Schlemm's canal: the effect of intraocular pressure

We have shown that in the excised eye when oil in the anterior chamber has made the trabecular mesh impermeable, resistance to circumferential flow along the canal increases moderately as transmural pressure is increased. At the same time, resistance to outflow from the canal through the collector channels increases markedly as transmural pressure is increased from low levels to 35 or 40 mm Hg. We have suggested that the usual primary defect in open-angle glaucoma is reduced facility of the inner canal wall and that collapse of the canal with reduction of filtering area and plugging of collector channels is a secondary effect.

Chronic experimental glaucoma in primates. I. Production of elevated intraocular pressure by anterior chamber injection of autologous ghost red blood cells

Chronic elevations of intraocular pressure (IOP) were produced in rabbit and monkey eyes by anterior chamber injection of autologous fixed red blood cells. The method confirms the possibility of secondary glaucoma due to trabecular obstruction by ghost cells in eyes with intraocular hemorrhage. In primates, decreased aqueous outflow may result from direct obstruction by free cells and macrophages as well as swelling of trabecular endothelium following phagocytosis of cellular debris. IOP elevations for from 2 days to greater than 1 month were produced in order to study the effects of elevated IOP on ocular tissues. The model has the advantages of producing IOP elevation easily and without associated intraocular inflammation. The extensive filling of the anterior chamber necessary to produce IOP rises in healthy animal eyes leads to the disadvantage of poor visibility of the optic disk. In rabbit eyes, chronic IOP elevation leads to corneal enlargement and ectasia, making IOP measurements difficult.

The anatomic basis for glaucoma

The greatest bulk of aqueous passes through a sieve-like trabecular wall to reach Schlemm's canal. The vacuoles of the endothelial lining of the trabecular wall are pressure-dependent one-way valves, adapted morphologically to provide for adequate aqueous transfer. Resistance to aqueous outflow seems to be most concentrated in the juxtacanalicular tissue which is also the site of the greatest mucopolysaccharide concentration and phagocytic activity. Normal intraocular pressure may be maintained through a balance between hydrophilic polymerized MPS and hydrophobic depolymerized MPS. It may be effected also by a phagocyte-induced, self-cleaning biologic filter mechanism of the trabecular wall. The use of electron microscopy and improved biochemical techniques have led recently to a greater understanding of the anatomy of the structures concerned with the outflow of aqueous from the anterior chamber. Despite these advances, no clear-cut differences between normal and glaucomatous eyes have emerged thus far. However, the future should bring more knolwedge as the stimulus of conflicting theories leads to progress.

Obstruction of aqueous outflow by lens particles and by heavy-molecular-weight soluble lens proteins

Enucleated human eyes were perfused via the anterior chamber at 25 mm Hg pressure with lens particles (whole lens homogenates) in one series of experiments and with soluble lens proteins from human cataractous lenses in another series. Adding 1% of a homogenate of a single cataractous lens to the anterior chamber induced a 68% decrease in outflow. Perfusion with HMW soluble lens proteins (1 mg/ml; MW more than 150 million) caused a 60% decrease in outflow in 1 hr. In neither series was the obstruction to outflow relieved by subsequent irrigation of the anterior chamber with balanced salt solution or alpha-chymotrypsin. The results show that both lens particles and soluble lens proteins can directly obstruct the aqueous outflow pathways of human eyes. Such obstruction may be a significant factor in certain lens-induced glaucomas.

The pathogenesis of glaucoma in Sturge-Weber syndrome

What is the cause of glaucoma in Sturge-Weber syndrome? Looking for the answer to this puzzling question, we examined 21 patients with the disease. Sixteen patients had gglaucoma: three bilateral and 13 unilateral. Episcleral hemangiomas were visible in all glaucomatous eyes. In general, the more extensive the hemangioma, the more severe was the glaucoma. During gonioscopy, blood could easily be made to reflux into Schlemm's canal of glaucomatous eyes. Often the canal separated into multiple fine channels. Episcleral venous pressure, which we measured in 11 patients, was high in all glaucomatous eyes. These observations suggest that glaucoma in Sturge-Weber syndrome is caused by elevated episcleral venous pressure. Most likely, veins draining aqueous from the canal of Schlemm are part of an intrascleral or episcleral hemangioma. The canal of Schlemm itself may be part of the hemangioma. Arteriovenous shunts in the hemangioma raise episcleral venous pressure, which in turn elevates intraocular pressure.

Mathematical consideration of aqueous outflow after trabeculo-electropunture (TEP)

The obtainable postoperative intraocular pressure following a TEP in a glaucoma patient with an intraocular pressure of 30 mm Hg is calculated in accordance with the concept that there is no outflow of aqueous humor along Schlemm's canal. If aqueous humor reaches all collector channel openings in the surgical area without resistance, the postoperative intraocular pressure reads 14.75 mm Hg. This intraocular pressure is obtained at a point when a collector channel of 1 mm length and of 32 mum in diameter or two collector channels of 27 mu in diameter or five collector channels of 22 mu in diameter establish connection with the trabecular opening.

[Tonometry and tonography for open Schlemm's canal (author's transl)]

In 13 eyes tonometry was performed during trabeculotomy at the beginning, after preparation of the outer scleral lamella and after the excision of the outer wall of the canal of Schlemm. In 4 eyes this was combined with tonography. There was no significant change of the averaged i.o. pressure and the resistance to aqueous outflow. Thus in primary open angle glaucoma the resistance to aqueous outflow must be located primarily in the trabecular meshwork.