Schlemm’s canal is a circular, vessel-like structure that plays a specialized role in the eye’s fluid management. This microscopic channel encircles the front of the eye as a component of its drainage apparatus. Its primary function is to collect fluid from the eye’s anterior chamber and return it to the bloodstream, a process that maintains the eye’s internal environment.
The Eye’s Drainage System
The eye’s drainage system enables the constant turnover of a clear fluid called aqueous humor. This fluid fills the space between the cornea and the lens, providing nourishment to these tissues. Schlemm’s canal is located at the iridocorneal angle, the junction where the clear cornea meets the white part of the eye, known as the sclera.
The pathway for fluid drainage is precise. Aqueous humor first flows through a spongy, porous tissue called the trabecular meshwork. This meshwork acts as a filter, regulating the fluid’s passage into Schlemm’s canal and presenting the initial resistance to outflow so fluid does not drain too quickly.
Once the aqueous humor passes through the trabecular meshwork, it enters Schlemm’s canal. From this circular channel, the fluid is directed into smaller collector channels. These channels connect to the episcleral veins, which are part of the body’s blood circulation.
Regulating Intraocular Pressure
The balance between the production and drainage of aqueous humor maintains the eye’s internal pressure, known as intraocular pressure (IOP). This pressure is necessary for the globe to maintain its spherical shape and for its internal structures to remain in their correct positions. A healthy eye maintains an IOP between approximately 11 and 21 mm Hg.
The ciliary body, a structure located behind the iris, is responsible for continuously producing new aqueous humor. To maintain equilibrium, the drainage system must remove fluid at a rate equivalent to its production. This pressure can fluctuate slightly and is often higher in the morning.
The resistance provided by the trabecular meshwork and the efficiency of Schlemm’s canal are the determining factors in how well this balance is maintained. Any disruption to this equilibrium can lead to changes in intraocular pressure, which can have significant consequences for the eye’s health.
Connection to Glaucoma
Glaucoma, a group of progressive optic neuropathies, is closely linked to the function of the eye’s drainage system. The most common form, primary open-angle glaucoma (POAG), occurs when aqueous humor does not drain properly. This dysfunction originates in the outflow pathway, specifically with increased resistance within the trabecular meshwork.
In a glaucomatous eye, the trabecular meshwork becomes less efficient at filtering fluid. This can be due to factors like the accumulation of extracellular material, a reduction in the number of cells lining the tissue, or a general stiffening. This obstruction prevents aqueous humor from effectively reaching Schlemm’s canal, causing a backup of fluid.
The consequence of this impaired drainage is a gradual increase in intraocular pressure. This elevated pressure damages the fibers of the optic nerve, which transmits visual information to the brain. This damage is irreversible and leads to a progressive loss of vision, typically starting with peripheral sight. The process is often asymptomatic until advanced stages.
Medical Procedures Targeting Schlemm’s Canal
Many modern surgical interventions for glaucoma directly address dysfunction in and around Schlemm’s canal. These procedures, categorized as Minimally Invasive Glaucoma Surgery (MIGS), aim to lower intraocular pressure by restoring the eye’s natural drainage system. They are less invasive than traditional glaucoma surgeries, involving smaller incisions and less tissue disruption.
One category of MIGS involves placing microscopic stents or shunts. A device like the iStent is inserted through the trabecular meshwork, creating a permanent bypass allowing aqueous humor to flow directly into Schlemm’s canal. This approach targets the primary site of resistance in open-angle glaucoma.
Other procedures focus on modifying existing structures. Canaloplasty is a technique that dilates Schlemm’s canal using a microcatheter to stretch it open and clear blockages. Alternatively, goniotomy or trabeculotomy involves removing a small section of the trabecular meshwork, creating a direct opening for fluid to enter the canal. These interventions aim to lower pressure and prevent further optic nerve damage.