Although it is recognized that not all glaucoma patients demonstrate elevated intraocular pressure and not all glaucomatous optic nerve damage is attributable to pressure damage per se, current standard glaucoma care is devoted almost exclusively to the reduction of intraocular pressure. Bringing pressure down into the normal or low normal range (17 mm Hg or less) can be expected to arrest progression or dramatically slow its course in the vast majority of cases. At the same time, it must be recognized that some unfortunate individuals, diagnosed late with end-stage nerve damage, with an unusually sensitive optic nerve, or who are primarily sensitive to non-pressure factors, will continue to show unabated visual field loss despite maximal pressure lowering. These are the individuals to whom future research regarding other non-pressure causative factors and their treatment must be directed.
Three methods for glaucoma pressure-lowering treatment are available: medical (usually eye drops), laser, and surgical. Since the threshold of pressure damage varies among patients, the only reliable indicators of glaucoma stabilization are the stability of the visual field and the prevention of optic nerve damage. In the usual glaucoma therapy, intraocular pressure may be lowered by any or all of the three methods (drops, laser, and surgery).
In a normal, non-glaucomatous population, intraocular pressure averages approximately 16 mmHg, and most (95%) will fall between 10 mmHg and 24 mmHg. In the glaucomatous population, the mean intraocular pressure is somewhat higher and the range much broader, even as high as 70 mmHg where arterial circulation to the eye begins to be compromised. Typically, however, the early untreated open-angle glaucoma patient will manifest an eye pressure in the mid-20s. Measurement of intraocular pressure at different times of the day establishes the degree of pressure variability before glaucoma therapy is started. The experienced ophthalmologist typically will determine a “target pressure” as a goal to achieve with pressure-lowering therapy, recognizing that the goal may have to be revised based on the future assessment of the visual field.
What Laser Therapy is Available for Open-Angle Glaucoma?
During the early 1970s, attempts were made with a variety of lasers to enhance aqueous humor outflow through the trabecular meshwork in open-angle glaucoma by puncturing the trabecular meshwork with the laser energy. Despite the failure of these procedures to create holes in the trabecular meshwork, a subsequent decrease in the intraocular pressure, several days to weeks following some of the procedures, was often observed. In 1979, Wise and Witter published a pilot study describing an argon laser procedure for the control of intraocular pressure. This technique, known as argon laser trabeculoplasty, has changed little since its original description.
Many theories attempting to explain the effect of the trabeculoplasty laser burns have emerged. It is now thought that a cascade of biological events that involves renewal of trabecular meshwork cells and accelerated turnover of the extracellular matrix, or tissues between the trabecular cells, enhance outflow through the trabecular meshwork following laser treatment. Argon laser trabeculoplasty is a relatively uncomplicated office procedure and has gained wide acceptance in the treatment of open-angle glaucoma. In approximately 80% of eyes treated with argon laser trabeculoplasty, a significant lowering of the intraocular pressure will be achieved.
However, the intraocular pressure-lowering effect will diminish over time and approximately 10% of initially successful treatments will fail with each year. In patients in whom the initial laser trabeculoplasty was successful, additional laser therapy may be warranted. A modification of this therapy, Selective Laser Trabeculoplasty, is also available to treat open-angle glaucoma.
What Laser Therapy is available for Angle Closure Glaucoma?
As described above, angle-closure involved the lens coming too close to the iris and blocking the passage of fluid into the front of the eye, where it drains into the circulation. Relief of the relative pupillary block allows the iris to move back and the anterior chamber drainage angle to open, allowing the escape of aqueous humor and lowering of the intraocular pressure. This is now commonly and simply done by fabricating a small hole in the iris with laser (laser iridotomy), which provides another route for fluid to enter the front of the eye. This equalizes the pressure between the posterior and anterior chambers and allows the iris to fall back to its normal anatomic position and away from the trabecular surface.
What Surgical Therapy is Available for Glaucoma?
Trabeculectomy or Filtering Surgery
Trabeculectomy is the most common operation for the control of elevated intraocular pressure in adult glaucoma. Various filtering procedures have been developed to shunt the aqueous humor from the anterior chamber to a reservoir under the conjunctiva on the surface of the eye. These procedures provide an alternative low-resistance pathway for aqueous humor egress from the eye. It is believed that the aqueous humor either filters through the conjunctiva from the reservoir, mixing with the tears, or it is absorbed by the blood vessels on the surface of the eye.
Postoperative management includes topical dilating drops and antibiotics for the first one to two weeks following surgery. Topical corticosteroids are also used to suppress inflammation. Corticosteroid therapy is thought to reduce scar formation and failure of the filtering bleb.
Youth, skin pigmentation, previous surgery, and secondary glaucoma greatly increase the risk of failure. The majority of surgical patients receive some form of additional chemical antimetabolite therapy, either during surgery by sponge application or post-operatively as subconjunctival injections. 5-Fluorouracil or Mitomycin-C is the most commonly used antimetabolite adjunct to trabeculectomy surgery.
In addition, a variety of artificial drainage devices are available that employ a plastic shunt tube to divert the aqueous humor from the anterior chamber into the space behind the eye, where it is reabsorbed. These glaucoma tube shunts are generally reserved for eyes in which trabeculectomy surgery has failed or in which failure is likely due to extensive scar tissue formation, such as neovascular glaucoma.