Excimer Laser Trabeculostomy

31 Excimer Laser Trabeculostomy

Michael S. Berlin

Case Presentations

Case 1: Gonioscopic Procedure for Excimer Laser Trabeculostomy Alone

A 71-year-old woman presented with open-angle glaucoma (OAG) and cataract in both eyes (OU). Her right eye was uncontrolled with maximally tolerated topical medications: latanoprost at bedtime and dorzolamide and timolol fixed-combination b.i.d. with an intraocular pressure (IOP) of 28 mm Hg. Her corrected visual acuity (VA) was 20/40 OU. Her visual field and optic disk cupping at 0.7 were consistent with moderate glaucomatous loss, with both showing progression. Surgical options were discussed, including lensectomy, trabeculectomy, lensectomy combined with trabeculectomy, and excimer laser trabeculostomy (ELT) with the expectation that eventual lensectomy might be forthcoming.

Excimer laser trabeculostomy under gonioscopic observation was performed on the patient’s right eye. Ten channels were created through the trabecular meshwork (TM) into Schlemm’s canal (SC) following the current 10-channel protocol.

The postoperative IOP was 21 mm Hg on day 1, whereas postoperative therapy included dexamethasone and Garamycin. Glaucoma medications were discontinued. Postoperative gonioscopy revealed laser-induced effects within the trabecular meshwork. Initially, the holes appeared round but over time appeared ovular with slight pigmentation around the edges. At 1 month post-ELT, the patient’s IOP was 16 mm Hg with no medications. At 3 months, her IOP was 14 mm Hg without medications. At 1 year, her IOP was 12 mm Hg, which was a 57% reduction from her preoperative IOP. Two years later, with visible crater openings into the SC and an unmedicated IOP of 16 mm Hg, the patient underwent lensectomy with intraocular lens (IOL) implantation. Five years later, her post-lensectomy IOP has remained within the range of 15 to 17 mm Hg, a 39% decrease in IOP that has been sustained without medications, even after a second intraocular surgery (Fig. 31.1).

Case 2: Combined Phacoemulsification and Gonioscopic ELT Procedure

A 73-year-old man presented for lensectomy with a dense cataract and OAG in his left eye with marked visual field (VF) defects and significant disk cupping, with a cup-to-disk ratio of 0.8. When first examined, his IOP was 25 mm Hg on the α-agonist Brimonidine. He was stable on a medication treatment regimen for nearly a decade with no further progression. At the time of cataract surgery, he was offered the option of adding ELT to potentially eliminate his need for postoperative glaucoma medications.

Phacoemulsification was performed through a 2.4-mm clear corneal incision without complication followed by ELT. The postoperative therapy included dexamethasone phosphate 0.1 mg/mL plus tobramycin 0.3 mg/mL fixed-combination eyedrops administered q.i.d. over a period of 4 weeks. His preoperative washout IOP was 27 mm Hg. His postoperative IOP was 12 mm Hg on day 1, 11 mm Hg at 1 month, 12 mm Hg at 1 year, 14 mm Hg at 2 years, and 13 mm Hg at 3 years, a 53.0% decrease in IOP. The patient’s VA improved from the preoperative 20/100 to 20/20 at 1 month, 20/25 at 1 year, and remained stable for the entire 3 years of follow-up. In addition to stable VA, the patient’s IOP has also remained stable over the past 3 years, and the patient has not required the use of any topical hypotensive medication during this period. Although the primary diagnosis and treatment was for a mature cataract, the addition of an ELT procedure was anticipated to lower the IOP more than cataract surgery alone to reduce the patient’s topical medication requirement. This goal was successfully achieved (Fig. 31.2).

Case 3: Combined Phacoemulsification and Endoscopic ELT Procedure

An 83-year-old man presented with moderate cataract and early to moderate OAG in his left eye on maximum tolerated medications including prostaglandin analogue, β-blocker, and carbonic anhydrase inhibitor, with progressive glaucomatous VF defects. His IOP was 15 mm Hg in his left eye on presentation. He had been treated with medications for over a decade, increasing from monotherapy to his current regimen. However, even on maximal medications, his VF defects continued to progress. He elected to proceed with phacoemulsification combined with endoscopic ELT. Preoperative washout IOP in his left eye was 29 mm Hg.

After combined phacoemulsification plus ELT, the patient’s vision improved from 20/40 to 20/25 and his IOP was 14 mm Hg without glaucoma medication at 1 week after the procedure and remained stable for over 2 years with no further progression of VF defects. His immediate postoperative therapy included dexamethasone phosphate 0.1 mg/mL and tobramycin 0.3 mg/mL fixed-combination eyedrops q.i.d. tapered over a period of 4 weeks. The ELT procedure in this patient was performed endoscopically, which often enables better visualization of the TM than a gonioscopic approach. In combined cases in which ELT follows rather than precedes lensectomy, endoscopic ELT is considered a better alternative than gonioscopic ELT due to the possibility of corneal edema or Descemet’s folds at the end of the cataract surgery, as is often seen following the removal of a dense cataract with a hard nucleus. In addition, the 2.4-mm clear corneal tunnel for the phacoemulsification readily enables an endoscopic approach without further enlargement of the incision.

Even though this patient’s cataract was considered only moderate, a combined procedure was preferred due to his progressive glaucomatous field loss while on maximum tolerated medical therapy. It was anticipated that cataract removal with an ELT procedure would reduce both the IOP and the medication requirements. An endoscopic approach for ELT is also preferred when visualization of the anterior chamber angle through a goniolens is inadequate, such as in patients with advanced corneal scarring, band keratopathy, or a failed corneal graft (Figs. 31.3 and 31.4).

The Procedure

Excimer laser trabeculostomy is a minimally invasive glaucoma surgery (MIGS) procedure. A 308-nm xenon chloride excimer laser is utilized to remove the tissue obstructing aqueous outflow due to its precision and its effectively nonthermal laser/tissue interaction properties. Additionally, ELT reestablishes physiological aqueous outflow through the TM in a manner that does not provoke a healing response, and consequently enables a sustained reduction in IOP. The ELT procedure is performed via a clear corneal incision in which direct visualization of the target tissue, using a goniolens or an endoscope, provides immediate feedback to the surgeon. ELT, when compared with more invasive glaucoma surgical procedures such as trabeculectomy, is almost as efficacious in both lowering the IOP and decreasing the need for pressure-lowering medication, while being far less traumatic. The sparing of conjunctiva is a major advantage of this technique, because the option of performing a subsequent trabeculectomy, if necessary, would not be precluded. This MIGS procedure, another option in the armamentarium of the glaucoma and cataract surgeon, offers a robust safety profile, rapid stabilization of IOP, and clinically verified long-term efficacy with minimal negative impact on the patient’s quality of life. As such, it may become a replacement option for topical glaucoma therapies, which are associated with high costs, compliance issues, and topical and systemic side effects with long-term use.

The ELT concept is similar to that utilized by 193-nm ultraviolet (UV) excimer lasers for corneal surface ablation. The 193-nm wavelength enables precise removal of corneal tissue, facilitating successful refractive surgery without thermal degradation of the corneal collagen to ensure clarity. However, this 193-nm wavelength is not useful for intracameral procedures because it is absorbed by the cornea and is not readily transmissible by fiber optics. In contrast, the 308-nm UV excimer-laser–generated light is fiber-optic transmissible, and after extensive preclinical experimentation it became the wavelength of choice for nonthermal, precisely targeted, ab interno fistulizing procedures and subsequently for ELT procedures. The initial ocular application of 308-nm excimer lasers was for nonhealing full-thickness ab interno sclerostomy. ELT was developed after careful measurements of the distance between the anterior chamber (AC) and the SC were taken and laser/tissue interaction details were quantified, enabling calculations for the creation of nonhealing ELT channels.1

Prior procedures aimed at eliminating the outflow obstruction localized to the juxtacanalicular trabecular meshwork (JCTM) and the inner wall of the SC region attempted to use mechanical devices and thermal lasers to perforate the TM. These have been shown to adequately bypass the outflow obstruction for the short term. However, they have been unsuccessful in the long term due to the amount of adjacent tissue damage related to the nature of the technique or device. Adjacent tissue damage evokes a healing response that eventually closes the openings. As laser technology evolved, several novel lasers were similarly used, but none enabled long-term patent openings. Krasnov2 reported moderate success using a 943-nm ruby laser to perform “trabeculopuncture.” Other laser trabeculopuncture attempts have included Hager’s3 use of an argon laser (488 + 514 nm) and Fankhauser’s group’s4 use of a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser (continuous wave of 1,064 nm). All have been unsuccessful due to early and late postoperative scarring. These and other laser trabeculopuncture attempts also limit prospective options for subsequent procedures,5,6 as they induce destruction of local tissue and scar formation. In addition, with large openings and markedly increasing aqueous outflow, the compositional alterations of the aqueous humor augment the tissue-destructive healing responses.

Unlike ELT, current clinical laser treatments for glaucoma are based on procedures to modify the TM’s function without physically bypassing TM flow obstruction. Following initial attempts using lasers ab interno to perforate the TM or to create full-thickness sclerostomies, Wise7 found that a continuous-wave, longpulsed, argon laser (488 + 514 nm) could successfully modify the TM to increase outflow without perforation. Their argon laser trabeculoplasty (ALT) procedure effectively lowered the IOP, but it functioned by creating thermal damage to the target tissue, causing coagulative necrosis of the TM.7 In contrast to ALT procedures, laser trabeculoplasty (LTP) is now more commonly performed with solid-state (532 nm, frequency-doubled Nd:YAG) and diode (810 nm) lasers.

Studies comparing the efficacy of these lasers demonstrate minimal differences in efficacy, longevity, or repeatability. The efficacy of LTP in lowering the IOP has been well documented in the literature.810 However, long-term studies have shown that the IOP-lowering efficacy of LTP decreases over time from a 77% success rate at 1 year, to 49% at 5 years, and finally to 32% at 10 years.11

Alternatively, selective laser trabeculoplasty (SLT) relies on selective absorption of short laser pulses to generate and spatially confine heat to pigmented targets within TM cells.12,13 Based on the principle of selective photothermolysis, SLT uses a Q-switched, frequency-doubled 532-nm Nd:YAG laser. Laser Q-switching enables an extremely brief and high-powered light pulse to be delivered to the target tissue, which is intracellular pigment. Intracellular energy absorption and the short duration of the pulse are critical in preventing collateral damage to the surrounding tissues.14

The ELT procedure was developed with the goal of creating a long-term, nonhealing, anatomic modification of the TM to bypass outflow obstructions. It was finalized once the parameters of the target tissue anatomy,15,16 localization of SC, and ablation rates for the 308-nm wavelength used on target tissue were determined. Target tissue anatomic considerations must specifically address decreasing trauma to the outer wall of the SC, so as to minimize healing responses. The outer wall endothelium contains fibroblasts, whereas the inner wall endothelium does not. Avoiding trauma to the outer wall is paramount to the successful long-term maintenance of outflow. Another anatomic consideration is the space between the inner and outer walls of SC, which can be less than 20 µm. The accuracy of a tool used to perforate the inner wall, such that it does not disturb the outer wall, must be of this same scale. The laser penetration depth is fixed by the number of pulses, similar to the penetration depth control in laser-assisted in-situ keratomileusis (LASIK). Perforation of the inner wall of the SC depends on the canal’s distance from the fiber tip, which may vary due to the angle of placement and the amount of pressure on the fiber. This distance was determined by numerous preclinical experiments.17,18 The ablation precision of the 308-nm excimer laser on this tissue, with 1.2 µm of tissue removal per pulse, enables the ELT procedure’s efficacy.

In a study of the effects of 308-nm excimer laser energy applied ab interno to the limbal sclera of rabbit eyes, long-term decreases in IOP were achievable.19 The use of this 308-nm wavelength, unlike that of earlier procedures with thermal lasers, enables laser tissue interactions that are less likely to evoke a cicatricial response in the TM or sclera. In addition, direct tissue contact of the fiber-optic delivery system ensures minimal exposure of adjacent tissue to radiation and maximizes ablation efficacy. The development of current ELT technologies and techniques are based on evidence that the TM and scleral tissue could be successfully removed without adjacent tissue damage, scar formation, or channel closure. High ablation accuracy enables the precise targeting of the TM through the inner wall of the SC without perforating the outer wall of the SC.

Conjunctiva sparing is another advantage of ELT because subsequent trabeculectomy would not be compromised. ELT, when compared with more invasive glaucoma surgical procedures (i.e., trabeculectomy), is almost as efficacious in lowering the IOP and reducing the use of glaucoma medication.19

The ELT procedure potentially enables a pneumatic canaloplasty. Both endoscopic and gonioscopic views of ELT have revealed gas bubble formation in the channel tissue and around the probe tip in the anterior chamber as a result of photoablation of the TM tissue. It is theorized that this process dilates the SC. When the ablation penetrates the outflow obstruction, gas is able to enter the SC through the newly formed channels in the TM. The pressure of this gas dilates the SC, displaces the SC’s outer wall from the probe, and dilates adjacent collector channels to improve aqueous outflow, lowering the IOP. Observing gas bubbles exiting the adjacent openings confirms the continuity of flow from the SC. This hypothesis has yet to be confirmed via real-time imaging or histological studies. Such studies will improve our understanding of these pneumatic effects on the procedure and enable modifications to potentially further improve outcomes (Fig. 31.5).

Rationale Behind the Procedure

The MIGS procedures use a microincision of 1 to 2 mm and can easily be combined with cataract surgery. The microincision facilitates the intraoperative maintenance of the anterior chamber, retains normal ocular anatomy, minimizes changes in refractive outcome (neutral to induce astigmatism), and adds to procedural safety. The invasive character of trabeculectomy and the incidence of serious complications after the procedure limit its use to later-stage and recalcitrant glaucoma patients. Issues regarding medication costs, patient compliance, and toxicity of preservatives suggest that medicinal therapeutic options also have limitations. Thus, there remains an unmet patient need for treatments that could effectively treat mild-to-moderate glaucoma. In recent years, several MIGS procedures have been developed to fill the gap left by traditional treatment options. All have advantages and disadvantages, but no current technique fulfills all of the following requirements: (1) ab interno microincision; (2) minimal trauma; (3) efficacy (i.e., both lowering the IOP and reducing medication use); (4) rapid recovery; and (5) high safety profile.

The ELT was first used in a clinical setting by Vogel and Lauritzen20 in 1997 following preclinical development by Berlin et al.18 The ELT treats the primary pathology responsible for most OAG by decreasing the outflow resistance at the JCTM and the inner wall of the SC.20,21 The TM itself is responsible for 60 to 80% of aqueous outflow resistance.22 Using specified laser parameters, ELT evaporates human tissue by means of essentially nonthermal photoablation, thus denaturing the organic structures without producing undesirable marginal necrosis.23 ELT excises the uveoscleral, corneoscleral, and juxtacanalicular meshwork, as well as the inner wall of the SC without damaging the outer wall of the SC or the collector channels.24 No filtering fistula or bleb is created.17,25,26

The ELT surgical procedure is performed on an outpatient basis under local anesthesia (e.g., topical, peribulbar, or retrobulbar). Following paracentesis and stabilization of the anterior chamber with a viscoelastic agent, a fiber-optic probe is introduced and advanced across the AC and brought in direct contact with the TM (Fig. 31.6). Probe placement is controlled by direct observation using either a goniolens or an endoscope. In the current protocols, six to 10 channels are created into the SC (Fig. 31.7).

Oct 29, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Excimer Laser Trabeculostomy
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