How does nonpenetrating glaucoma surgery work? Aqueous outflow resistance and glaucoma surgery

Histologic, experimental, and theoretical studies of the aqueous outflow pathways point toward the juxtacanalicular region and inner wall of Schlemm's canal as the likely site of aqueous outflow resistance in the normal eye. At least 50% of the aqueous outflow resistance in the normal eye and the bulk of the pathologically increased resistance in the glaucomatous eye resides in the trabecular meshwork and the inner wall of Schlemm's canal. The uveoscleral, or uveovortex, pathway, which accounts for perhaps 10% of the aqueous drainage in the healthy aged human eye, can become a major accessory route for aqueous drainage after pharmacologic treatment. Surgeries designed to incise or remove the abnormal trabecular meshwork of glaucoma address the pathologic problem of the disease. Surgeries that unroof Schlemm's canal or expand the canal, such as viscocanalostomy, probably cause inadvertent ruptures of the inner wall and juxtacanalicular tissue, thus relieving the abnormal outflow resistance of glaucoma. This review is a summary of current thought on the pathophysiology of aqueous outflow resistance in glaucoma and, in light of this, provides an interpretation of the mechanism of pressure reduction created by these new surgeries.