Pathophysiology of glaucoma

his might be good time to back up and look at the physiology of the eye and how the mechanistic concept of glaucoma might have come about.

Physiology

Two structures of the eye are not equipped with blood vessels: the lens and the cornea. Some might argue that the cornea is an integral part of the sclera but its characteristics are such as to render it separate — though firmly attached. At any event, it is a given that living tissue must be supplied with nutrients and perhaps oxygen in order to survive. It is the function of the blood supply of the body to supply these nutrients and to carry away by-products of metabolism; garbage if you will. But we’ve just said that the cornea and the lens do not have such a blood supply. What’s a body to do?

In the case of the lens, it is immersed within a fluid called the aqueous. This fluid is filled with nutrients which the lens absorbs through an active transfer mechanism. The lens also secretes metabolite's which, if allowed to accumulate, can become toxic. This fluid is manufactured within an intraocular structure called the ciliary body which lies behind the iris and surrounds the lens at the iris root (see Figure 1).

Figure 1. Circulation of aqueous

This fluid (aqueous) production chugs along at a more or less constant rate; I’ve seen no evidence to show that it is demand driven, though the presence of hypertension may increase aqueous production. Hence the analogy of a constantly running faucet is not far off. This fluid flows around the lens, through the pupil of the iris, churns around a little in the forepart of the eye called the anterior chamber and thence exits through a structure lying at the juncture of the iris root and the cornea called the trabecular meshwork. Behind this meshwork lies a more or less circumferential tubular structure called the canal of Schlemm. This canal is connected to the venous outflow of the eye through a series of interconnecting tubules called the collector channels. If the trabecular meshwork is visualized as a grating in a sink, then the analogy of the plugged sink is also apt — as far as it goes.

Figure 2. Trabecular meshwork

It is easy to understand from the above that if the outflow channels (drain) get plugged up somehow and the ciliary body (faucet) keeps producing aqueous, the IOP will rise within the eye. It is also easy — from observing the way the optic nerve fibers bend around the edge of the sclera (Figure 3) — to assume that pressure within the eye will be applied here (recall Pascal's Law) and the nerve function will be impaired. Think about what happens to your fingers when your elbow is pressed against the edge of a table for awhile and you get the idea.

Figure 3. Optic nerve cupping

But that doesn't explain normotensive glaucoma