Overview

In cross-section the meshwork is a wedge stretching from Schwalbe line to the scleral spur and root of the iris. Three regions are described from inner to outer: the uveal meshwork (large openings), the corneoscleral meshwork (sheets with smaller pores) and the juxtacanalicular (cribriform) tissue adjacent to the inner wall of Schlemm canal. Most normal resistance to aqueous outflow — and the pathological increase in primary open-angle glaucoma — lies in the juxtacanalicular tissue and inner wall of the canal.

The trabecular cells are phagocytic and maintain the extracellular matrix. Contraction of the longitudinal ciliary muscle, which inserts into the scleral spur, pulls on the spur and opens the meshwork, increasing outflow — the basis for the IOP-lowering effect of pilocarpine (a muscarinic agonist).

The angle is visible only by gonioscopy (a mirrored contact lens), because the light from it is totally internally reflected at the cornea. Its width determines the risk of angle closure.

Location: In the scleral sulcus at the iridocorneal angle, between Schwalbe line (anteriorly) and the scleral spur/iris root (posteriorly).

Anatomy

Histology: Collagen and elastic beams covered by flattened trabecular endothelial cells; juxtacanalicular region of loose extracellular matrix.

Function

Filter aqueous humour and conduct it to the canal of Schlemm (conventional outflow, ≈80–90%)
Generate the outflow resistance that sets intraocular pressure
Phagocytose debris from the aqueous

Physiology: By the Goldmann equation, IOP ≈ (aqueous inflow ÷ outflow facility) + episcleral venous pressure (≈8–10 mmHg). Outflow facility is normally ≈0.2–0.3 µL/min/mmHg and falls with age and in glaucoma.

Clinical significance

Primary open-angle glaucoma

The angle looks open, but outflow resistance in the meshwork is increased, raising IOP and damaging the optic nerve. Treatments: prostaglandin analogues (increase uveoscleral outflow), β-blockers, carbonic anhydrase inhibitors and α₂-agonists (reduce production), pilocarpine and Rho-kinase inhibitors (increase trabecular outflow), selective laser trabeculoplasty, and trabeculectomy.

Angle closure

Peripheral iris apposition to the meshwork blocks outflow. Narrow angles occur in hyperopic, small eyes and with a thickening ageing lens.

Pigment dispersion and pseudoexfoliation

Iris pigment or fibrillar material clogs the meshwork, causing secondary open-angle glaucoma.

Test yourself

Through which route does most aqueous humour leave the eye?

Uveoscleral outflow through the ciliary muscle
Posterior diffusion through the vitreous to the retinal veins
Across the corneal endothelium into the tear film
Trabecular meshwork → canal of Schlemm → episcleral veins

Answer: Trabecular meshwork → canal of Schlemm → episcleral veins

About 80–90% of aqueous drains by the conventional (trabecular) pathway into Schlemm canal and then to aqueous and episcleral veins; ≈10–20% leaves by the uveoscleral route, which prostaglandin analogues enhance.