Argon laser trabeculoplasty

Indications for laser trabeculoplasty

Laser trabeculoplasty has been shown to be effective in reducing intraocular pressure in uncontrolled open-angle glaucoma, both primary and secondary. Primary open-angle glaucoma, normal-pressure glaucoma, pigmentary glaucoma, and pseudoexfoliative glaucoma respond best to this treatment. In juvenile glaucoma and secondary glaucomas, such as neovascular and inflammatory, the results of laser trabeculoplasty are generally worse. The necessary conditions are transparency of the ocular media and good visibility of the trabecular meshwork. Corneal opacity and developed peripheral anterior synechiae may impede laser surgery. To perform laser trabeculoplasty, it is necessary to master the technique of gonioscopy and clearly recognize the structures of the anterior chamber angle.

Laser trabeculoplasty technique

Since the introduction of argon laser trabeculoplasty (ALT) in 1979 by Witter and Wise, the technique has undergone only minor changes. Dots measuring 50 µm are applied to the trabecular meshwork with energy up to 1000 mW, sufficient to cause minimal pigment bleaching. A minimal amount of energy is used to destroy the tissue.

Laser coagulants should be applied at the border of the pigmented and non-pigmented parts of the trabecular meshwork. One operation with application of about 100 points over the entire 360° circle or two operations with 50 points applied over 180° semicircles can be performed. One- or three-mirror Goldmann goniolenses or Rich goniolenses are used during this operation.

To minimize the likelihood of transient intraocular pressure peaks, local a-adrenergic agonists (apraclonidine and brimonidine) are prescribed before and after surgery. To prevent inflammation after laser treatment, a glucocorticoid is used locally 4 times a day for a week.

The patient's intraocular pressure is measured 1 hour after the operation. If the intraocular pressure peaks, carbonic anhydrase inhibitors or hyperosmotic drugs are prescribed orally. The patient is re-examined 1 week and 1 month after the intervention. During the last examination, a conclusion is made on the effectiveness of laser therapy.

Mechanism of action of laser trabeculoplasty

The developed theories of decreasing intraocular pressure using laser therapy have not been confirmed. Probably, the degree of pigmentation of the trabecular meshwork is of decisive importance for the successful outcome of laser trabeculoplasty. Expressed pigmentation is a good harbinger of a successful operation. Histologically, it has been shown that thermal action of an argon laser causes melting and deformation of the trabecular bundles. According to the first theory, these contraction burns in the angle area mechanically promote a wider opening of the trabecular meshwork bundles, thereby facilitating the outflow of moisture. According to the second theory, laser irradiation stimulates the division of endothelial cells of the trabecular meshwork. Since these cells act as phagocytes in the angle area, it was believed that endothelial cells clear the intratrabecular spaces from detritus, which can be the cause of impaired outflow of intraocular fluid in glaucoma.

The effectiveness of laser trabeculoplasty

After argon laser trabeculoplasty, intraocular pressure usually decreases by 20-30% of the initial level. Not all patients respond to laser trabeculoplasty. Positive prognostic factors for a satisfactory response are: pronounced pigmentation of the trabecular meshwork, age (older patients) and diagnosis (pigmentary glaucoma, primary open-angle glaucoma and exfoliation syndrome).

Over time, the effect of argon laser trabeculoplasty fades. In long-term studies (5-10 years), the lack of effect of argon laser trabeculoplasty was observed in 65-90% of cases. Reoperation after full circular argon laser trabeculoplasty gives at best a short-term effect with 80%

By fading within a year. Due to structural damage to the outflow system during argon laser trabeculoplasty, repeated treatment may result in a paradoxical persistent rise in intraocular pressure. Gaasterland used repeated application of argon laser to the structures of the anterior chamber angle in animals to create an experimental model of open-angle glaucoma. In cases where rapid or significant (i.e., more than 30% of the pre-treatment pressure) reduction in intraocular pressure is required, argon laser trabeculoplasty is not the method of choice. Drug therapy or filtering surgery are better suited to achieve such goals.

The current treatment algorithm for glaucoma in the United States is to start with medications, then argon laser trabeculoplasty, and finally filtration surgery. This algorithm is only a guideline; treatment should be individualized for each patient to ensure optimal results. There are studies that have re-examined the effects of some open-angle glaucoma treatments. The GLT study compared argon laser trabeculoplasty with medications as the initial treatment for newly diagnosed primary open-angle glaucoma. After 2 years, 44% of patients who underwent argon laser trabeculoplasty alone were followed up, compared with only 20% of patients who were treated with timolol. In a follow-up study with a mean follow-up of 7 years, 20% of patients who underwent argon laser trabeculoplasty were followed up, compared with 15% of patients who received timolol. Although there were methodological flaws in the design of this study, it confirmed that, at least for certain patients, argon laser trabeculoplasty may be an initial treatment option.

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