Section 6-E: Optic nerve examination

One of the hallmarks of glaucoma is the optic nerve damage, which is characterized by cupping of the optic nerve. Even a normal optic nerve has a small amount of cupping. However, the patients with glaucoma tend to have larger cupping than normal subjects. As discussed in Chapter 1, the cup-to-disc ratio of normal subjects is typically around 0.2 to 0.3 (Figure 6-14). The cup-to-disc ratio is often measured both in the vertical and horizontal position to estimate the amount of cupping and amount of optic nerve damage (Figure 6-15). The cup size is simply the area of the optic nerve that is not occupied by the optic nerve fibers (an empty space). However, with glaucoma, there is progressive loss of optic nerve fibers, and consequent increase in the cup size of the optic nerve. When an eye doctor says there is a cup-to-disc ratio close to 1.0, this refers to almost complete cupping and an advanced damaged optic nerve from glaucoma (Figure 6-16). Conversely, if the cup-to-disc ratio is 0.3 or less, then this refers to a relatively healthy looking optic nerve. While there is no one cup-to-disc ratio that separates normal from glaucoma, the cup-to-disc ratio greater than 0.6 or 0.7 is suspicious of glaucoma and often requires further testing to rule out glaucoma. As glaucoma progresses, the cup-to-disc ratio enlarges (as more optic nerve fiber dies off), and the patient may start to develop peripheral vision loss. A small fraction of glaucoma patients, if detected late or inadequately treated, may become blind in one or both eyes with a complete loss of optic nerve fibers.

	Figure 6-14. Normal optic nerve - 0.3 C/D ratio (same as figure 1-6).
	Figure 6-15. The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio. This C/D ratio here is 0.4. (same as figure 1-5).
	Figure 6-16. Glaucomatous optic nerve – 0.9 C/D ratio. (same as figure 1-7)

In glaucoma the position of the blood vessels within the optic nerve can shift with progressive cupping that can be an important feature of glaucoma progression. Other important optic nerve findings include hemorrhages or bleeding around the optic nerve (Figure 6-17). The optic nerve bleeding is especially common in normal tension glaucoma. It often indicates an ongoing damage to the optic nerve and inadequate control of glaucoma.

Figure 6-17. Optic nerve hemorrhage. There is a flame-shaped hemorrhage of the optic nerve located at the arrow at 5 o’clock. (figure 3-6)

Optic nerve can be viewed using a slit lamp with an appropriate lens by an eye doctor. However, in order to objectively document the status of the optic nerve one has to take stereo photographs of the optic nerve, which can be used as a baseline for comparison in the future. Therefore, it is important to take an optic nerve photographs at initial exam and periodically afterwards. The stereo photographs of the optic nerve are taken with a fundus camera (a camera designed to take pictures of the retina and optic nerve of an eye) in a doctor’s office. There are different models of fundus camera, and in 2006 most fundus cameras can be modified to take digital pictures (Figure 6-18).

Figure 6-18. A Stereo Disc Camera (3-Dx, Nidek Co. Ltd, Fremont, CA). A fundus camera such as this is used to take stereo optic nerve photographs of glaucoma patients. It has a digital camera-back which is connected to a computer and hard disc storage device (computer not shown).

Beginning in the early 1990’s, a number of computerized optic nerve imaging devices has been introduced commercially to aid the eye doctor in documenting the optic nerve damage. In 2006, these devices include HRT2 (Heidelberg Retina Tomograph 2, by Heidelberg Engineering, Inc. Vista, CA, Figure 6-19) Stratus OCT (Ocular coherence tomography by Carl Zeiss Meditec, Dublin, CA, Figure 6-20), and GDx (or scanning laser polarimeter by Carl Zeiss Meditec, Dublin, CA). These sophisticated instruments provide an accurate map of the optic nerve and quantitative analysis of the optic nerve cupping. In addition, devices such as Stratus OCT and GDx provide the thickness of the nerve fiber layer around the optic nerve, which is related to the amount of cupping. In general, the greater amount of cupping (or the larger the cup-to-disc ratio), the thinner the nerve fiber layer. This is because as you lose more optic nerve fibers, the optic nerve cup gets larger and the nerve fiber layer becomes thinner. Your eye doctor may utilize one or more of these computerized optic analyzers (as well as optic nerve photographs) to evaluate the amount of optic nerve damage in glaucoma to diagnose and follow progression of glaucoma over time. Therefore, it is useful to obtain optic nerve photographs and/or computerized optic nerve imaging on a periodic, on-going basis. Undoubtedly, there will be improved or new optic nerve imaging devices in the future that may further enhance our ability to diagnose and follow patients with glaucoma.


Figure 6-19: Heidelberg Retina Tomograph 2 (HRT2). It uses a scanning laser to map out the topography (surface contour) of the optic nerve. The images are then analyzed using a computer to assess the amount of cupping and glaucoma optic nerve damage.


Figure 6-20: Stratus Ocular Coherence Tomography (OCT). It uses a coherent laser source to scan a cross-sectional picture of the retina and optic nerve. A computer analysis of the cross-sectional picture allows it to measure the thickness of the nerve fiber layer, which correlates with the amount of cupping and glaucoma optic nerve damage.


References:

1. Allingham RR, Damji K, Freedman S, Moroi S, Shafranov G. Ch 2: Intraocular Pressure and Tonometry. In: Shield’s Textbook of Glaucoma. 5th Ed. Lippincott Williams and Wilkins, Philadelphia, p36-58, 2005.

2. Alward, WLM. Ch 5: Optic Nerve Head Evaluation. In Glaucoma: The Requisites in Ophthalmology, Mosby, St. Louis, p. 46-55, 2000.

3. Alward, WLM. Color Atlas of Gonioscopy, Mosby, St. Louis, 1994.

4. Brandt, JD. Corneal Thickness in Glaucoma Screening, Diagnosis, and Management. Current Opinion in Ophthalmology, 15: 85-89.

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