Optic Nerve Structure and Function
The optic nerve is what connects the eye to the brain. The optic disc is the end of the optic nerve that can be seen within your eye when you eye doctor uses certain lenses and instruments. The optic disc can progressively change in appearance and progressively lose function when it is affected by glaucoma.
All of the approximately one million nerve fibers that receive light signals from the outside world pass through the optic nerve to reach the brain. In glaucoma the number of living, functioning nerve fibers decreases at a rate much faster than would occur through the normal aging process. Their death may be due to increased intraocular pressure (IOP), lack of blood flow, some other mechanism not yet discovered, or a combination of mechanisms. When the nerve fibers die they leave an empty space where they used to be. If enough nerve fibers die the empty space on the optic disc becomes visible to the ophthalmologist examining the patient.
Video | Anatomy: Optic Nerve
Optic Nerve Testing
Because the optic disc tends to change in appearance over time in patients with glaucoma, your eye doctor may have optic disc photographsor optic disc imaging tests done. These tests enable your doctor to compare your disc appearance at every check-up with the baseline photos and/or imaging test results taken during an earlier visit. Having a basis for comparison makes it easier to detect change.
Optic disc photographs and imaging tests allow your eye doctor to see structural changes due to glaucoma. Visual field testing or perimetryallows your eye doctor to see functional changes. Various machines can be used for this purpose. The most commonly used method involves asking the patient to look at a central target while flashes of light or other kinds of visual stimuli are projected all around the central target. The patient has to press a button whenever a stimulus is seen. The machine then records and analyzes what stimuli were seen and what were not and the result is sent to the eye doctor for interpretation. The test is not painful and not too uncomfortable but it can be tiring or stressful due to the intense concentration required. It is helpful to get a good night’s sleep the night before your test. The test takes from 2 to 20 minutes per eye depending on the machine, the test program used, and the speed of the patient’s responses.
Videos | Optic Nerve Testing
References:
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
- South East Asian Glaucoma Interest Group. Asia-Pacific Glaucoma Guidelines. Sydney, Australia, 2003-2004, SEAGIG.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma 2nd Ed. Savona, Italy, 2003, EGS.
