Anatomy and Location of Schlemm's Canal
Schlemm's canal is an integral part of the eye's conventional outflow pathway. It is located in the iridocorneal angle (the angle of the anterior chamber) situated between the cornea and the iris.
The canal forms a circular, interconnected network of endothelial-lined tubular spaces embedded within the limbus. The endothelial lining is continuous with that of the surrounding veins.
Aqueous Humor Outflow Pathway
Intraocular pressure (IOP) is determined by the balance between aqueous humor production and its drainage. Aqueous humor is produced by the ciliary body and flows through the drainage structures, where Schlemm's canal plays the primary role in ultimate fluid removal.
Sequence of Aqueous Outflow:
- Posterior and anterior chambers (camerae posterior et anterior bulbi) — filled with circulating aqueous humor.
- Spaces of Fontana (trabecular meshwork) — fenestrated spaces formed by collagen beams within the trabecular meshwork (pectinate ligament). Fluid must filter through these spaces before reaching the sinus.
- Schlemm's canal (sinus venosus sclerae) — collects aqueous humor filtered through the trabecular meshwork.
- Collector channels — drain aqueous humor away from Schlemm's canal.
- Episcleral venous plexus (superficial scleral veins) — the final destination of the conventional outflow system into the systemic venous circulation.
Clinical Significance and Pharmacological Effects
Intraocular pressure regulation heavily depends on the patency of the conventional outflow pathway: the iridocorneal angle, the trabecular meshwork (spaces of Fontana), and Schlemm's canal.
- Direct muscarinic agonists (e.g., pilocarpine): Cause contraction of the ciliary muscle and the sphincter pupillae, resulting in miosis (pupillary constriction). This pulls the peripheral iris away from the iridocorneal angle, opening up the trabecular meshwork and improving access to Schlemm's canal. Effect: Increased aqueous humor outflow and reduced intraocular pressure.
- Muscarinic antagonists (anticholinergics/mydriatics): Cause mydriasis (pupillary dilation). The iris bunches up at the root and can mechanically block the iridocorneal angle. Effect: Decreased aqueous humor outflow and a significant risk of acute angle-closure glaucoma.