Monday, February 27, 2006

Section 1-A: Glaucoma: Optic Nerve Disease. Introduction

Figure 1-1. The eye.
The eye is a fascinating organ which allows us to make visual observations about the world around us (Figure 1-1). The complex images that we see are transmitted to the brain for processing by way of the optic nerve. Approximately 1.2 million nerve fibers, or axons, make up each human optic nerve (Figure 1-2). The optic nerve travels from the back surface of each eye and joins together to form the optic chiasm. The nerve fibers that entered the chiasm as part of the optic nerves, travel from the chiasm to the lateral geniculate body but are now called optic tracts. Finally, visual input is transmitted from the lateral geniculate body and travel as optic radiations to an area of the brain called the occipital lobe. Visual images from the retina (the “film” of the eye) travel through the optic nerve, optic tract, and eventually to the visual part of the brain. There the images are processed and interpreted by the brain. Any disease process which affects the optic nerve could disrupt this input, leading to visual loss.
Figure 1-2. This is a healthy optic nerve. Notice the healthy rim of nerve tissue with a central cup.

The optic nerve can be damaged by a number of disease entities. The general term for any condition that damages the optic nerve is “optic neuropathy.” Optic neuropathies may be caused by vascular (blood flow) problems, inflammation/infections, metabolic disorders, trauma, tumors, or hereditary diseases. In each case, the result of the disease process is visual loss (temporary or permanent), which can affect central and/or peripheral vision. Each type of optic neuropathy has characteristic signs and symptoms of visual loss. Some cases are temporary, while others are permanent. Some are less noticeable by patients, especially when the peripheral vision is affected rather than central vision.

Ophthalmologists (M.D.s specializing in medical and surgical care of the eye) are trained in differentiating various optic neuropathies based on signs and symptoms. Neuro-ophthalmolgists are ophthalmolgists with additional training in various diseases of the optic nerve. Similarly, glaucoma specialists are ophthalmologists with additional training in diagnosis and treatment of glaucoma (Figure 1-3).

Figure 1-3.Examination of the eye with a goniolens to evaluate glaucoma

Figure 1-4. A goniolens is a special lens with mirrors used to examine the drainage structure of the eye

Glaucoma is one type of optic neuropathy. It is a disease of the optic nerve, often caused by high intraocular pressure (IOP) resulting from poor drainage of fluid from the eye. There are various types of glaucoma but all share the common feature of optic nerve damage which causes irreversible vision loss. Glaucoma differs from other optic neuropathies by a few key features. The main distinguishing feature of glaucoma is “cupping of the optic nerve,” which describes the appearance of the optic nerve when it is damaged by glaucoma.

Another distinguishing feature of glaucoma is that the treatment is aimed at lowering the intraocular pressure (IOP). The eye has a certain pressure determined by the production and drainage of fluid (called aqueous humor) within the eye. The eye fluid is continuously produced and drained from the eye (Figure 1-4). In patients with glaucoma, normal drainage is impaired, often leading to elevation of IOP.

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