Technique

Patient preparation

Preoperatively, the patient maintains their usual medication regimen. Additionally, the surgeon may choose to use pilocarpine 2% 30 minutes to 1 hour prior to the LTP procedure, which can help to further expose the trabecular meshwork and prevent IOP spikes. Topical 2 agonists are the most effective preventive for postlaser IOP spikes. Topical fluorescein, often used in conjunction with Goldmann applanation tonometry, is a chromophore for most LTP wavelengths; therefore, preoperative IOP is measured with a tonometer not requiring fluroescein or a suitable time period is allowed to elapse between IOP measurement and laser treatment. The procedure is performed under topical anesthetic.

Procedure

General preparation for LTP should be the same as that for any laser treatment of the anterior segment (see Ch. 29). While the patient is seated at the laser–slit-lamp system, a coated three-mirror Goldmann gonioscopy lens or a specially constructed trabeculoplasty lens such as the Ritch or Karichoff lens is attached to the eye with methylcellulose gonioscopy solution (e.g. Goniosol®). For SLT, a Latina or other suitable SLT lens is used to view the angle. The surgeon examines the entire circumference of the angle to ensure proper orientation in the trabecular meshwork and good visibility of the angle structures. In order to identify markers, it can be helpful to locate a pigmented portion of the trabecular meshwork or the scleral spur (see Ch. 6). The helium–neon aiming beam is focused onto the pigmented trabecular meshwork.

Laser trabeculosplasty.  The laser delivery system is first calibrated to ensure a focal spot of 50  m.This 50  m spot is used with 0.1 second exposure time.45 The power is titrated to the visible effect. In an eye with moderate or average trabecular pigmentation, the initial power of 400–500 mW is increased in 100-mW increments (to a maximum of 1000 mW) until a slight blanching effect occurs or small bubbles form at the point of laser impact. Less energy is required for more heavily pigmented angles. Reducing the power below 500 mW in lightly pigmented eyes, however, seems to decrease the treatment’s effectiveness.46

Orientation may be difficult in patients with little or no pigment in the angle. The surgeon should clearly identify the region

between the anterior border of the ciliary body and Schwalbe’s line. The middle of the trabecular meshwork lies near the midpoint of this region. If possible the scleral spur should be identified and visualized 360°. Pigmentation anterior to Schwalbe’s line may resemble trabecular pigmentation and confuse the surgeon. Identifying and following the scleral spur helps to eliminate this confusion.The lens is held so that the laser beam is focused clearly as a sharp circular spot.Asking the patient to gaze toward the mirror (e.g., if the mirror is to the right during the examination the patient is asked to look to the right) may assist in viewing the angle, especially when the iris is convex. Poor or astigmatic focusing diffuses the laser energy and increases tissue damage unnecessarily.47 Although treatment at the level of the ciliary body, scleral spur, posterior meshwork, and anterior meshwork have all been proposed, fewer complications and equal effectiveness result from treating at the junction of the trabecular pigment band and anterior meshwork (Fig. 31-2).

Laser applications are then positioned 3–4° apart so that approximately 20–25 spots are created per quadrant. Debate continues over how many quadrants should be treated. There is less postoperative inflammatory response and fewer pressure spikes when there are fewer spots.48,49 Most physicians advise initial treatment of 180°.50 If only 180° is treated, the patient is re-evaluated in 4–6 weeks. If the IOP drop is inadequate, the remaining 180° is treated.26 If the IOP drop is adequate, the patient is observed until a lower IOP is necessary, at which time the second 180° is treated. A few patients will achieve a substantial drop in IOP with a second 180° treatment, even though treatment of the first 180° was disappointing. When considering eyes that have had the second 180° treated, there appears to be little difference between the long-term efficacy of initial treatment of 180° and 360° (Fig. 31-3).

The patient should be closely monitored on the day of surgery and on the first postoperative day for pressure spikes and iritis.The final effect of LTP may not be evident for 4–6 weeks.

Selective laser trabeculoplasty.  The 400- m spot size – compared to the 50- m spot size of ALT (Fig. 31-4) – covers most of the angle structures and iris root. However, the short pulse duration enables selective absorption by the intracellular melanin target. The other, non-pigmented, tissues irradiated by the beam are not affected.

The energy level for SLT treatment is set initially at 0.8 mJ.A test pulse is delivered at the set energy. If the surgeon visualizes bubble