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Practitioners begin to offer SLT as first-line glaucoma therapy
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Traditionally, the first line of intervention in the treatment of open-angle glaucoma has been, and for the most part still is, medical therapy. In 1979, Wise and Witter were the first to describe and introduce argon laser trabeculoplasty (ALT) for the treatment of several forms of open-angle glaucoma as an intermediate alternative to filtration surgery.
ALT first-line surgery since 1979
Since then, ALT has been the primary choice of intervention for open-angle glaucoma not controlled with maximum medical therapy. The standard continuous wave argon thermal laser (λ=488-514 nm) uses a spot size of 50 µm with a power of 1,000 to 1,500 mW for a pulse duration of 0.1 second. Due to the long pulse duration, heat is dissipated from the pigmented treatment area to the nonpigmented adjacent areas, leading to subsequent collateral damage.
The exact mechanism of ALT’s pressure-lowering ability is still controversial. Two theories, mechanical and cellular, have been proposed to play a role in the reduction of intraocular pressure. The mechanical theory suggests that the thermal coagulative damage from the laser causes trabecular meshwork shrinkage and contraction, leading to adjacent openings, resulting in increased aqueous outflow and decreased outflow resistance. The cellular theory states that the damage leads to activation of macrophages that phagocytose and clear the trabecular meshwork debris, thereby increasing outflow.
However, ALT has been known to cause irreparable permanent coagulative damage to the ultrastructure of the trabecular meshwork and its surrounding tissues, which may limit future re-treatments. Also, one of the most serious complications after ALT is a spike in IOP.
SLT introduced in mid-1990s
In 1995, Mark Latina and colleagues introduced a new technique to reduce IOP using a 532-nm frequency-doubled Q-switched Nd:YAG laser to selectively target the pigmented cells of the trabecular meshwork, hence the name selective laser trabeculoplasty (SLT).
In contrast to ALT, SLT uses a single-pulse, short duration of 3 ns (3 billionths of a sec) with a spot size of 400 µm and an energy level ranging from 0.2 to 1.7 mJ. The large spot size allows treatment to the entire height of the trabecular meshwork. Due to the short pulse duration, there is no coagulative damage to the adjacent untreated cellular network, as shown by histopathological studies, therefore making SLT a safe and repeatable procedure with good outcomes every time.
Kramer in 2001 published histological comparisons of ALT vs. SLT. Dramatic, crater-like defects in the trabecular meshwork were documented following ALT, but negligible crack-like defects were found after SLT. Much less dramatic comparisons were published by Cvenkel et al. in 2003.
ARVO studies on SLT
At the Association for Research in Vision and Ophthalmology (ARVO) meeting held in May 2003, scientists from all over the world presented their ongoing research studies, which included some very encouraging results on SLT.
Bovell and colleagues conducted a randomized clinical trial comparing the efficacy of ALT and SLT and found that SLT was equivalent to ALT in lowering IOP at 3 years.
Lamar and colleagues studied the thermomechanical effects on the trabecular meshwork of three different trabeculoplasty lasers. They used a diode-pumped, Q-switched, frequency-doubled Nd:YAG laser (532 nm, 0.1 s pulse duration, 50 µm spot size); a frequency-doubled, Q-switched, Nd:YAG laser (532 nm, 3 ns, 400 µm spot size: used in SLT); and a Ti:Al2O3 laser (790 nm, 7 µs pulse duration, 175 µm spot size). Electron microscopy revealed no observable structural damage to the trabecular meshwork with the Ti:A1203 laser, unlike the diode laser, which produced shrinkage and coagulative damage at all powers. An important finding was that the Ti:Al2O3 laser also did not produce structural or thermal damage when used at the threshold energy level. The authors therefore suggested the possible use of the Ti:Al2O3 laser as a safe, repeatable procedure similar to SLT.
Melamed and colleagues studied the safety and efficacy of SLT as primary treatment for open-angle glaucoma patients. They tested 45 eyes of 31 patients and found that the mean IOP change was 31.6% 12 months post-SLT with very few initial side effects. They concluded that SLT is a safe and effective treatment for ocular hypertension and open-angle glaucoma.
Hodge and colleagues compared the safety and efficacy of SLT to ALT in patients with pseudoexfoliative syndrome and found that at the end of 3 years, SLT lowered IOP by 8.0 mm Hg and ALT by 6.2 mm Hg.
Said and colleagues reported results on a 4-year prospective study on SLT. They found a 46.99% average IOP reduction after 4 years with SLT, making it a very effective treatment, and suggested that SLT could be an alternative to ALT.
Personal experience with SLT
We’ve had the opportunity to treat many patients (1,000+) this year with SLT, and the results are very promising. The initial trials on SLT were performed primarily on a Caucasian population, and the energy level required to reach therapeutic levels was 1.0 mJ or more. We have designed a new approach to treatment in a non-white population that uses a lower energy level (0.4-0.5 mJ) and multiple treatments.
We have used as low as 0.3 mJ in patients with a dense pigmented trabecular meshwork. Our SLT patients on average receive about two re-treatments of this low power energy level. On a rare occasion, we have given patients three re-treatments.
We are seeing a significant drop (range of 5 to 7 mm Hg) in IOP 1 week after initial treatment. The second treatment is done about 4 to 6 weeks after the first. The energy level is increased from 0.4 to 0.6 mJ. If the trabecular meshwork is very pigmented, we keep the energy level low.
The only complication noted in 10% of our patients treated with SLT was fleeting episodes of iritis. Peripheral anterior synechiae (PAS) has not yet been seen in our patients treated with SLT, unlike ALT where the occurrence of PAS is very high following treatment.
Due to the significant drop in IOP with SLT, our patients who have been newly diagnosed with glaucoma are given the option of SLT as the primary treatment regimen instead of glaucoma medication. The patients are educated about all risks and benefits of medical treatment as well as SLT, and about half of them choose SLT over medication. We highly recommend this procedure to our patients who are noncompliant with medication. Once this procedure is performed, compliance becomes less of an issue because the patient no longer needs to use eye drops. If the pressure is unchanged after the first treatment, the patient has the option of having the treatment performed again.
For Your Information:
- Jerome Sherman, OD, FAAO, is a Primary Care Optometry News Editorial Board member, the Distinguished Teaching Professor at the State University of New York and in private practice at the Eye Institute and Laser Center. He can be reached at SUNY, 33 W. 42nd St., New York, NY 10036; (212) 780-5004; fax: (212) 780-4980; e-mail: jsherman@sunyopt.edu.
- Shital V. Shah, OD, is an ocular disease resident who can be reached at SUNY College of Optometry; (212) 780-4007; e-mail: sshah@sunyopt.edu.
- Sanjeev Nath, MD, is surgeon director of the Eye Institute and Laser Center of New York and can be reached at 56 E. 66th St., New York, NY 10021; (212) 861-0800; fax: (212) 628-4065.