Optic nerve features important in glaucoma assessment

"While we were all taught about the importance of assessing the cup/disc ratio, it's critical that one recognize much more sensitive and specific characteristics of this disease," said Dr. Greenfield, who is professor of ophthalmology at the University of Miami Miller School of Medicine.

Those features include the integrity of the neural rim, the presence of neural rim pallor (and its magnitude if present), the size of the optic disc, the prominence of the scleral ring, the presence of peripapillary atrophy, the presence of optic disc hemorrhage, and clinically detectable nerve fiber defects, he said.

Dr. Greenfield cited the ISNT rule, which emphasizes the characteristics of the physiologic rim. The rule states that the thickest portion of the neural rim is located in the inferior quadrant, with the thickness decreasing successively in the superior, nasal, and temporal portions of the rim. A neural rim whose thickness deviates from the ISNT rule may raise one's index for suspicion for early glaucomatous optic nerve damage, Dr. Greenfield said.

"This rule is true of a large disc (also referred to as a macrodisc), or a small-sized optic nerve (microdisc)," he said.

"A very important sign of glaucomatous optic nerve damage is the presence of an optic disc hemorrhage," Dr. Greenfield said. "While the mech-anisms still remain somewhat unclear, many ran-domized clinical trials have dem-onstrated that eyes that manifest optic disc hemorrhage are much more likely to go on to develop glaucomatous progression."

It's not necessary to examine the optic nerve through a dilated pupil to visualize a hemorrhage, he said. It can be done by using a handheld indirect lens at the slit lamp.

"We were taught as residents that eyes with glaucoma have cupping in excess of pallor and that the neural rim generally has a healthy pink color," he said. "By contrast, eyes that have non-glaucomatous optic neuropathy tend to have significant rim pallor that exceeds the magnitude of cupping. Yet there are various kinds of optic neuropathy that are associated with optic atrophy and secondary cupping of the optic nerve. Very often these tend to resemble the glaucomatous optic nerve. One may see atrophic optic disc cupping in eyes following arteritic ischemic optic neuropathy; nonarteritic ischemic neuropathy rarely is associated with optic nerve cupping."

Measurement of optic disc size is important when assessing the cup/disc ratio, Dr. Greenfield said.

"Large physiologic discs have a larger cup/disc ratio that resembles glaucoma; in contrast small glaucomatous optic nerves may not have appreciable cupping," he noted. Size can be determined by use of computerized imaging devices such as Heidelberg retina tomography (HRT) or optical coherence tomography (OCT), as well as with a direct ophthalmoscope, using the 5° aperture.

"Vertically, the diameter of the optic nerve is between 1.8 and 1.9 mm," he said. "It is a bit smaller in the horizontal diameter, ranging between 1.7 and 1.8 mm." If one uses a 78- or 90-D handheld lens to measure the size of the optic nerve at the slit lamp, a correction factor of 1.1 or 1.3, respectively, must be employed.