To fully understand the pathophysiology of glaucoma, it becomes imparative to understand route of inflow as well as outflow of the aqueous humor. It should be remembered that the eye is considered a leaking system of aqueous humor.
The Trabecular Meshwork
The trabecular meshwork is a spongy porous tissue lined with trabeculocytes. It is located around the base of the cornea just beyond the line of Schwalbe. It sits just above the anterior fenestrated iris. It runs the course of the entire corneal base of the eye. It is avascular.The trabecular meshwork is divided into three, they include the following-
1) The inner uveal meshwork: This part is closest to the anterior chamber. It is radially oriented and contains trabeculae.
2) The corneoscleral meshwork: This part consists oflarge amount of elastin and often described as the ciliary muscle tendon.
3) The Juxtacanacular meshwork (Cribiform meshwork): This part is close to the canal of Schlem (Sinus Venosus Sclerae). It consists of a thin strip of tissue covered by mono-layer endothelial cell. It recieves support by the connective tissues ground substances full of glycoaminoglycans and glycoproteins.
Canal Of Schlem (Sinus Venosus Sclerae)
The canal of Schlem is acircular canal lined with endothelium that runs around the eyeball at the corneoscleral junction. It is positioned within the internal scleral sulcus and posteriorly related to the scleral spur. The inner endothelial wall of the sinus is related to the trabecular meshwork and the anterior chamber; but there is no direct passage way between the trabecular meshwork, the anterior chamber cavity and the sinus.
The scleral sinus is drained by 25-30 collector canals via the deep scleral venous plexus, then into the intrascleral plexus & the episcleral plexus which enters the larger circulation via the anterior ciliary veins.
A few of the collector canals bypass the deep scleral venous plexus and pass directly into the sclera via the subchoroidal vessels. These collector channels are known as aqueous veins.
85-95 % of aqueous outflow takes place through the trabecular meshwork via the Sinus Venosus Sclerae.
The Uveoscleral pathway
The junction between the uveal portion of the eye and the sclera is linked by the ciliary body smooth muscles. This area has been noted to play a role in outflow of aqueous humor. This area provides about 5-15 % drainage of the aqueous humor. It is known as the 'Extracanalicular' pathway. The aqueous humor is drained into the orbital blood vessels. Recently, the uveoscleral pathway has assumed a very important mechanism in the outflow mechanism of the aqueous humor.To be continued...
Dr Ezebuiroh Victor Okwudiri
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