Non-Penetrating Deep Sclerectomy Decreases IOP Without Increasing Outflow Facility

Abstract

Abstract: : Purpose: To determine the effect on aqueous humor dynamics of deep sclerectomy glaucoma surgery. Methods: Two forms of deep sclerectomy glaucoma surgery were investigated. In Study A, 8 patients underwent bilateral glaucoma and cataract surgery by one surgeon, (JSS), with one randomly chosen eye receiving viscocanulostomy and the other, control eye, a standard trabeculectomy. In Study B, one surgeon, (MEY), performed a new deep sclerectomy procedure in 20 eyes and a standard trabeculectomy in 20 control eyes. The deep sclerectomy used a tight scleral flap closure to minimize transcleral flow and an internal silicone tube shunt/space maintainer to maximize the area of exposure to the underlying uvea. After 42 months in Study A and 3 months in Study B, 4 minute tonography was done on all eyes using a Medtronics pneumatonometry unit. In Study B, 10 eyes of both groups also underwent pre-operative tonography and 9 eyes of both groups underwent pre and post-opertive fluorophotometry to determine the anterior chamber aqueous humor flow rate, F_a. Results: In both Study A and B, the effect of the deep sclerectomy procedure on IOP was comparable to that of the standard trabeculectomy group. However, the tonographic outflow flacility, C_tot, after deep sclerectomy was half that of the standard trabeculectomy group, (p ≤ 0.01, paired t test). In Study B, when comparing pre and post-op C_tot and F_a no significant effect of deep sclerectomy was found. A calculation of the presure independent outflow, F_u, showed a marked increase after deep sclerectomy. Conclusions: Standard trabeculectomy decreased IOP entirely by increasing C_tot. However, viscocanulostomy in Study A and deep sclerectomy, using an internal shunt/space maintainer, in Study B, had little effect on C_tot and acted mainly by increasing F_u. This is strong evidence that these newer surgical procedures create an internal shunt of aqueous humor back to the uveal blood in response to the protein colloid osmotic pressure driving force of the blood. This is an entirely different mechanism of action from that of standard trabeculectomy. Since these newer procedures are not dependent on a transcleral drainage route they offer many advantages over standard trabeculectomy.

Keywords: outflow: ciliary muscle • aqueous • uvea