What is Schlemm’s canal?
Schlemm’s canal is a cylindrical vessel that collects fluid (aqueous) inside the eye and drains it into blood vessels. Schlemms’s canal drains approximately 80% of the aqueous fluid outside of the eye. It is a major route involved in maintaining a normal eye pressure. The corneoscleral outflow pathway goes through Schlemm’s canal. If the aqueous outflow through Schlemm’s canal is blocked, it can cause elevated intraocular pressure which can lead to glaucoma. Glaucoma is an eye disease that causes progressive damage to the optic nerve and its retinal ganglion cells, leading to decreased peripheral vision and poor visual acuity. Glaucoma is the second leading cause of preventable, irreversible blindness in the United States.
Key Takeaways
- Schlemm’s canal is a vessel that drains aqueous fluid from the eye into systemic circulation.
- Schlemm’s canal allows the eye to maintain fluid homeostasis and normal eye pressure.
- If Schlemm’s canal is obstructed, the intraocular pressure may increase and lead to glaucoma.
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Understanding Schlemm’s Canal
Schlemm’s canal is a vessel that transports aqueous fluid. Aqueous humor is the transparent fluid between the lens and cornea that provides nutrients to the cornea and lens. Aqueous humor plays an important role in eye pressure. If there is too little aqueous humor in the eye, the eye pressure is low (hypotony). If there is too much aqueous humor inside of the eye, the intraocular pressure becomes high. The amount of aqueous fluid may become excessive if too much fluid is produced by the ciliary body, or if the fluid is not draining out of the eye properly. Schlemm’s canal is the main way that aqueous fluid drains outside of the eye into systemic blood vessels. If the Schlemm’s canal is blocked, less aqueous fluid is drained and the intraocular pressure becomes elevated.
Schlemm’s canal is the major route of aqueous outflow. It is part of the conventional (corneoscleral) outflow pathway, which drains 80% of the aqueous fluid. The conventional pathway relies on three main structures to drain fluid: the trabecular meshwork, Schlemm’s canal, and episcleral veins. The conventional aqueous outflow pathway is pressure-dependent, meaning that the pressure in the eye must be higher than the episcleral venous pressure for the pathway to be open.
Another aqueous outflow pathway is the unconventional (uveoscleral) pathway. Schlemm’s canal is not used in this pathway. The unconventional pathway drains aqueous fluid through the ciliary muscle bundles and into the suprachoroidal space. The unconventional pathway only drains 20% of the intraocular fluid. It is pressure-independent and so it is always actively draining fluid.
Structure And Function Of Schlemm’s Canal
Schlemm’s canal is a circular canal lined by a single layer of endothelial cells. Schlemm’s canal receives aqueous fluid from the trabecular meshwork, a spongy tissue with many holes that allows aqueous fluid to pass through. The fluid flows from the trabecular meshwork, into Schlemm’s canal, and eventually into the episcleral veins. After making its way through the episcleral veins, the aqueous fluid then travels to the ophthalmic veins, cavernous sinus, internal jugular vein, superior vena cava, and finally into the right atrium of the heart.
Schlemm’s Canal: A Therapeutic Target
Blockage of Schlemm’s canal can decrease aqueous fluid drainage and the subsequent increase in intraocular pressure. Prolonged elevated pressure can cause glaucoma, which is a leading cause of preventable blindness. Certain glaucoma medications are designed to increase the amount of aqueous humor that drains through Schlemm’s canal.
There are two classifications of eye drops that reduce the intraocular pressure by increasing outflow through the Schlemm’s canal. The two glaucoma medications that activate Schlemm’s canal are:
- Rho Kinase Inhibitors:
- Netarsudil (Rhopressa) is a newer glaucoma medication. It increases the trabecular meshwork outflow and enlarges the inner wall of Schlemm’s canal. This allows more intraocular fluid to flow out of the eye.
- Pilocarpine:
- Pilocarpine is an older glaucoma treatment that increases conventional outflow through Schlemm’s canal by contracting the ciliary muscle. It is rarely used today because it causes side effects such as headache and numbness of the extremities. It also increases the risk of a retinal detachment.
