BERMÚDEZ VALLECILLA MC, MORALES FERNÁNDEZ L, MARTÍNEZ DE LA CASA JM, GARCÍA FEIJOO J
Hospital Clínico San Carlos. Glaucoma Dept.
Laser trabeculoplasty (LTP) is a technique in which laser energy is applied over the trabecular mesh in the form of separate impacts, usually at the half angle (180º) in each treatment.
LTP has become a very important tool for treating open angle primary glaucoma (OAPG). This procedure reduces intraocular (IOP), improving the drainage of aqueous humor.
In 1979, Wise and Witer demonstrated that argon laser trabeculoplasty (ALT) could reduce IOP (1).
LTP comprises several modes such as argon laser trabeculoplasty (ALT) , diode laser trabeculoplasty and selective laser trabeculoplasty (SLT) (2).
Historically, LTP was indicated when it was necessary to reduce IOP in glaucoma patients with the maximum tolerated medical treatment and open angle. At present, most ophthalmologists continue to begin treatment with medication before opting for LTP, although in some cases the latter can be considered as a first stage of treatment. Patients who do not tolerate or complied with medical treatment could be candidates for LTP.
LTP reduces IOP in patients with open angle primary glaucoma (OAPG), pigmentary glaucoma and pseudoexfoliation syndrome. The response of aphakic and pseudophakic eyes could be poorer than phakic eyes.
When LTP is efficient, it usually reduces IOP 20-25% (2).
Since its appearance, ALT has been a valid tool for treating open angle glaucoma patients (3). It has been used as first choice treatment as well as in poorly controlled glaucoma as a stage prior to surgery (4).
The ideal patient for ALT is a patient with open angle or pseudoexfoliation glaucoma, over 60 and with good angular pigmentation. Initially, ALT is not indicated for advanced glaucoma patients with evident papillary and/or perimetric damage. Even so, this therapy can be a good option for very elderly patients or those who do not accept surgery.
Juvenile, uveitic, aphakic, closed angle, secondary, neovascular glaucoma or glaucoma with high onset IOP are not susceptible to LTP due to very small benefits or none at all (5).
Fig. 1: Angle with pigment deposit in pigmentary glaucoma.
Aqueous humor exit obstruction can be caused by several ocular structures. The trabecular mesh increases 2 or 3 times its thickness with age, mainly at the expense of collagen. There also is central area thinning with degenerative signs in collagen and elastic fibers. In addition, the trabecular mesh endothelium becomes thinner and loses cells, increasing intra-cytoplasmatic inclusions (6).
Several mechanisms have been proposed to improve drainage after successful LTP. It is said that the heat energy produced by the laser absorption by the pigments produces a contraction of the trabecular mesh collagen with subsequent shortening of the treated mesh, leading to increased space between 2 treated areas or expansion of Schlemm’s canal due to central mesh stretching (1).
The trabecular mesh cells also release chemical mediators, specifically interleukin 1b and tumor necrosis factor which improve the drainage of aqueous humor by inducing specific matrix metalloproteases (2).
The mechanical theory proposed for ALT does not explain how SLT works because the trabeculum does not exhibit the cicatrizations which account for the opening of the trabeculae. It is suggested that after treatment macrophages are captured and these eliminate cellular remains and extracellular melanine from the mesh. In addition, said macrophages would stimulate the production of cytokins which enhance the porosity of Schlemm’s canal walls and stimulate the syntheses of extracellular matrix.
After treatment with ALT and SLT the structural differences at the trabecular level have been studied with electronic microscope. After treatment with argon, the trabecular mesh exhibits craters that correspond to laser impacts and cause a complete destructuring of trabecular fibers and spaces. The intra-cellular pigment granules appear intact. On the other hand, after treatment with SLT, the trabecular architecture appears entirely respected without evidence of craters. The pigment granules of endothelial cells appear fragmented and dispersed, without evidence of coagulative damage (5).
Fig. 2: Pigment in posterior crystalloids in pigment glaucoma.
A ful clinical history and exhaustive ocular examination must be made. The angle must be open to gonioscopy. For LTP to be efficient, there must be pigment in the trabecular mesh and this in turn will determine the power to be applied. If more pigment is found on the trabecular mesh, lower energy will be required to obtain the desired effect.
The patient must be informed about possible complications and an informed consent must be signed.
Topical anesthetic must be administered before the procedure and the Goldmann lens or specific trabeculoplasty lens must be used to visualize the angle.
For ALT, a 50um and 0.1 seconds of exposure are focused through the lens at the juncture of the nonpigmented trabecular mesh and the posterior pigmented part. Treatment in the posterior trabecular mesh tends to produce inflammation, pigment dispersion, prolonged IOP elevation and anterior synechiae.
The power must be adjusted to achieve trabecular mesh whitening or formation of bubbles. If a large bubble is formed the power must be reduced.
SLT begins with 0.7 mj energy which is gradually increased to achieve the desired effect. The exposure time is preestablished at 3 ns, allowing the energy to be selectively captured by the pigmented cells of the trabeculum, respecting adjacent structures.
Fig. 3: Laser impacts on the trabecular mesh.
The spot is also predetermined at 400 um, significantly higher than in ALT. this spot occupies virtually all the angle, making the procedure easier to carry out even for ophthalmologists with limited experience (7). In both procedures sufficient introduction of IOP is achieved with less risk of initial hypertension if only half of the circumference is treated applying about 50 shots (8).
It is recommended to begin with the lower half, leaving a space of 2 spots between each impact.
Topical corticoid treatment 3 or 4 times a day during one week shall be prescribed. In this period an assessment shall be carried out.
The IOP increase can be significant but generally lasts under 24 hours. Increased IOP is more frequent if 360° are treated with high power. This symptom appears in about 20% of patients.
IOP increases are of particular concern in patients with advanced glaucoma damage.
It has been demonstrated that the use of 1% apraclonidine or 0.2% brimonidine prevent post-treatment hypertension. Oral hyper-osmotic agents can be useful if IOP does not go down with topical medication.
Between 1% and 3% of ALT patients experience permanent IOP increases and in most cases require filtrating surgery.
Anterior chamber inflammation
According to some studies there is greater inflammation in patients treated with SLT due to the large size of the spot which, in some cases, can impact the ciliary body on the base of the iris. Even so, a majority of studies have not found significant differences between the inflammation produced by both techniques or demonstrated lower inflammation due to the lower energy applied with SLT (9).
Bleeding is due to the application of laser on iridian root vessels and may cause hyphema.
Peripheral anterior synechiae (PAS)
The incidence of PAS varies between 20% and 40%. Normally these synechae are small and do not produce pathology although, if large, could cause partial obstructions of the trabeculum.
ALT as well as SLT are frequently applied for treating OAPG. Many studies have focused on the magnitude of IOP reduction for comparing both techniques and have reported similar results for both modes of treatment.
Damji et al found similar results in a six-month period and a sample of 36 eyes. A prospective study by Hollo found similar results and a further prospective study by Martínez de la Casa et al, which treated 20 eyes with ALT and 20 eyes with SLT, found that 6 months after treatment IOP reduction in both groups was similar. Finally, a larger series comprising 176 eyes by Damji reported equivalent success rates with IOP reductions above 20% at month 6 after treatment.
Comparing the magnitude of IOP reduction at each point in time after treatment, the initial IOP reduction (day 7) was higher in the group treated with SLT. In addition, this initial finding was a significant medium term success predictor for these patients, but not for those treated with ALT. It could be thought that this difference might be due to the specific action of the selective laser on the pigmented cells of the trabecular mesh. Whereas SLT stimulates the exit of aqueous humor without structural damage to the mesh, ALT first requires structural damage and subsequent healing of the tissue to increment the exit flow of aqueous humor, and this may take longer. This suggests that SLT is a better option for patients requiring faster IOP reduction after trabeculoplasty.
In summary, both SLT and ALT are safe, effective and equivalent techniques for IOP reduction in the treatment of OAPG. Larger reduction of initial IOP (first week) was observed in SLT patients, and this could be a good medium term success predictor for SLT patients but not ALT patients (1).
At least 6 weeks must elapse before assessing the results and deciding on the need of additional treatment.
Better results are obtained with elderly patients with OAPG and pseudoexfoliative glaucoma. Patients with pigmentary glaucoma may experience initial IOP reduction but, when pigment continues to be released, ocular hypertension may reappear (2).
Numerous studies demonstrate that baseline IOP is the best success predictor in laser treatments due to the finding that with a lower baseline IOP the laser will have lower effect (10).
A faster IOP reduction has been demonstrated with ALT in pseudo-exfoliative than in pigment glaucoma at the beginning but, in the course of the follow-up, results even out. This effect seems to be explained by the fact that the thermal effect produced by ALT is absorbed in angles with high melanine levels. With SLT, long-term results are similar with both types of glaucoma (11).
Some recent publications described the use of SLT as the first option in some patients with OAPG (12).
Studies demonstrate that long-term results for both treatments (5 years) are similar (13).
Re-treatment consists in performing laser treatment in an eye which was already treated through its 360°. The success rate is much lower and complications more frequent. It appears that with SLT the results of re-treatments are better than with ALT due to the lower destructuring that the Nd-YAG laser produces in the trabecular mesh.
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