rgon laser trabeculoplasty is the procedure probably closest to being equivalent to pneumatic trabeculoplasty in it’s affect and possible mode of action. Clinical data shows that, overall, PNT produces the same or similar reduction in IOP with fewer serious side effects and has the advantage of being both totally non-invasive and repeatable with high success rates in repeated treatment.

The pertinent scientific literature

Argon laser trabeculoplasty (ALT) was introduced as a therapy for primary open-angle glaucoma in 1979.[1] ALT was first viewed as an alternative to surgery after failure of medications to control progression of primary open-angle glaucoma.[2-6] Subsequently, argon laser trabeculoplasty was considered an adjunct or alternative to medication. With confirmation of the pressure- lowering effect of argon laser trabeculoplasty in patients whose intraocular pressure was not controlled by medical therapy, the concept of starting therapy for primary open-angle glaucoma with argon laser trabeculoplasty emerged.

With this treatment, the argon laser is used to make a series of burns in the trabecular meshwork that results in lowered IOP by increasing aqueous outflow in a manner still not fully understood. A search of the NLM database using the keywords — argon/laser trabeculoplasty — resulted in the retrieval of 703 references to medical literature of which 226 pertained to complications and 6 to mechanism of action.

One of the more comprehensive examinations of ALT in newly diagnosed glaucoma patients was carried out by the Glaucoma Laser Trial Research Group beginning in 1987 reporting out after 2 years and again after 7 years of follow-up.[7-14]

In the Glaucoma Laser Trial, mean intraocular pressure was consistently lower, by 1 to 2 mm Hg, for eyes initially treated with laser trabeculoplasty than for eyes initially treated with medication.[12] Eyes initially treated with laser trabeculoplasty generally required less medication for intraocular pressure control than eyes initially treated with medication. It was noted that neither argon laser trabeculoplasty alone, nor argon laser trabeculoplasty with medications as needed, nor medication alone was a cure-all. Two years after the start of treatment, 56% (137 of 244) of eyes initially treated with laser trabeculoplasty required the addition of one or more medications for control of intraocular pressure, and 70% (171 of 244) of eyes initially treated with timolol required new or additional medication to control intraocular pressure.

Coakes found that pressure reduction with ALT is seldom greater than 30% and eyes with an IOP of more than 28 mm Hg are unlikely to be controlled for any length of time without additional treatment. ALT is probably most suitable either for older patients whose glaucoma is not well controlled on medical treatment, or for patients with newly diagnosed open angle glaucoma, but without advanced field loss, as a primary treatment or in combination with a non-miotic topical medication. In many it will defer surgery, in some indefinitely.[15]

Side effects of argon laser trabeculoplasty observed in the Glaucoma Laser Trial included transient increases in intraocular pressure after argon laser trabeculoplasty treatment and formation of peripheral anterior synechiae.[7, 8] Intraocular pressure increase of  greater than 5 mm Hg occurred in 34% (91 of 271) of eyes initially treated with laser trabeculoplasty after one or both argon laser trabeculoplasty sessions; these increases were associated with moderate or heavy pigmentation of the trabecular meshwork. Higher rates of formation of peripheral anterior synechiae were associated with brown iris color and placement of laser burns posterior to the junction of the pigmented and unpigmented anterior trabecular meshwork. Throughout follow-up, these 93 eyes had similar or better intraocular pressure reduction, similar visual field status, and similar medical and surgical history as the 171 eyes initially treated with laser trabeculoplasty that were free of peripheral anterior synechiae three months after treatment with ALT.[12]

Finnstrom reported an incidence of rapidly healing corneal burns in 80% of patients in his series.[16] Hong showed lasting morphological changes in the endothelial cells which may have significance later in life.[17]

Clinical data supporting substantive equivalence

It is unclear what the mechanism of action is in ALT. Suggestions from alteration of the Blood-Aqueous Barrier to the release of prostaglandins from trabecular injury to release of gelatinase under the same circumstances have been posited.

The mechanism of action of PNT is likewise unclear. But there is supporting evidence to show that it too acts on the trabecular meshwork but in a more indirect way. This evidence comes in the form of measured increases in accommodative amplitude in early presbyopes undergoing PNT, albeit of a temporary nature. There is corroborating evidence from the studies of Schachar[18-21] and Thornton in which expansion of the sclera over the ciliary body either by means of implanted plastic ring segments or radial incisions was accompanied by a measured decrease in post-surgical IOP. The mechanism may well involve stretching of the zonule which stretching produces some form of change in the trabecular meshwork either physiologically, through chemical mediators, or through a mechanical opening of the trabecular pores. That chemical mediators may be involved is suggested by the fact that patients who respond well to latanoprost (Xalatan) also seem to do well with PNT.

Here is a comparison of PNT with an ALT study done in 1982.[22] The series consisted of 59 eyes, all of which had POAG and all of which had not been subjected to surgery other than iridectomy or cataract extraction. All were on medications and candidates for surgical intervention. This early ALT series was chosen because then ALT was recommended for patients who were surgical candidates because they were not in control. It was also shortly after the introduction of ALT. In the group of PNT patients, none had had any glaucoma surgeries nor previous ALT.

Both ALT and PNT can be considered non-invasive procedures designed to lower IOP. They are non-invasive in the sense that the eye is not opened. However, it would be more accurate to describe ALT as quasi-noninvasive because while it does not open the eye, it introduces a high radiant energy laser beam into the eye which does physical damage. This damage is not limited to the trabecular meshwork as we have seen. Corneal burns have resulted as have anterior synechiae formed. In addition, ALT is accompanied by a spike or increase in IOP of sufficient magnitude as to require an increase in or an addition to the medical regimen to control. Levene reported a frequency of 3.1% vision loss and 1.8% emergency surgical intervention in a series of 400 eyes associated with these increases in IOP.[23] This has not been the case with PNT. Neither the current study(s) nor the aborted study from Duke recorded any increase in IOP following PNT.

Re-treatment of eyes following ALT has a checkered record. Brown reported a success rate of 38% in his series.[24] Others have reported a success rate of only 8%. Avalos reports 33% of patients in his study have undergone repeated PNT procedures with significant drops in IOP lasting upwards of 3 months. Repeat PNT procedures show a similar drop in IOP on subsequent treatments as in the initial treatment. In the current series(Bores), the repeat rate is somewhat lower at 30%.

Asymmetric responses to PNT were seen in 31% of treated eyes. This compares with the 50% asymmetry reported by Lieberman following ALT.[22]

In this current series (Bores), all but 2 patients (4 eyes) had previously established diagnoses of POAG and were already on treatment. Of these 4 eyes all (100%) had IOP’s of 16 or less at 1 year. With this small cohort, we do not know whether the effects of having pneumatic trabeculoplasty early in the course of treatment for primary open-angle glaucoma are the same as when pneumatic trabeculoplasty is administered later. Hence, the benefits of pneumatic trabeculoplasty as an initial treatment cannot necessarily be expected to accrue to eyes treated for several years with topical medications and then treated with pneumatic trabeculoplasty.

It is clear from the data obtained in the initial clinical trial, that PNT lowers IOP in glaucoma patients at least as well, if not better than ALT and with much greater safety since no complications such as those reported to occur following ALT. No increases in IOP immediately following PNT have ever been recorded, either in the preliminary study done by Avalos, the Phase I trial done by Bores, or in the 6(8) eyes treated in the Duke study.

1. Wise, J.B. and S.L. Witter, Argon laser therapy for open-angle glaucoma. A pilot study. Arch Ophthalmol, 97(2): p. 319-22; 1979. 
2. Schwartz, A.L., D.C. Love, and M.A. Schwartz, Long-term follow-up of argon laser trabeculoplasty for uncontrolled open-angle glaucoma. Arch Ophthalmol, 103(10): p. 1482-4; 1985. 
3. Wilensky, J.T. and L.M. Jampol, Laser therapy for open angle glaucoma. Ophthalmology, 88(3): p. 213-7; 1981. 
4. Wise, J.B., Long-term control of adult open angle glaucoma by argon laser treatment. Ophthalmology, 88(3): p. 197-202; 1981. 
5. Thomas, J.V., R.J. Simmons, and C.D.d. Belcher, Argon laser trabeculoplasty in the presurgical glaucoma patient. Ophthalmology, 89(3): p. 187-97; 1982. 
6. Wilensky, J.T. and R.N. Weinreb, Early and late failures of argon laser trabeculoplasty. Arch Ophthalmol, 101(6): p. 895-7; 1983. 
7. GLTRG, The Glaucoma Laser Trial. 1. Acute effects of argon laser trabeculoplasty on intraocular pressure. Glaucoma Laser Trial Research Group. Arch Ophthalmol, 107(8): p. 1135-42; 1989. 
8. GLTRG, The Glaucoma Laser Trial (GLT). 2. Results of argon laser trabeculoplasty versus topical medicines. The Glaucoma Laser Trial Research Group [see comments]. Ophthalmology, 97(11): p. 1403-13; 1990. 
9. GLTRG, The Glaucoma Laser Trial (GLT): 3. Design and methods. Glaucoma Laser Trial Research Group. Control Clin Trials, 12(4): p. 504-24; 1991. 
10. GLTRG, The Glaucoma Laser Trial: 4. Contralateral effects of timolol on the intraocular pressure of eyes treated with ALT. GLT Research Group. Ophthalmic Surg, 22(6): p. 324-9; 1991. 
11. GLTRG, The Glaucoma Laser Trial (GLT): 5. Subgroup differences at enrollment. Glaucoma Laser Trial Research Group. Ophthalmic Surg, 24(4): p. 232-40; 1993. 
12. GLTRG, The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow-up study: 7. Results. Glaucoma Laser Trial Research Group. Am J Ophthalmol, 120(6): p. 718-31; 1995. 
13. GLTRG, The Glaucoma Laser Trial (GLT): 6. Treatment group differences in visual field changes. Glaucoma Laser Trial Research Group. Am J Ophthalmol, 120(1): p. 10-22; 1995. 
14. GLTRG, The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow-up study: 7. Results. Glaucoma Laser Trial Research Group. Am J Ophthalmol, 120(6): p. 718-31.; 1995. 
15. Coakes, R., Laser trabeculoplasty. Br J Ophthalmol, 76(10): p. 624-6; 1992.
16. Finnstrom, K., Laser treatment for open angle glaucoma. A one year follow-up. Acta Ophthalmol (Copenh), 63(1): p. 23-7; 1985. 
17. Hong, C., Y. Kitazawa, and T. Tanishima, Influence of argon laser treatment of glaucoma on corneal endothelium. Jpn J Ophthalmol, 27(4): p. 567-74; 1983. 
18. Schachar, R.A., et al., In vivo increase of the human lens equatorial diameter during accommodation. Am J Physiol, 271(3 Pt 2): p. R670-6; 1996. 
19. Schachar, R.A., Zonular function: a new hypothesis with clinical implications. Ann Ophthalmol, 26(2): p. 36-8; 1994. 
20. Schachar, R.A., et al., A physical model demonstrating Schachar's hypothesis of accommodation. Ann Ophthalmol, 26(1): p. 4-9; 1994. 
21. Schachar, R.A., T. Huang, and X. Huang, Mathematic proof of Schachar's hypothesis of accommodation. Ann Ophthalmol, 25(1): p. 5-9; 1993. 
22. Lieberman, M.F., H.D. Hoskins, Jr., and J. Hetherington, Jr., Laser trabeculoplasty and the glaucomas. Ophthalmology, 90(7): p. 790-5; 1983. 
23. Levene, R., Major early complications of laser trabeculoplasty. Ophthalmic Surg, 14(11): p. 947-53; 1983. 
24. Brown, S.V., J.V. Thomas, and R.J. Simmons, Laser trabeculoplasty re-treatment. Am J Ophthalmol, 99(1): p. 8-10; 1985.