Selective Laser Trabeculoplasty

Selective laser trabeculoplasty (SLT), introduced by Latina and Park in the mid-1990s and FDA approved in 2001, has been a mainstay of glaucoma therapy for nearly two decades. SLT uses a 532-nm Q-switched, frequency-doubled Nd:YAG laser with a short-pulse duration of 3 ns. The quick-pulse duration allows SLT to achieve selective photothermolysis, which is the process by which energy can be selectively absorbed by a pigmented cell population within a tissue to cause only localized damage and permit target selectivity, thereby reducing collateral damage.¹ Selective photothermolysis is achieved due to the fact that the 3-ns pulse duration of SLT is quicker than the thermal relaxation time of melanin, which is approximately 1 µs. Therefore, SLT laser energy, which is selectively absorbed by melanin within the trabecular meshwork (TM), is not spread collaterally as a photocoagulation or thermal burn within the TM. The effect is “biologic” or “inflammatory” in effect or better described as selective photothermolysis.

The mechanism by which SLT lowers intraocular pressure (IOP) is not completely understood and is likely multifactorial.² Numerous studies have shown that SLT increases aqueous outflow through the TM.³ Since limited structural damage occurs to the TM, the mechanical and structural theories, which have been suggested to explain the mechanism of action of argon laser trabeculoplasty (ALT), do not fully apply to SLT. Moreover, SLT has been demonstrated to induce biological changes that modulate increased aqueous outflow through the TM, including changes in gene expression, cytokine secretion, matrix metalloproteinase induction, and TM remodeling.¹,⁴

Recent studies described light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy findings from cadaver eye sections that were treated with either ALT or SLT using power ranging from 0.4 to 2.0 mJ. Eyes treated with ALT demonstrated significant disruption of TM architecture. Eyes treated with SLT showed normal TM architecture on light microscopy, but transmission electron microscopy did show some disruption of TM cells with cracked extracellular pigment granules even at low power settings. On SEM, TM treated with high power (2.0 mJ) showed more obvious tissue destruction. This suggests that treatment with SLT does have the potential to cause structural damage to the TM and that dose titration remains important.²,⁵

INDICATIONS

When considering a laser procedure into the anterior chamber angle, it is critical to ensure that the patient has an open angle. POAG and OHT are the two strongest indications to consider SLT due to the fact that studies show positive predictors for success include the following:

Pre-laser IOP with higher IOPs being more predictive of greater IOP lowering.⁶,⁷
Fewer number of medications the patient is on at the time of the SLT. As with eye drops, typically SLT earlier in the course of therapy leads to more robust lowering of IOP.
Some research indicates that lack of prostaglandin analogue drop use at the time of SLT may increase chances of robust IOP lowering.⁸

Despite higher pre-laser IOPs being likely the most predictive factor of SLT success, there still is an indication for NTG. The IOP-lowering effect is more modest in NTG compared to POAG, with studies showing a 12% to 15% IOP reduction with 27% to 41% fewer medications one to two years postoperatively.⁹,¹⁰ Additionally, SLT also has a positive effect on diurnal IOP fluctuations, which may be an additional benefit in NTG patients.¹¹,¹²

Pigmentation in the TM has been shown in some studies to be another predictive factor for SLT success with heavier pigmentation being more predictive of greater IOP lowering.⁷ Patients with pigmentary glaucoma have similar success rates with SLT as patients with other types of open-angle glaucoma.²,¹³,¹⁴ However, caution must be taken when considering SLT in pigmentary glaucoma, as the high levels of pigmentation can cause an overproduction of the biologic or inflammatory SLT effect. Studies have suggested there may be an increased complication rate, including eye pain, inflammation, IOP spikes, and a greater need for surgical intervention, after SLT in patients with highly pigmented TM.²,¹⁴ In a case series of four patients with post-SLT IOP spikes lasting four days to three months, three patients had pigmentary glaucoma and the other had a
heavily pigmented angle. Three of these patients eventually required trabeculectomy. Lowering power settings and treating 180° or less may be necessary in patients with heavily pigmented angles.²,¹⁵

Patients with pseudoexfoliative glaucoma seem to have similar IOP-lowering efficacy, failure rate, and adverse event rate compared with patients with other openangle glaucomas. The presence of pseudoexfoliation does not seem to be a risk factor for IOP spikes or complications after SLT.²,¹⁶

Many patients with PAC or PACG will require additional therapy beyond the angle-closure treatment to maintain IOP control.²,¹⁷ One study examined subjects with at least 180° of open angle after peripheral iridotomy and found a 17% IOP reduction with SLT.¹⁸ Consequently, even though it is counterintuitive, SLT may be an additional IOP-lowering option in patients with PAC or PACG whose anterior chamber angles have been opened with angle-closure therapy and the TM remains healthy without synechiae.²,¹⁸

CONTRAINDICATIONS

The presence of less than 90° to 180° of visible posterior pigmented TM on gonioscopy likely is a contraindication for performing SLT. Numerous secondary glaucomas, including inflammatory, neovascular, angle recession, and juvenile, are either absolutely or relatively contraindicated due to the potential for worsening the condition (inflammatory glaucoma), needing other therapy (neovascular glaucoma), or likely noneffectiveness of treatment (angle recession and juvenile glaucoma).

The effect of SLT on the corneal endothelium may be transient, and long-term effects are probably negligible in normal corneas. However, in compromised corneas and corneas with pigment deposits on the endothelium, there may be a risk of corneal endothelial compromise, especially after repeated SLT.¹⁹ Therefore, it may be wise to limit the number of shots and energy when considering SLT in a patient with a compromised corneal endothelium.

INFORMED CONSENT CONSIDERATIONS

Informed consent should include a description of the procedure in plain language. For example, “A laser will be used to place gentle laser pulses in the drainage area of the eye. The laser energy will work on the pigmented area of the drain which in turn will recruit the body’s own natural immune cells to travel up to the area and clean out the debris in the drain. This will help lower the eye pressure which is the main known modifiable risk factor in glaucoma.”

Potential alternative treatments, such as eye drops, monitoring, minimally invasive glaucoma surgery, or invasive surgery, should be listed. Risks and complications inherent to any laser procedure, including SLT, include but are not limited to increased eye pressure; eye inflammation; irritation or pain during or after the procedure; temporary blurring of vision, swelling of the cornea, hyphema or microhyphema; as well as ineffectiveness of the procedure.

A note from the counseling clinician should be included and phrased similar to “I have counseled this patient as to the nature of the proposed procedure, the attendant risks involved, and the expected results.” The patient’s and doctor’s names should be printed and signed with the date as well as a witness signature (typically a staff member).

PREOPERATIVE CARE

If not already performed, angles should be viewed via gonioscopy to assess visible structures, amount of pigmentation in the TM and any pathology such as peripheral anterior synechiae.

The amount of pigmentation in the TM will affect starting energy level with lighter pigmented meshworks requiring higher energy levels and heavier pigmented meshworks requiring lower energy levels.

Approximately 15 to 30 minutes prior to the procedure, one or two drops of an alpha-agonist such as brimonidine (0.1%-0.2%) or apraclonidine 1% should be instilled into the selected eye to blunt any potential IOP spike. Pilocarpine 1% may also be used to help move the iris out of the angle and visualize more of the TM and angle structures. Immediately before the procedure, proparacaine should be instilled into both eyes; instillation in the fellow eye will help control blinking during the procedure.

SETTINGS AND PROCEDURE

A laser lens (Latina lens or Rapid SLT lens) is required. The procedure has been studied and theoretically can be performed without a laser lens by applying transscleral laser pulses near the perilimbal scleral area over the area of the TM.²¹ This technique however is experimental and has not reached widespread clinical practice. The use of a lens has several advantages: (1) it helps stabilize the eye and minimize eye movements and prevents the upper lid from blocking the doctor’s view; (2) it helps to focus and concentrate the laser energy; and (3) it provides an easy and inconspicuous method of staunching a rare angle bleed and potential hyphema. The most effective way to stop bleeding is to apply gentle pressure with the laser lens for 30 to 60 seconds.

A small amount of cushioning agent should be instilled into the lens well. While 1% carboxymethylcellulose (Celluvisc) drops can be used, a gel formulation such as Genteal or Systane gel (both hypromellose 0.3%) has the advantage of better contact
with the SLT lens and less potential for air bubbles thus providing a better view through the lens. After applying the laser lens to the patient’s anesthetized eye, the initial treatment mirror should be placed in the initial treatment position.

SLT Settings

Due to the laser spot size and pulse duration being internally fixed and not adjustable for the doctor, there are less laser settings to dial in for an SLT compared to other laser procedures. Typical starting energy setting is between 0.8 and 1.0 mJ and is adjusted based on TM pigmentation. Angles with heavy pigmentation (grade 3 or 4) may require titrating the energy down to 0.4 to 0.7 mJ depending on the patient and tissue reaction during the procedure. Angles with light pigmentation (trace pigmentation or less) may require titrating the energy up to 1.1 to 1.4 mJ or more.

As with every laser procedure, the laser shot counter should be reset or “zeroed” before starting, with the last thing done before beginning the procedure is turning the laser on.

The Procedure

Key Procedural Points

Magnification—12.5 to 20x (medium mag)
Illumination tower variable to wherever produces the best view
Laser lens mirror placement:
- Latina lens 1 mirror placed at 9:00 (for clockwise rotation) or 3:00 (for counterclockwise rotation) if desiring to treat the inferior 180° first which is typical
- Rapid SLT lens 4 mirrors placed at 12:00, 3:00, 6:00, and 9:00
Approximately 100 nonoverlapping laser pulses for 360° treatment, and 50 laser pulses for 180° treatment
Desired tissue reaction = lowest laser energy, which produces champagne bubbles approximately 50% to 75% of the time
360° treatment for POAG, OHT, NTG
180° treatment for pigmentary or pseudoexfoliative glaucoma

Many of the same skills and slit lamp settings necessary for gonioscopy are utilized in SLT. Magnification is usually high enough to visualize angle structures such as the TM but not too high as to cause the treating clinician to lose orientation. The illumination tower should be positioned in a manner that gives the best view possible with minimal glare and reflections. Clinically speaking, it has been the author’s experience that the illumination tower on the opposite side of the mirror (e.g., working
in the right mirror with the illumination tower slightly to the left) generally gives the brightest, clearest view while minimizing reflections and glare.

Laser lens placement on the eye depends on which laser lens is being used. Traditionally, the inferior 180° is the first half of the eye treated as it is the most open half of the angle and usually contains the most pigment in the TM. With the 1-mirror Latina lens (Figures 12.1 and 12.2), the mirror should initially be placed at 9:00 (if moving clockwise) or 3:00 (if moving counterclockwise). The single-aiming beam should be placed to cover the entire width of the TM, and 8 to 12 nonoverlapping laser pulses are usually placed in the mirror followed by rotating the mirror to the next location. The Latina lens typically requires 8 to 12 mirror rotations during a 360° treatment session. With the 4-mirror Rapid SLT lens (Figures 12.3 and 12.4), the mirrors should be placed at 12:00, 3:00, 6:00, and 9:00. It is recommended to start in the 12:00 mirror since it provides a direct view of the inferior TM and continue to the 3:00 mirror, 6:00 mirror, and then 9:00 mirror. The aiming beam should again be placed to cover the entire width of the TM (Figure 12.5), and 10 to 15 nonoverlapping laser pulses should be placed in each mirror, giving approximately 50 shots after completing the 4 mirrors. If treating 360°, the Rapid SLT lens should then be rotated 45° one time to place the 4 mirrors in the oblique locations, and treatment continues with the 10:30, 1:30, 4:30, and finally 7:30 mirror locations. Approximately 90 to 110 laser pulses are typically placed for a 360° SLT treatment session, with 45 to 55 laser pulses for a 180° SLT treatment session.