Learning More . . . Seeing More

Optometric Management, Jul 2006

Panel members discuss the practical and clinical challenges to evaluating glaucoma patients.

Robert N. Weinreb, M.D.: Glaucoma affects both ocular structure and function, and I consider both types of testing complementary. Together, they provide optimal information for evaluation of patients for glaucoma or its progression. Structural changes often appear before any loss of function, but this is not always the case. How do you approach the two?

Donald L. Budenz, M.D., M.P.H.: Our goals have changed a lot as we've learned more about what glaucoma is and what we need to measure. Having structural and functional information is important. We still rely on both. Once we know what we're looking for, the challenge is to determine how best to measure it.

Glaucoma's evolving definition

Dr. Wcinrcb: I would like to open the discussion with our working definition of glaucoma. It frames our use of instruments to manage the disease. What is glaucoma?

Robert D. Fechtner, M.D.: Each of us struggles to answer that question, but generally, we agree glaucoma is a progressive optic neuropathy. It often has a characteristic pattern, so we identify it by the structural and functional damage it causes within the eye.

Dr. Weinreb: One characteristic you didn't mention was intraocular pressure.

Dr. Fechtner: That's right. I think we've gone far beyond the era where we define glaucoma based on intraocular pressure. Certainly, pressure can damage the eye, and we've all seen cases of very high pressure, but it doesn't fit into the definition because a patient with glaucoma can have any pressure at any visit.

Jeffrey M. Liebmann, M.D.: I think of glaucoma as a neurodegenerative disorder that's characterized by damage to retinal ganglion cells and their axons, which results in changes in the contour of the optic nerve that we see as cupping.

Intraocular pressure, which is a causal risk factor for the onset of glaucoma and a risk factor for progression, remains important because it allows us to gauge the effectiveness of our treatment.

Dr. Weinreb: So glaucoma is nor only a progressive optic neuropathy, or optic nerve disorder, but we also are characterizing it as neurodegenerative, which implies that the damage goes beyond the eye to the entire visual pathway. The disease damages the retinal ganglion cells, their axons, the retinal nerve fiber layer and optic nerve axons.

Biologically, we know glaucoma is a disease that causes progressive changes and loss in the retinal ganglion cells, which lead to characteristic changes in the optic nerve head and, finally, loss of vision. Clinically, what we see is loss of retinal nerve fibers and visual field loss.

Dr. Liebmann: Damage to the retinal nerve fiber layer is often the first detectable sign of disease. Patients lose ganglion cells and the axons early in the disease process, before we can detect changes in optic nerve appearance. In clinical practice, it's difficult to look at the nerve fiber layer, and that WLIS the primary driving force behind the development of scanning laser polarimetry. This technology enables us to look at the eye in ways not possible with the slit lamp, helping us evaluate patients at risk for the disease, as well as those who have progressed.

Challenges of slit lamp examination

Dr. Weinreb: How can we best examine the retinal nerve fiber layer? As we have said, we often cannot detect changes clinically with slit lamp biomicroscopy and a hand-held lens or with photographs.

We can expand our senses, so to speak, by using technology that enables us to detect things that are not necessarily detectable by the human eye, as well as to quantify changes that we cannot quantify with clinical examination. How do you perform structural and functional testing for glaucoma management?

Dr. Fechtner: The optic nerve is relatively straightforward to examine - we learned to do that as residents - but the nerve fiber layer is much more difficult, particularly because we're trying to detect the absence of nerve fiber layer. We're looking for the loss of the structure that signals the biological process of glaucoma. This is quite challenging clinically.

I try to examine the nerve fiber layer in each patient using the slit lamp biomicroscope and a supplemental hand-held binocular indirect lens with a red-free filter. It's an important part of the clinical exam, and a very difficult clinical skill to master. Technology can really make a difference in our ability to see the nerve fiber layer and to enhance glaucoma care.

Dr. Wcinreb: Why is it so difficult to examine the retinal nerve fiber layer?

Murray Fingeret, O.D.: Some patients have small pupils or cataracts, making the clinical examination difficult. What's more, the retinal nerve fiber layer is a moving target; we often cannot see the whole picture - just a small piece.

With the optics we have and slit lamp limitations, we see a wedge or a dark area of the retina. Using the slit lamp is very difficult with any type of lens, but if we take into account the patient's movement, a small pupil and lens changes, we often don't see loss of retinnl nerve fibers.