Laser Trabeculoplasty

Anatomy and Physiology

Anatomy of Angle of the Anterior Chamber

Clinically, the structures are visualized by gonioscopic examination. The structures forming the angle of the anterior chamber are as follows (posterior to anterior):

• Ciliary Body Band
  • It is formed by the anterior-most part of the ciliary body between its attachment to the scleral spur and iris insertion.
  • It appears to be a grey or dark brown band
  • on the gonioscopy. It is the most posterior landmark.
• Scleral Spur
  • This posterior part of the scleral sulcus is related to the ciliary body posteriorly and corneoscleral meshwork anteriorly.
  • It appears as a white band on the gonioscopy.
• Trabecular Meshwork
  • It appears as a band anterior to the scleral spur. It has 2 clinical parts: anterior non-pigmented and posterior pigmented. The posterior is the draining part; hence, it is pigmented.
  • It is responsible for 90% of the aqueous humor outflow.
  • It has 3 components:
    • The uveal meshwork: the innermost portion consists of cord-like endothelial cells, and the intertrabecular spaces are relatively large, around 70 microns in diameter, offering little resistance to the passage of aqueous matter.
    • The corneoscleral meshwork: lies anterior to the uveal meshwork and forms the thickest portion of the trabecular meshwork. The intertrabecular spaces are lesser, around 35 microns, offering greater resistance.
    • The juxtacanalicular meshwork: the outer part of the trabecular meshwork, lines the endothelium of the Schelmm canal. The intertrabecular spaces are around 7 microns, offering maximum resistance to aqueous outflow.
• Schwalbe Line
  • It appears as a fine ridge just in front of the trabecular meshwork, formed by the prominent end of the Descemet membrane of the cornea.
  • It marks the anterior limit of the structures forming the angle of the anterior chamber.[3]

Physiological Aspects of Outflow

The aqueous humor flows from the posterior chamber via the pupil in the anterior chamber. It exits the anterior chamber via the following routes:

• Trabecular meshwork - about 90% of aqueous flows through this route
• Uveoscleral pathway - about 10% of aqueous flows via the uveoscleral pathway
• Iris - some aqueous fluids also drain via the iris

Indications

Indications for laser trabeculoplasty include: 

• In patients with OAG, uncontrolled by medical therapy, the IOP remains above the target IOP despite the maximum medical therapy that can be tolerated.[4]
• In patients with OAG with poor compliance to medical treatment, the IOP remains above the target IOP, and the glaucomatous optic atrophy worsens.
• Patients with OAG with poor tolerance to medical therapy
• Patients with pseudoexfoliation or pigmentary glaucomas
• Angle-closure glaucoma with a patent iridotomy
• When the glaucoma surgery needs to be deferred or delayed

Contraindications

Contraindications for laser trabeculoplasty inclue: 

• Inflammatory glaucoma: post-laser inflammations will increase, leading to further IOP spikes
• Advanced glaucoma
• Poor visualization of the TM is due to synechiae, as the site of action is trabecular meshwork

Equipment

Laser System

• Argon green laser light is used in ALT
• Q-switched Nd: YAG laser is used in SLT

Lenses used: A contact lens with a mirror to visualize the anterior chamber angle (gonioprism) is used in laser trabeculoplasty. The lens should have an anti-reflective coating on its front surface.

• Goldmann type 3-mirror lens has 1 mirror inclined at an angle of 59°.
• Thorpe 4-mirror gonioscopy lens has all the mirrors are inclined at 62°. It has the advantage of simultaneous viewing of all the quadrants of the angle of the anterior chamber.
• Ritch trabeculoplasty lens has 2 mirrors are inclined at 59° to view the inferior quadrants and 2 at 64° to view the superior angles. It has a 17 diopter planoconvex lens which provides a magnification of 1.4, thus reducing a laser spot size.
• Latina lens- this lens is designed explicitly for SLT. It has a single mirror at a 63° angle. It has a magnification of 1.

Personnel

The team performing a laser trabeculoplasty may include:

• Attending and resident surgeons
• Nursing staff
• Operative room technicians

Preparation

History

A thorough medical and ocular history should be taken.

Examination

The preoperative examination must include:

• Gonioscopic examination of the angle of the anterior chamber- visibility of trabecular meshwork without indentation is noted as this is the target structure for trabeculoplasty. If the iris approach is steep, but the trabecular meshwork is visible by asking the patient to look towards the mirror, this implies sufficient angle area is available for treatment. Any synechiae should be ruled out because it will obstruct access to the trabecular meshwork, hence a contraindication to the procedure. The degree of pigmentation of the trabecular meshwork should be noted because it will influence the initial energy level chosen for the procedure.[5][6]
• Intraocular pressure measurement- to record a baseline IOP before the procedure.
• Central corneal thickness
• Optic nerve evaluation using slit-lamp biomicroscopy for glaucomatous damage
• Visual field charting

Medications

1.  IOP lowering agents- either apraclonidine or brimonidine, both being alpha-adrenergic agonists. It decreases the chance of an IOP spike in the immediate post-op period.[7]
2. Topical anesthetic agents –immediately before the procedure to anesthetize the eye.

Informed consent should be obtained before the procedure. The patient should understand all the risks, benefits, and alternatives to the procedure. Managing the expectations and addressing questions about the patient's procedure is essential. Also, the possibility of failure of the procedure should be explained.

Technique or Treatment

Mechanism Of Action  

Several theories have been proposed, and the mechanism is as follows:

1. Cellular - suggests that the reduction in IOP occurs due to the cellular activity stimulated by the laser; there is increased recruitment of macrophages in the trabecular meshwork, which aids in remodeling the extracellular matrix, thus allowing for increased aqueous outflow.
2. Cytokine production- there is increased expression and secretion of IL-1 beta and TNF alpha in the first 8 hours after treatment; these mediate increased trabecular stromelysin expression, which leads to remodeling of the juxtacanalicular extracellular matrix of the trabecular meshwork. This improves the normal outflow facility, thereby decreasing IOP.
3. The increased conductivity of the Schlemm canal- SLT leads to a 3-fold increase in Schlemm canal cells conductivity, thus increasing the transendothelial fluid flow across Schlemm canal cells.
4. ALT- causes coagulative necrosis of the trabecular meshwork.
5. SLT- causes cracking of intracytoplasmic pigment granules and disruption of the endothelial cells of the trabecular meshwork

 Technique

Complications

The complications that can manifest with laser trabeculoplasty are as follows:

1. IOP rise - the most common complication in patients undergoing laser trabeculoplasty. The frequency of IOP spikes can be reduced by two-thirds using prophylactic alpha-adrenergic agonists about 30-60 minutes before the procedure.[9] A severe and frequent IOP rise occurs with:
   • The use of higher energy levels,
   • 360° angle treatments,
   • Posterior placement of laser beam,
   • Heavy pigmentation of the angle
   • A low aqueous outflow facility. These IOP spikes are usually transient, occur in the first hour, and resolve with medical treatment. However, in cases with sustained IOP rise, surgical management is needed.[10]
2. Low-grade iritis
3. Formation of PAS
4. Corneal edema- occurs due to HSV reactivation. The inflammatory cascade following laser therapy reactivates the virus. 
5. Hyphema

Clinical Significance

The effect of either type of laser trabeculoplasty decreases gradually over time. A prospective study that randomized patients to 180° of SLT versus ALT, did not find any statistically significant difference in IOP lowering between the 2 types of laser trabeculoplasty. The effect of trabeculoplasty on the diurnal curve shows that both ALT and SLT decrease diurnal IOP fluctuation. In different subgroups of patients of POAG, both ALT and SLT have been found efficacious compared to medical treatment. In pseudoexfoliation glaucoma, SLT is equally efficacious in pseudophakic eyes compared to phakic ones, whereas ALT is generally performed in phakic eyes. Laser trabeculoplasty is a cost-effective procedure that can be performed as an adjunct to medical therapy or to control IOP when the surgical treatment is delayed. It is gaining popularity as a primary mode of treatment in several OAG patients.[11][12]

Enhancing Healthcare Team Outcomes

Glaucoma leads to irreversible damage to the optic nerve, thus vision, which can adversely impact a person's life personally and socially. Hence, an interprofessional approach should be adopted to identify, treat, and delay glaucoma progression. The clinician should carefully examine to uncover early signs of glaucoma. If found, appropriate steps should be adopted. A physician in the periphery should refer the patient to a higher center for management. Patient counseling plays an important role. A combined effort by the treating doctor, nurses, and pharmacist should explain the necessity of seeking early treatment. The importance of medication compliance should also be presented. Family members of a patient diagnosed with glaucoma should be encouraged to get themselves screened. Patients undergoing laser procedures should understand the importance of the procedure and any potential complications. Any misconception regarding the laser procedure should be clarified, and the patient should receive an explanation regarding the safety and efficacy of the procedure. An interprofessional approach is key to addressing the global burden of glaucoma. There should be cooperation at each level to prevent the attack by glaucoma.