GLAUCOMA

A more in-depth discussion of the current status of glaucoma diagnosis and management

A famous glaucomatologist once described the treatment of glaucoma as "delaying the inevitable progression toward blindness". Though pessimistic, this viewpoint was based on common clinical experience in an age when few medications or surgeries were successful at halting the process of glaucoma. Thankfully, recent advances in glaucoma diagnosis and treatment have made this the exception rather than the rule. This article will review a few of these advances.

Glaucoma is best understood as an optic neuropathy characterized by progressive damage to the optic nerve and typical visual field loss (loss of peripheral vision). In the center of the optic disc (the portion of the optic nerve that is visible inside the eye) is an excavation or "cup" that is normally only 30% of the surface area of the disc. The "cup-to-disc ratio" (CDR or C/R) is a way to document the size of the cup. As nerve fibers are lost due to progressive glaucoma damage, the CDR enlarges as more of the nerve head is taken up with cup instead of healthy nerve tissue. Documented enlargement of the cup is diagnostic of glaucoma, while a large cup or asymmetry of the cups between the eyes (one cup much larger than the other) are suggestive, but not diagnostic of glaucoma. Nerve fibers tend to be lost first from the top and the bottom of the optic disc resulting in specific patterns of visual field loss. Initial glaucomatous visual field loss is often "nasal" (on the nose side of each eye's field of vision) and so is not noticed (less so if one's nose is big!). This gradually progresses around and toward the center of vision until only a central island of vision is left ("tunnel vision"). In the end stages, complete blindness can occur.

Elevation of the pressure inside the eye (intraocular pressure or IOP) was once thought to cause all glaucoma, but we now know that this is only one (if the most important) of many risk factors. In fact, up to a third of glaucoma patients never have an abnormally high IOP (so-called "low or normal tension" glaucoma). Recently, the first gene causing a form of glaucoma was identified. As more genes are identified, we may realize the exciting goal of earlier and more accurate diagnosis, followed by therapy directed at the underlying genetic/biochemical cause of the glaucoma, rather than at the effect (elevated IOP).

Automated visual field testing has undergone a revolution in the past couple of years. Improvements in the computer software of the Humphrey perimeter (called SITA = Swedish Interactive Testing Algorithm) have allowed the test time to be cut in half with improved reliability, so patients get less tired and we get better information. The machine uses a "smart" strategy that adjusts the speed of the test based on the patient's responses, thus reducing patient anxiety. Patients do still need to be reminded that the test is mapping out their side vision so they must keep their eyes looking straight ahead and only use their side vision to "see" the lights. The machine is trying to find the dimmest light that can be seen peripherally, so even a normal person will not see half the lights and even the "seen" lights may only appear as a dim glimmer. Once this is understood, most patients do a very good job of taking the test, improving the reliability of the results.

There are several "hot" new visual field tests that are not yet commonly used. SWAP (Short Wave Automated Perimetry, also known as "blue-on-yellow") can be performed on a properly upgraded Humphrey perimeter. Unlike standard perimetry which flashes a white spot of light onto a white background, SWAP projects a blue light onto a yellow background. It can catch subtle glaucoma damage earlier than standard perimetry, which may be helpful in deciding when to treat persons with high intraocular pressure but normal appearing optic nerves (a "glaucoma suspect"). Another exciting new technique is called Frequency Doubling Technology (FDT). Because the machine required for this test is relatively small and the test only takes a couple of minutes to perform, FDT may be very useful in screening large populations for glaucoma.

Optic nerve imaging is a rapidly developing technology allowing one to compare computerized measurements of the optic disc and/or nerve fiber layer to determine if glaucoma damage is present or has worsened. Examples of these instruments include the GDx Nerve Fiber Analyzer, the Heidelberg Retinal Tomograph (HRT) and Ocular Coherence Tomography (OCT). Eye Health Northwest has recently purchased a GDx Nerve Fiber Analyzer. This new technology allows us to objectively measure the retinal Nerve Fiber Layer (NFL), the tissue that is damaged by glaucoma. In the past, this thin, semi-transparent tissue could only be subjectively evaluated under magnification with the naked eye. The GDx uses the change in birefringence as near-infrared polarized laser light passes through the NFL to calculate the actual thickness of the NFL. The GDx generates this measurement of the NFL in just seconds through an undilated pupil. By comparing these results to a database of healthy, glaucoma-free eyes and to the patient's previous exams, we can assess whether glaucomatous damage has occurred or progressed. The exam is objective so the patient is not required to respond in order to have a valid and accurate result. This is an advantage over other tests, such as the visual field, which requires a subjective response from the patient. Also, it may be more sensitive to the subtle changes in the NFL that occur early in the course of glaucoma. This means better diagnostic decisions, including earlier diagnosis and modification of therapy. This test does not eliminate the need for regular examinations and visual field testing. It does provide one additional and very useful means of diagnosing and monitoring glaucoma.

Many new medications are now available for lowering IOP. Ten years ago a patient with uncontrolled glaucoma may have ended up on 5 medications before proceeding to laser or surgery. Many of these medications had noxious side effects and had to be taken several times a day. Now we have medications with fewer side effects that can be taken only once or twice a day (e.g., Alphagan, Xalatan, Lumigan and Travatan). Combinations drugs are also coming out, decreasing the number of bottles a patient needs to keep track of (e.g., Cosopt = Timoptic + Trusopt). This all serves to improve the quality of life of the glaucoma patient who would otherwise be tied to a rigorous schedule of drops. At the cutting edge of glaucoma research are medications that seek to protect the optic nerve from pressure damage, rather than lowering the pressure itself. Treatments aimed at the genetic defects underlying most glaucoma may be seen in the not-too-distant future.

Research has shown that laser and surgical procedures can be considered much earlier in the treatment of glaucoma and in some cases may precede use of medications. Trabeculectomy is the most commonly performed surgery for glaucoma. This procedure was developed in the 1950's and the actual surgical technique has not changed much since that time. What has changed is the use of medications and laser to modify the eye's healing response during and after surgery. During surgery a trap door is fashioned in the white of the eye (the sclera) to allow the fluid in the eye to drain out under the clear covering of the eye (the conjunctiva) where it is absorbed back into the bloodstream. If this trapdoor heals shut, the surgery will fail and the eye pressure will rise again. Frequent steroid drops are used to suppress this healing and the sutures holding the trap door down may be cut with the laser to allow more fluid out (suturelysis). Mitomycin C (MMC) and 5-Fluorouracil (5 FU) are chemotherapy drugs that also happen to inhibit or kill fibroblasts, the cells that cause healing and scarring after surgery. MMC is applied to the eye during surgery while 5 FU is given as one or more subconjunctival injections in the days and weeks after surgery. While these techniques do have side effects such as increased risk of infection and the possibility of an eye pressure that is too low (hypotony), they have greatly increased the success rate of trabeculectomy.

And finally, that question topmost in your minds: what about marijuana? Legalization of marijuana for medicinal use in Oregon has generated a flood of patients interested in treating their glaucoma with it. The active ingredient in marijuana does lower IOP, but not as well as commonly available glaucoma medicines. Due to its short duration of action, you must smoke a joint every 3 hours to maintain a therapeutic effect. This is certainly an impractical treatment schedule, not to mention the adverse effect of a life lived constantly "under the influence". Research is underway to test an eye drop made from the active ingredient.