A topical hypotensive agent, most commonly brimonidine or apraclonidine, is given before the procedure to reduce the risk of a postoperative IOP spike. A gonioscope designed for laser treatment is placed on the eye with methylcellulose, allowing the surgeon to visualize anatomic landmarks, with the goal of recognizing the trabecular meshwork. Care must be taken to identify the Schwalbe line, as this line can be mistaken for pigmented trabecular meshwork in some patients. Individuals with a heavily pigmented trabecular meshwork have a higher risk of postoperative IOP spikes; thus, lower energy settings should be used for these patients.
Common laser settings are shown in Table 13-2. In argon laser trabeculoplasty, a beam is focused at the junction of the anterior nonpigmented and the posterior pigmented edge of the trabecular meshwork (Fig 13-1A). Care must be taken to avoid applying the laser too far posteriorly, as this will promote the formation of focal PAS. The power setting should be titrated to achieve blanching of the trabecular meshwork or small bubble formation. If a large bubble appears, the power is reduced and titrated to achieve the desired endpoint. Approximately 40–50 spots are applied over 180° of the TM. Usually, only half of the TM is treated to reduce the risk of postoperative IOP spikes and to allow for a second treatment if necessary in the future.
Table 13-2 Laser Trabeculoplasty Settings
Figure 13-1 Illustration shows positioning of laser spots on the trabecular meshwork for argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT).
Selective laser trabeculoplasty targets intracellular melanin. A frequency-doubled (532-nm) Q-switched Nd:YAG laser with a fixed 400-μm spot size and 3-nanosecond pulse is applied to the TM (Fig 13-1B), with laser energy titrated to the appearance of cavitation bubbles or just below that. Treatment can be applied to 180°–360° of the TM, but most surgeons treat 360° in 1 session. Typically, 80–120 spots are applied over 360°.
MicroPulse laser trabeculoplasty applies energy to the trabecular meshwork at the same wavelength as in SLT. Unlike SLT, MLT uses a thermal laser application that is substantially longer but has a 15% duty cycle, meaning that energy is delivered in a sinusoidal pattern, “on” for 15% of the application time and “off ” for 85%. This allows the tissue to cool during the application of each individual laser spot and may decrease the thermal injury to both the target and adjacent tissues. Approximately 120–140 confluent spots are applied over 360° of the TM.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.