Laser Surgery for Glaucoma: A Status Report

SLT has demonstrated significant advantages.

BY ANDREW RABINOWITZ, MD

As the landscape of healthcare continues to morph, and the burden of paying for medications shifts more to the consumer and away from the healthcare companies, patients and doctors are looking for alternative and cost-effective treatments to manage the glaucomas. One treatment that reduces the need for medications and that has regained popularity over the past five years is laser trabeculoplasty The procedure is currently performed using selective laser trabeculoplasty (SLT), argon laser trabeculoplasty (ALT) and mul-tipulse diode laser trabeculoplasty (MDLT).

In this article, I will compare the relative merits of the three types of trabeculoplasty, discuss my own preferences in using trabeculoplasty in practice, and explain why I believe that trabeculoplasty offers certain specific advantages in the current healthcare environment.

In the area of reimbursement, which is always a consideration when a major investment of capital such as purchase of a sophisticated laser is involved, all trabeculoplasty is billed under CPT code 65855. Since Jan. 1, 2008, trabeculoplasty is approved to be performed in an ASC if the laser is located in the ASC. Physician reimbursement can vary depending on whether the procedure is performed in an ASC (where a facility fee can also be claimed), a physician's office or a hospital outpatient department. On average, physician reimbursement is approximately $300 per treatment and the ASC facility fee averages about $136 nationally.

Increasing Use of Trabeculoplasty

When I first entered ophthalmology in the early 1990s, laser trabeculoplasty was used only with patients who had an uncontrolled intraocular pressure in spite of using three to four different types of medications on a daily basis. Frequently, these eye drops were dosed three to four times daily. It was not unusual for a glaucoma patient to have to instill medications 10 to 15 times a day. Our mentors taught us that if one medication was not able to lower intraocular pressure adequately, then a second medication be added. If a combination of two types of medication were inadequate, than a third and if necessary a fourth medication would be added. This was known as the “stepped care” approach to glaucoma management. It was not until the patient was “maximally medicated” that the option of laser trabeculoplasty was introduced.

In recent years, I have observed a profound paradigm shift with regard to application of laser trabeculoplasty in the management of glaucoma. Laser trabeculoplasty has evolved from a treatment applied only to advanced glaucomas to a treatment applied at all stages of the disease, including as an initial treatment when eye drops are not tolerated by the patient.

Increasingly, patients are inquiring about alternatives to eye drops as a method of controlling their glaucoma risk. Many patients are considering laser treatment for glaucoma earlier in the disease cycle due to the cost of obtaining the medications on a monthly basis.

Today, it is a far less common event for a patient to be placed on four medications before entertaining the option of laser trabeculoplasty. Both physicians and patients are responsible for this sea change.

From the physician perspective, it is clear that patient compliance falls dramatically when a patient s treatment regimen is upped from two to three types of medications.

Compliance with one drug regimen dosed once daily is dramatically higher than multi-drug regimens. Compliance with two drug regimens each dosed once daily is perhaps the plateau beyond which compliance falls off profoundly. Drug regimens requiring three or more drugs are widely acknowledged to be less firmly adhered to, and thus less likely to help patients achieve their target intraocular pressure.

Even conservative ophthalmologists will now entertain the option of laser trabeculoplasty when two drugs fail to lower intraocular pressure to the target level. Ophthalmologists who treat elderly patients with physical and mental limitations such as Parkinsons disease or Alzheimer's disease will frequently consider laser trabeculoplasty if the complexity or the demands of the drug regimen exceed the capabilities of the patient.

A growing number of doctors will offer laser trabeculoplasty in patients who do not attain an ideal intraocular pressure on one medication, and some forward-thinking surgeons will even offer laser trabeculoplasty if a patient fails to tolerate one medication.

From a patient perspective, laser trabeculoplasty offers the opportunity to treat their disease without the growing expense of multi-drug regimens. Many glaucoma patients have a number of concurrent diseases. They are responsible for purchasing their medications for blood pressure control, blood sugar control and for cardiac stability. Often, the added expense of eye drops creates an unacceptable burden. Fortunately, laser trabeculoplasty can allow patients to undergo adequate treatment at a more reasonable annual cost if they include trabeculoplasty in their treatment strategy. For the first time in my professional career, patients are now “requesting” SLT as a way of minimizing their need for eye drops to lower intraocular pressure.

ALT Came First

It's productive to discuss the history of laser trabeculoplasty and the evolutionary progress from ALT to SLT.

ALT was first introduced in the management of the open-angle glaucomas in the late 1970s. By 1979, ALT had gained acceptance as an effective tool to use to lower intraocular pressure adjunctively to topical and oral medications. Over the subsequent two decades, argon lasers were routinely used to perform trabeculoplasty to enhance conventional aqueous outflow and thereby lower IOP

The mechanism by which ALT works has not yet been clearly elucidated. It is thought to lower IOP by causing an immediate “mechanical” effect on the trabecular meshwork. ALT produces a thermal stimulus to the targeted tissue. The primary result of this stimulus is the tightening of the trabecular meshwork — known as circumferential shortening. This procedure augments conventional aqueous outflow.

ALT may also promote IOP reduction by invoking a long-term metabolic effect on the meshwork. Together, the thermal and metabolic sequelae of ALT propelled laser trabeculoplasty to the forefront of glaucoma management in the final two decades of the twentieth century.

Because ALT works by causing thermal “burns” to the trabecular meshwork, it is thought to have only limited repeatability. This limitation is due to the fact that the thermal stimulus can result in trabecular scarring. Each session of ALT provides improvement in aqueous outflow, yet also results in irreversible damage to the trabecular meshwork cells.

In general, a patient can safely undergo two to three sessions of argon laser trabeculoplasty over a period of two to 10 years. Beyond the third treatment, the IOP reduction achieved with ALT is limited at best. It is also possible that repeated sessions beyond the third treatment of 180 degrees can lead to elevated intraocular pressure by causing excessive meshwork scarring.

An Advance to SLT

The lack of repeatability of ALT prompted the study and ultimate introduction of SLT. Unlike ALT, SLT uses much lower energy and thus does not cause irreversible tissue scarring (Figure 1).

Figure 1. ALT histology (left) shows permanent trabecular mesh-work damage while SLT histology shows no destruction of the trabecular meshwork.

SLT uses a Q-switched, frequency-doubled (532) Nd:YAG laser (Lumenis) that targets melanocytes in the pigmented trabecular meshwork. The pigmented trabecular meshwork is thought to be the outflow tract through which aqueous humor leaves the eye.

The mechanism of action of SLT is not entirely clear. It is thought to involve the release of cytokines (chemical messengers) that trigger macrophage recruitment. Macrophages are cells that work as large “garbage men” to remove debris that may be blocking the trabecular meshwork (outflow tract.) SLT results in a relative cleaning of the trabecular meshwork. This allows fluid to leave the eye more easily and thereby lowers eye pressure.

Unlike ALT, SLT treats the meshwork without causing thermal or coagulative tissue damage. SLT delivers laser energy in short “pulses” rather than ALT's continuous wave. The amount of laser energy applied to the trabecular meshwork by SLT is less than 1% of that applied by ALT. In addition, the larger SLT “spots” do not have to be as precisely placed as the smaller ALT spots (Figure 2).

Figure 2. The larger SLT spots can be placed in the area of the meshwork while the small ALT spots must be precisely placed to avoid peripheral anterior synechia.

The single most important difference between SLT and ALT is that SLT, due to its low energy and limited tissue damage, can be repeated for an apparently indefinite number of sessions. This potential for repeatability has prompted a sea change in the application of laser trabeculoplasty. Because this tool can be used in early, middle and later stages of glaucoma, its applications continue to expand in the fight against glaucomatous sight loss.

Mark Latina, MD, is widely credited as the father of SLT. Dr. Latina's efforts should be viewed as giant leap forward in the management of the open-angle glaucomas.

MDLT: A Third Option

A third type of laser can also be used to perform trabecu-loplasty. This laser is known as Multipulse Diode Laser Trabeculoplasty. MDLT uses pulsed diode laser energy to provide deposition of the laser energy into the deeper layers of the meshwork as opposed to the surface cells. MDLT does not result in a visible tissue response but can provide IOP reduction comparable to that obtained with SLT and ALT.

To date, there have been no major studies comparing ALT to SLT to MDLT. It is widely thought, however, that when any of these energies are delivered in a proper fashion to the desired tissues, the response is an excellent reduction in IOP. All three of these treatment modalities can result in clinically significant IOP reduction. The lower energies that are applied to the mesh-work by SLT and to a similar extent by MDLT suggest that these modalities can offer true long-term repeatability that was not possible with the thermally traumatic argon lasers.

A Key ALT Study

The landmark study that evaluated argon laser trabeculoplasty was the Glaucoma Laser Treatment Trial (GLT). The GLT was a large scientific study that was performed to attain a better understanding of the benefit of laser trabeculoplasty.

At the time the GLT was conducted, SLT and MDLT had not yet been introduced. Thus, the results of the GLT reflect only the benefits of A LT. We believe, however, that the benefits of SLT would be as good as or better than those obtained with ALT.

The results of the GLT study suggest several important facts:

1. Initial treatment with of newly diagnosed patients with open-angle glaucoma was at least as effective as initial treatment with eye drops.
2. Laser trabeculoplasty is relatively successful in patients more than 50 years of age with primary open-angle glaucoma, especially in cases associated with pseudoexfoliation and pigmentary dispersion.
3. Laser trabeculoplasty has poor success in eyes with congenital or juvenile-onset glaucoma, inflammatory glaucoma and post-traumatic glaucoma with angle recession injury.
4. The amount of IOP reduction obtained increases as baseline pressure increases.
5. Eye pressure tends to drift back toward the baseline following treatment.
6. The average benefit in eye pressure control lasts two to five years. By two years after ALT treatment, about 46% of treated patients remain better controlled. The failure rate is approximately 10% per year and, by 10 years after ALT, half of treated eyes have undergone filtering surgery.

SLT vs. ALT

A 1999 study published in the British Journal of Ophthalmology found that patients with previous ALT treatments showed a greater decrease in eye pressure when treated with SLT compared to eyes treated with a second ALT procedure. The study concluded that SLT was equal to ALT in lowering eye pressure for the initial six months after treatment.

The US trials for the SLT laser, which the FDA approved in 2001, found a mean pressure reduction of 3 mmHg at 26 weeks among treated patients. There are several ongoing studies in the United States and worldwide that are evaluating the long-term benefits of SLT. It is now widely accepted among glaucoma specialists that SLT and MDLT are at least “as effective” as ALT.

Indications for SLT

There are many reasons to use SLT in patients with elevated eye pressure and glaucoma. Many patients cannot tolerate the eye drops that are necessary to control their eye pressure. Some patients can tolerate their eye drops but have difficulty getting the drops into their eyes. Some patients have difficulty remembering to use their medications on a daily basis. Many patients who can tolerate the eye drops, and who can instill them on a regular basis, have financial difficulty obtaining the drops on a monthly basis. Thus, SLT offers an option for patients who for many reasons are not good candidates for the use of drops.

As patients age, they become less tolerant of multi-drug regimens. The historic “stepped-care” approach to adding eye drop upon eye drop to patients whose intraocular pressures are sub-optimally controlled has recently come under significant scrutiny. With appropriate application of low-energy, safe and repeatable SLT, patients can now be spared drug regimens that frequently expanded to three to four medications daily.

The absence of scarring, and the apparent repeatability, allow us to use SLT much earlier in the disease cycle than we traditionally used ALT. Historically, we reserved ALT until a patient failed maximal medical therapy (as many drops as a person could tolerate). Knowing that there is no limit to the number of sessions during which SLT can be performed allows us to use it sooner in the treatment cycle.

Patients who have undergone “complete” ALT treatment (360 degrees) remain excellent candidates for SLT. This is due to the fact that the mechanism of action of the two treatments is quite different. Unlike ALT, which, as earlier noted, causes thermal or coagulative changes to the trabecular meshwork, SLT's mechanism of action involves stimulating “chemical” reaction within the cells of the trabecular meshwork (the outflow channels of the eye.)

In my experience, I achieve the greatest impact by performing SLT in 180° sessions. I pre-treat patients with Iopidine 0.5% 30 to 45 minutes prior to the procedure.

Subsequent to the SLT, I instill one to two drops of prednisolone acetate 1% as well as another dose of Iopidine 0.5%. Although many surgeons do not use postoperative anti-inflammatory medications, I continue to use a four-day course of postoperative steroid or non-steroidal antiinflammatory eye drops.

There is significant debate among surgeons regarding the use of anti-inflammatory drops following SLT. Specifically, the mechanism of action of SLT is to invoke cytokine release, which results in enhanced aqueous outflow. Some surgeons believe that it is important to stimulate a low level of inflammation to maximize the cytokine release. These surgeons suggest that using anti-inflammatory drugs following SLT can blunt the impact of the procedure. I prefer to use antiinflammatory agents routinely following ALT, SLT and MDLT. I have not found that the results of the procedure are compromised with the use of these agents. In fact, I find that the use of a short course of steroid drops (QID x four days) allows the patient to recover from the procedure with minimal inflammatory discomfort. It is important to note, however, that I have not performed a randomized comparison of the results of SLT with and without the use of postoperative anti-inflammatory drugs.

More Studies Are Needed

It will be important in future studies to compare the efficacy of SLT compared to ALT and MDLT. Additionally, prospective, randomized study of SLT with and without the use of postoperative anti-inflammatory medications will be helpful in creating national and international standards. Until such standards evolve, it will be helpful for surgeons to share their successes and surprises in applying laser trabeculoplasty to the glaucomas.

In addition to improving IOP control during the daytime hours, it is possible that the enhanced outflow achieved with laser trabeculoplasty may provide improved intraocular pressure control in the overnight hours when pharmacologic agents may not provide adequate intraocular pressure control.

In my experience, many patients who undergo SLT achieve dramatically improved intraocular pressure control. Surprisingly, however, many patients also, suggest that their sight is also qualitatively improved following successful treatment. It is not clear whether this subjective improvement is a placebo effect or whether there is actual improvement in functional vision. This comment from patients, that they see better following the SLT treatment, occurs more frequently than would be expected. Although I have not compared pre-treatment visual acuity, visual field function and contrast sensitivity with post-treatment values, I think that a prospective, randomized study of the objective and perhaps subjective changes would be of value in gaining a better understanding of the role of laser trabeculoplasty in the management of glaucoma.

When Trabeculoplasty Is Not Enough

When a glaucoma patient reaches a point in the disease when a combination of medications plus laser trabeculoplasty are not able to adequately lower intraocular pressure, the discussion turns to surgical alternatives. Classically, a trabeculectomy or perhaps an aqueous shunt would be the most likely initial surgeries to discuss with the patient.

However, these two procedures carry much greater risks than laser trabeculoplasty. Additionally, these surgeries must be approached with great caution in patients with other ocular diseases such as diabetic microvascular disease as well as incipient cataracts. Any glaucoma surgery, even cases that go well, can alter the patient's functional vision. The alteration in vision following successful glaucoma surgery can range from mild to severe and from transient to permanent. It is imperative that the patient be aware of these surgical consequences even in the face of what we would consider “successful” glaucoma surgery.

In hopes of creating a less “dangerous” glaucoma surgery, the team at iScience has crafted the iStent canaloplasty procedure. This procedure attempts to lower IOP by enhancing outflow through an intact Schlemm's Canal. Success with this procedure would allow for improved IOP control without the creation of a filtering bleb, as well as with a lower risk of postoperative hypotony. Finally, there is ongoing work with the Trabectome device from NeoMedix that attempts to increase trabecular outflow without creation of a filtering bleb. Both the iScience canaloplasty and Trabectome continue to offer promise for advancing glaucoma treatment.

While surgeons continue to attempt to advance the surgical science of glaucoma treatment, there exists an excellent, repeatable, and reliable tool known as laser trabeculoplasty. With proper technique and patient selection, SLT, MDLT and ALT can be used to improve intraocular pressure control in patients with elevated IOP. OM

Andrew Rabinowitz, MD, is a glaucoma specialist at Barnet Dulaney Perkins Eye Centers, a multi-location practice based in Phoenix. He can be reached via e-mail at andrewrabinowitz@aol.com. He has no financial interest in any product mentioned in connection with this article.