Channelography and mechanism of action in canaloplasty

Abstract

Canaloplasty lowers the intraocular pressure (IOP) by restoring the natural outflow system. The success of canaloplasty depends on the function of this system. To evaluate the natural outflow system regarding canaloplasty by two clinical tests, provocative gonioscopy and channelography and to describe the mechanism of action of canaloplasty. Provocative gonioscopy evaluates the pattern of blood reflux which is induced by ocular hypotension as the result of a reversed pressure gradient between the episcleral venous pressure and IOP following paracentesis. In channelography the transtrabecular diffusion and the filling properties of the episcleral venous system are assessed by a microcatheter and a fluorescein tracer. Blood reflux varied greatly in glaucomatous eyes and showed an inverse correlation with the preoperative IOP. The higher the IOP, the poorer the blood reflux. The filling qualities of the episcleral venous system and diffusion through the trabecular meshwork were different. Poor trabecular passage and good episcleral fluorescein outflow indicates patent distal outflow pathways, poor trabecular passage and poor episcleral fluorescein outflow indicates obstructed trabecular meshwork and closed collector channels and good trabecular passage together with poor episcleral fluorescein outflow suggests that the site of impairment is mainly in the distal outflow system. The quality of blood reflux and the characteristics of the episcleral filling and the transtrabecular diffusion by fluorescein represent the clinical state of the outflow pathway and help in the prediction of the surgical outcome in canaloplasty. The mechanism for canaloplasty is not yet completely clarified; currently under discussion are circumferential viscodilation, permanent distension of the inner wall of Schlemm's canal using a suture and a Stegmann canal expander.