Argon Laser Trabeculoplasty

From Kahook's Essentials Of Glaucoma Therapy
  1. Essentials of Glaucoma Therapy
  2. Lasers
  3. Argon Laser Trabeculoplasty
Primary authors
  • Elizabeth T. Viriya, MD
  • Joseph R. Zelefsky, MD

Argon laser trabeculoplasty (ALT) is a treatment option for open-angle glaucoma, involving the application of laser energy to the trabecular meshwork (TM) to lower intraocular pressure (IOP). It can be performed as first-line therapy or in conjunction with medical therapy. ALT can be used to lower pressure either before or after filtering surgery as demonstrated by the Advanced Glaucoma Interventional Study (AGIS).[1]

Efficacy

On average, ALT reduces IOP by 25% from baseline.[2] This reduction, however, wanes over time. Failure—defined as IOP reduction from baseline of less than 3 mm Hg, IOP greater than 20 mm Hg, or anatomical or visual field progression—occurs nearly 25% of the time in the first year after treatment and at a rate of 7% to 10% per year afterwards.[3]

Mechanism of Action

The exact mechanism of action for ALT is unknown, although ALT increases aqueous outflow up to 50%.[4] Two popular theories as to how ALT lowers IOP are the mechanical theory and the biological theory. The mechanical theory states that the thermal energy delivered by the laser leads to coagulative necrosis and mechanical stretching of trabecular meshwork, thereby improving aqueous outflow.[5] The biological theory states that upregulation of macrophage activity, as well as the release of inflammatory cytokines (IL-1a, IL-1b, TNF-alpha),[6] leads to remodeling of the extracellular matrix of the trabecular meshwork, thereby improving aqueous outflow facility.

Preoperative Evaluation

Because argon laser treatment is applied directly to the trabecular meshwork, it is important to perform gonioscopy to visualize and evaluate the TM preoperatively. Chronic angle-closure glaucoma and other conditions involving angle obscurations, such as media opacities and iridocorneal endothelial (ICE) syndrome, are contraindications for ALT. Laser iridoplasty may be indicated in cases of narrow angles if it allows better access to an open angle.

Active uveitis is another contraindication because ALT can exacerbate intraocular inflammation. Because IOP spikes can occur after the procedure, ALT must be used with caution in cases of end-stage glaucoma where such sudden changes in IOP can worsen vision or lead to progression of visual field damage.

Patient Consent

As with all surgical procedures, a detailed discussion involving the risks and benefits of the procedure must be done prior to performing ALT. Patients should be advised that reduction of IOP may not occur until 4 to 6 weeks after the ALT. During this time period, patients must continue using all of their ocular antihypertensive medications. Patients must also be aware that even with effective ALT, the IOP reduction wanes over time, and ­continued follow-up will be necessary.

Procedure

IOP should be measured prior to performing ALT. Some advocate that this measurement of IOP should be performed without the instillation of fluorescein on the ocular surface because the fluorescein may absorb laser energy.[7]

Topical antihypertensive medications should be administered 30 to 60 minutes before ALT to blunt post-ALT IOP spikes. Commonly used prophylactic medications include apraclonidine, pilocarpine, and brimoni-dine.[8][9]

A topical anesthetic should be instilled into the operative eye and the patient is positioned comfortably at the slit lamp. A gonioscopy lens, such as a 3-mirror Goldmann lens, the Ritch trabeculoplasty lens, or Karickhoff lens, should be applied to the operative eye using a coupling gel. Care should be taken to prevent formation or retention of an air bubble between the lens and the eye.

Although the laser settings vary from surgeon to surgeon, commonly used parameters are 50 μm spot size, 800 mW, 0.1 seconds of exposure, and 50 to 100 total burns.[10][11] Care should be taken to ensure that the laser burns are equally spaced. The wavelength most commonly used in multiple trials ranged from 488 to 514 nm, within the green light component of the visible spectrum.[12][13] Laser spots are targeted at the junction of nonpigmented TM and the pigmented TM posterior to it. Coagulation of posterior TM results in formation of peripheral anterior synechiae and more severe postoperative inflammation. A well-focused laser beam is achieved when the front plane of the goniolens is perpendicular to the laser delivery path. The aiming beam should appear small and round. If the aiming beam is not focused properly, the laser strikes a larger area and coagulates less effectively.

The power of the laser burns should be titrated to the level required to induce slight blanching of the TM or formation of small bubbles. Typically, one starts with a low power, such as 700 mW, and the power can be adjusted by 50- to 100 -mW intervals to achieve the desired effect.[14] If large bubbles form, the power should be reduced. The amount of energy required tends to be inversely proportional to the degree of TM pigmentation; heavier ­pigmented TM requires less power than lighter pigmented ones.

How much trabecular meshwork needs to be treated? Multiple trials have demonstrated that applying 50 burns to 180 degrees of TM is as effective in achieving IOP control and preventing progression of glaucoma as compared to applying 100 burns to 360 degrees of TM.[15][16][17] Based on this, most clinicians treat a maximum of 180 degrees of TM when performing ALT.

It is common practice to instill one drop of a topical steroid, such as prednisolone acetate 1%, immediately after completing the ALT treatment. IOP should be checked at approximately 1 to 2 hours after the procedure.

Postoperative Care

Topical steroids should be administered during the first postopera-tive week to limit postoperative inflammation. Various trials involving the use of fluorometholone 0.25%, prednisolone acetate 1%, or loteprednol 0.5% postoperatively for 1 week or less revealed no differences between the type of steroids used with respect to long-term outcomes.[18][19]

The frequency of follow-up is at the clinician’s discretion. Given the fact that the maximal effect of the ALT may not manifest until 4 to 6 weeks postoperatively, clinicians should wait at least 6 weeks before declaring the procedure a success or failure.

Complications

The most common adverse effect associated with ALT is inflammation. Postlaser inflammation is generally mild, localized to the anterior chamber, and occasionally associated with mild hyperemia and photophobia. Treatment with a short course of topical steroids or NSAIDs effectively resolves the inflammation in most cases.[20] This treatment can be supplemented with oral analgesics if patients remain symptomatic.

Acute IOP spikes can occur after ALT. The incidence of an IOP spike of at least 10 mm Hg is 12%.[21] When IOP spikes occur, they tend to transpire within the first hour postoperatively. IOP spikes occur more frequently in cases of higher power burns, higher total administered power, greater pigmentation in the trabecular meshwork, placement of laser burns more posteriorly, and treatment of 360 degrees in one session.[22]

Peripheral anterior synechiae can develop as a result of ALT. This can be minimized by careful placement of the laser burns and avoiding treatment posterior to the pigmented TM.

Corneal epithelial or endothelial thermal burns can develop from improper focusing of the laser beam. Additionally, corneal decompensation with subsequent corneal edema may develop. Typically, this is associated with higher power settings and/or pre-existing corneal disease such as Fuchs’ endothelial dystrophy or Chandler’s syndrome.

Hyphema, resulting from laser-induced damage to the TM, is a rare complication of ALT.[23] Hyphema can be managed intraoperatively by applying pressure to the globe with the goniolens to minimize the bleeding.

Treatment Success and Failure

Occasionally, ALT is not successful at achieving the expected IOP reduction by 6 weeks postoperatively. Factors that contribute to decreased ALT efficacy include age less than 50 years, higher preoperative IOP, and previous failed laser trabeculoplasty.[24] Patients with traumatic glaucoma and juvenile or congenital glaucoma tend to respond poorly to ALT. Factors associated with better response to ALT include heavier pigmentation of the trabecular meshwork and African descent. Patients with pigmentary glaucoma­ and exfoliative glaucoma respond very well to ALT.

Retreatment

In the event that further IOP reduction is needed after ALT is per-formed, reapplication of ALT can be considered. In patients who were previously treated with 180 degrees of ALT, the other 180 degrees should be treated. ALT can even be considered after 360 degrees of TM has been treated, but the success rate of retreatment is low. The majority of patients do not respond well to ALT retreatment.[25][26]

Conclusion

ALT is an effective treatment for open-angle glaucoma. It can be used either as first-line therapy or in conjunction with medical therapy. When successful, the effects can last years and potentially reduce the amount of medication needed by the patient, thereby improving compliance and limiting side effects. As with any procedure, careful follow-up is critical in identifying short- or long-term adverse effects, the nonre-sponders, and those whose initial reduction in IOP from ALT become inadequate over time.

Key Points

  • ALT can be performed at any stage of glaucoma, ranging from· primary­ therapy to treatment after filtering surgery.
  • Preoperative gonioscopy is imperative to visualize the angle and determine the likelihood of response to ALT.
  • Use caution when applying the argon beam so that treatment does not extend posterior to the junction of the nonpigmented and pigmented trabecular meshwork.
  • Use prophylactic medications to blunt IOP spikes and inflammation that commonly occur postoperatively.
  • Continued follow-up after ALT is necessary because the effect of ALT wanes over time.

References

  1. ↑ Ederer F, Gaasterland DA, Dally LG, et al. The Advanced Glaucoma Intervention Study (AGIS): 13. Comparison of treatment outcomes within race: 10-year results. Ophthalmology. 2004;111:651-664.
  2. ↑ Spaeth GL. Ophthalmic Surgery: Principles and Practice. Philadelphia, PA: WB Saunders; 1990:309-312.
  3. ↑ Shingleton BJ, Richter CU, Bellows AR, et al. Long-term efficacy of argon laser trabeculoplasty. Ophthalmology. 1987;94:1513-1518.
  4. ↑ Wilensky JT, Jampol LM. Laser therapy for open angle glaucoma. Ophthalmology. 1981;88:213-217.
  5. ↑ Wise JB, Witter SL. Argon laser therapy for open-angle glaucoma. A pilot study. Arch Ophthalmol. 1979;97:319-322.
  6. ↑ Stein JD, Challa P. Mechanisms of action and efficacy of argon laser trabeculoplasty and selective laser trabeculoplasty. Curr Opin Ophthalmol. 2007;18:140-145.
  7. ↑ Stamper RL, Lieberman MF, Drake MV. Becker-Shaffer’s Diagnosis and Therapy of the Glaucomas. 8th ed. St. Louis, MO: Mosby; 2009:447-452.
  8. ↑ Barnes SD, Campagna JA, Dirks MS, et al. Control of intraocular pressure elevations after argon laser trabeculoplasty: comparison of brimonidine 0.2% to apraclonidine 1.0%. Ophthalmology. 1999;106:2033-2037.
  9. ↑ The Brimonidine-ALT Study Group. Effect of brimonidine 0.5% on intraocular pressure spikes following 360% argon laser trabeculoplasty. Ophthalmic Surg Lasers. 1995;26:404-409.
  10. ↑ Rouhiainen HJ, Teräsvirta ME, Tuovinen EJ. The effect of some treatment variables on the results of trabeculoplasty. Arch Ophthalmol. 1988;106:611-613.
  11. ↑ Rolim de Moura CR, Paranhos Jr A, Wormald R. Laser trabeculoplasty for open angle glaucoma. Cochrane Database Syst Rev. 2007;(4):CD003919.
  12. ↑ Gupta D. Glaucoma Diagnosis and Management. Philadelphia, PA: Lippincott Williams & Wilkins; 2005:255-266.
  13. ↑ Smith J. Argon laser trabeculoplasty: comparison of bichromatic and monochromatic wavelengths. Ophthalmology. 1984;91:355-360.
  14. ↑ Rouhiainen HJ, Teräsvirta ME, Tuovinen EJ. The effect of some treatment variables on the results of trabeculoplasty. Arch Ophthalmol. 1988;106:611-613.
  15. ↑ Schwartz LW, Spaeth GL, Traverso C, et al. Variation of techniques on the results of argon laser trabeculoplasty. Ophthalmology. 1983;90:781-784.
  16. ↑ Eguchi S, Yamashita H, Yamamoto T, et al. Methods of argon laser trabeculoplasty, complications and long-term followup of the results. Jpn J Ophthalmol. 1985;29:198-211.
  17. ↑ Klein HZ, Shields MB, Ernest JT. Two-stage argon laser trabeculoplasty in open angle glaucoma. Am J Ophthalmol. 1985;99:392-395.
  18. ↑ Kim YY, Glover BK, Shin DH, et al. Effect of topical anti-inflammatory treatment on the long-term outcome of laser trabeculoplasty. Fluorometholone-Laser Trabeculoplasty Study Group. Am J Ophthalmol. 1998;126:721-723.
  19. ↑ Shin DH, Frenkel RE, David R, et al. Effect of topical anti-inflammatory treatment on the outcome of laser trabeculoplasty. The Fluorometholone-Laser Trabeculoplasty Study Group. Am J Ophthalmol. 1996;122:349-354.
  20. ↑ Herbort CP, Mermoud A, Schnyder C, et al. Anti-inflammatory effect of diclofenac drops after argon laser trabeculoplasty. Arch Ophthalmol. 1993;111:481-483.
  21. ↑ Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial. I. Acute effects of argon laser trabeculoplasty on intraocular pressure. Arch Ophthalmol. 1989;107: 1135-1142.
  22. ↑ Rosenblatt MA, Luntz MH. Intraocular pressure rise after argon laser trabeculoplasty. Br J Ophthalmol. 1987;71:772-775.
  23. ↑ Shields MB, Krupin T, Ritch R. Glaucomas. St. Louis, MO: Mosby; 1996:1582.
  24. ↑ AGIS Investigators. The Advanced Glaucoma Intervention Study (AGIS): 11. Risk factors for failure of trabeculectomy and argon laser trabeculoplasty. Am J Ophthalmol. 2002;134:481-498.
  25. ↑ Richter CU, Shingleton BJ, Bellows AR, et al. Retreatment with argon laser trabeculoplasty. Ophthalmology. 1987;94:1085-1089.
  26. ↑ Feldman RM, Katz LJ, Spaeth GL, et al. Long-term efficacy of repeat argon laser trabeculoplasty. Ophthalmology. 1991;98:1061-1065.
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