Aqueous Humor Dynamics and Glaucoma Overview

Aqueous Humor Drainage

Conventional Outflow (Trabeculocanalicular)

• Accounts for ~80% of aqueous humor outflow.

• Involves:

  • Trabecular meshwork (Iridial, Uveal, Corneoscleral, Juxtacanalicular).

  • Canal of Schlemm.

  • Collector channels (Internal/External).

  • Associated vessels (Aqueous veins, Intrascleral plexus, Episcleral veins).

Trabecular Meshwork

• Triangular-shaped cross-section.

• Extends from Descemet’s membrane to the scleral spur.

• Consists of 3-5 layers anteriorly and 15-20 layers posteriorly.

• Composed of:

  • Cords: Connective tissue matrix, longitudinally oriented collagen fibers, endothelial cell lining.

Juxtacanalicular Tissue

• Surrounds Schlemm’s canal.

• Composed of endothelial cell meshwork with gel-filled spaces.

• Major site of outflow resistance (80-90%).

Canal of Schlemm

• Narrow, circular channel (~36 mm circumference).

• Lined by endothelial cells with an incomplete basement membrane.

• Contains pores and pinocytotic vesicles.

Collector Channels

• Internal collector channels of Sondermann increase canal surface area.

• External channels (25-35) connect the canal to the deep intrascleral venous plexus and episcleral veins.

Unconventional Outflow (Uveoscleral)

• Accounts for 10-15% (up to 35% in some cases) of drainage.

• Aqueous humor flows through connective tissue spaces of the ciliary muscle into supraciliary and suprachoroidal spaces.

• Fluid diffuses through the sclera or drains into the uveal vascular system and vortex veins.

Comparison of Aqueous Humor Outflow Pathways

Aqueous Veins

• Approximately 8 external collector channels connect directly to episcleral veins.

• Located near the limbus, usually inferonasally.

• Run 1-10 mm before connecting with an episcleral vein.

Flow Dynamics

• Most aqueous humor diffuses passively.

• Active transport of substances occurs via vesicles.

• Large molecules transported via transcellular channels.

• Nerve terminals influence permeability.

Intraocular Pressure (IOP)

• Normal IOP: 10-20 mmHg (average: 13-15 mmHg).

• IOP = F / C + PV where:

  • F = aqueous fluid formation rate.

  • C = outflow facility.

  • PV = episcleral venous pressure.

Measurement

• Tonometry estimates IOP.

• Methods include contact (Goldmann, Shiotz) and non-contact (Pulsair) tonometry.

• Corneal thickness affects measurements; for each 10 µm increase, IOP increases by ~0.2 mmHg.

Dynamics

• Pulsatile variation of 2-3 mmHg.

• Diurnal variation: Higher readings in the early morning.

Regulation

• Homeostatic control maintains IOP within a narrow range.

• Neural mechanisms: Pressure-sensitive nerve fibers in ciliary nerves; autonomic and sensory innervation.

Anterior Chamber Angle

• Formed by the iris root, connective tissue in front of the ciliary body, and trabecular meshwork.

• Assessed using gonioscopy.

Gonioscopy

• Allows visualization of angle structures (Iris, Ciliary Body, Scleral Spur, Trabecula, Schwalbe’s Line).

• Grading of Angles:

  • Wide Open (3): All structures visible.

  • Narrow (Grade 2, 1): Ciliary body or scleral spur not seen.

  • Closed (Grade 0): Schwalbe’s line only.

• Anatomical risk factors for closed angle: anterior chamber depth/volume, iris thickness/curvature, lens vault.

Age-Related Changes

• Increased outflow resistance.

• Decreased trabecular cell numbers.

• Increased thickness of trabecular sheets.

• Increased fusion of trabecular cords/sheets.

• Increased debris in meshwork pores.

• Proliferation of juxtacanalicular tissue.

Glaucoma

• Often caused by decreased aqueous outflow.

• Can be associated with high or normal IOP.

• Drug treatments aim to increase uveoscleral outflow (latanoprost), decrease aqueous production (CAIs, beta blockers, alpha agonists), or increase conventional outflow (pilocarpine).