Monday, April 24, 2006

Section 3-C: Optic nerve damage.

The primary function of the optic nerve is to convey visual signals received by the retina to the brain. As described in Chapter 1, specific sections of the optic nerve are responsible for transmitting particular parts of the visual field to the brain. If part of the optic nerve is damaged by glaucoma, the visual signals from a portion of the visual field are no longer reported to the brain and a blind-spot (called scotoma) is formed.

Eye doctors may directly observe damage to the optic nerve damage by looking through the pupil with an ophthalmoscope. A healthy optic nerve is pink, and the ratio of the diameter of the optic cup to that of the optic disc in a healthy eye is generally less than 0.5 (see Chapter 1-B for more discussion). When the optic nerve is damaged many of the individual fibers that make up the nerve are lost and the optic nerve becomes excavated or “cupped.” As glaucoma progresses and more optic nerve tissue is lost, the optic cup grows larger (Figure 3-5).

Figure 3-5. Progressive cupping of the optic disc. Early progression of cupping can be seen by comparing sequential photographs of the optic nerve. The optic cup which was initially not visible (A) has enlarged over time (B).

The superior and inferior aspects of the optic nerve are preferentially damaged by glaucoma. These parts of the nerve are responsible for peripheral vision. Consequently, as the optic cup enlarges, it typically becomes vertically elongated. In advanced stages of glaucoma the portion of the nerve that carries central vision may also become involved.

Bleeding or hemorrhage of the optic nerve is another sign of damage from glaucoma. There is a characteristic appearance to optic nerve hemorrhage that is due to the anatomy of the eye. When the optic nerve bleeds in glaucoma, the blood typically collects along the individual nerve fibers that radiate outwards from the nerve (Figure 3-6.) An optic nerve hemorrhage is a sign that glaucoma may not be under good control, and additional therapy may be necessary to bring the glaucoma under control.

The cause of the optic nerve damage in glaucoma is not well understood. In some patients that have elevated intraocular pressure, the progression of glaucoma is halted when the pressure is lowered to a more normal level with medicines or surgery. High intraocular pressure may directly damage the optic nerve and cause disease in this subset of patients. In other patients, the optic nerve is injured at pressures that are not elevated (or in the normal range). The optic nerves of these patients (so-called “normal tension glaucoma” patients) appear to be sensitive to damage at intraocular pressures that most people tolerate without any harmful effects. Studies have shown that lowering the intraocular pressure to low normal or even sub-normal levels can halt the progression of normal tension glaucoma. Additional mechanisms such as poor blood circulation to the optic nerve may be responsible for optic nerve damage in normal tension glaucoma.

Figure 3-6. Optic nerve hemorrhage. There is a flame-shaped hemorrhage of the optic nerve located at the arrow at 5 o’clock


References:
Alward, WLM. Aqueous system anatomy and physiology. In Glaucoma: The Requisites in Ophthalmology. JH Krachmer (ed.) Saint Louis: Mosby, Chapter 2, 2000.

Anderson, DR. Collaborative Normal Tension Glaucoma Study. Current Opinion in Ophthalmology. 14: 86-90.

Millar, JC, Gabert, BT, Kaufman, PL. Aqueous Humor Dynamics. In Duane’s Clinical Ophthalmology. W Tasman and EA Jaeger (eds.) Saint Louis: Lippincott Williams & Wilkins, Chapter 45, 2005.

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