ASSIGNMENT 3: INTRA OCULAR PRESSURE & TRABECULAR MESHWORK

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NOTE 2: FOR MORE INFORMATION REGARDING GLAUCOMA READ ASSIGNMENT #1

DEFINING INTRA-OCULAR PRESSURE

Intra-ocular pressure (IOP) is the pressure exerted by the aqueous humor on the globe wall. No matter where on the globe wall the pressure is measured, it should be the same. This is just like the pressure exerted by air on a tennis ball. Both can be treated as spheres and as a rule the pressure is equally exerted every where. The normal range of intra-ocular pressure is between 12-21 mm Hg.

AQUEOUS HUMOR FLOW DYNAMICS

The factor that is most important in determination of IOP is the resistance to the flow of aqueous humor. The aqueous humor is produced by the ciliary body. This eventually finds its way to the anterior of the lens and drains into the trabecular meshwork in the anterior chamber. The anterior chamber is the space between the cornea and the lens (Do not confuse this with anterior segment which is the part of the eye from the cornea to the lens).

The picture shows how the aqueous humor flows from the ciliary body to between the lens and iris, over the iris into the trabecular meshwork and finally drains into the canal of Schlemm.

TRABECULAR MESHWORK

Trabecular meshwork is a sieve like structures. It has many layers. Think of it as your drain's sieve only multi layered. Each layer has opening of different sizes. The flow of aqueous through the meshwork is a major source of resistance to its outflow. This resistance is directly proportional to the intra-ocular pressure. The more the resistance, the greater the intra-ocular pressure.

The histology slide above shows the trabecular meshwork (TM). As you can see it is composed of large spaces that are surrounded by cells. As you travel through the meshwork (from left to the right of the slide), the spaces get smaller and smaller.

This is a electron microscopic appearance of the trabecular meshwork. This is a saggital section (the cut in the plane of the saggital suture) view. Notice the opening of different sizes.

INTRA-OCULAR PRESSURE -THE OTHER SIDE OF GLAUCOMA

It has already been established that intra ocular pressure is only a risk factor in glaucoma. It is not an etiologic factor. It is possible to have intra-ocular pressure in the normal range and still end up with the disease. The reason IOP is still so central to glaucoma is that it is, for now, the only factor that can be controlled in the disease that is helpful for the patient. In other words lowering of the IOP is the ONLY effective way of therapeutically dealing with glaucoma (short of surgery).

A novel approach is to study the physiology of trabecular meshwork and figure out how the resistance to outflow can be lowered, thus help lowering the IOP.

Your task is to look for evidence that points to this mechanism of lowering the IOP