Medical Health Encyclopedia

Intraocular Eye Pressure. Previously, it was believed that glaucoma was almost always due to an abnormal rise in intraocular pressure.

Glaucoma is a condition of increased fluid pressure inside the eye. The increased pressure causes compression of the retina and the optic nerve which can eventually lead to nerve damage. Glaucoma can cause partial vision loss, with blindness as a possible eventual outcome.

Increased IOP is, indeed, present in most cases of glaucoma, but some patients have normal IOP, which is usually maintained at measurements of 10 to 20 mm Hg. Measurements above this, however, do not necessarily predict glaucoma. For example, only about 10% of people with IOP levels between 21 and 30 mm Hg will actually develop glaucoma. This still puts such individuals at considerable risk for glaucoma, however.

Primary Open-Angle Glaucoma

Most people with glaucoma have the form called primary-open-angle glaucoma (also called chronic open-angle glaucoma). Open-angle glaucoma is essentially a plumbing problem.

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The disease process may occur as follows:

• The drainage angle remains open, but tiny drainage channels in the trabecular meshwork pathway become clogged. This pathway is responsible for most aqueous humor fluid outflow. An imbalance then occurs as fluid continues to be produced but does not drain out efficiently. Experts have still not definitely determined the precise area in the pathway where the blockage is most likely to occur. (In rare instances the pressure is high because the eye produces too much aqueous humor.)
• The fluid in the eye’s anterior chamber builds up and increases pressure within the eye. This is called intraocular pressure (IOP).
• The intraocular pressure exerts force on the optic nerve at the back of the eye.
• Over time, the persistent pressure or other factors irreversibly damages the delicate long fibers of the optic nerve, called axons, which convey images to the brain.
• As these axons die, the small cup-like head of the optic nerve may eventually collapse into an enlarged irregular shape.