (3 months). At 3 months or later the AL reduction was related to postoperative IOP and to the amount of IOP reduction (p<0.05, stepwise multiple regression). 10.2% (4/39) of trabeculectomy patients had hypotony after 3 months, with a mean AL reduction (-0.39 (0.11)) that was statistically significantly lower (p<0.01) than the other trabeculectomy eyes (-0.14 (0.15)). CONCLUSIONS: There is a small but statistically significant decrease in AL after both trabeculectomy and GDD surgery, greater in eyes that are hypotonous after surgery. The authors suggest that AL reduction can be predicted after 3 months by the formula: AL reduction (mm) = -199+0.006 x IOP reduction+0.008 x final IOP.