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Dr.MEGHA GULATI, Dr.Bibhuti Prassan Sinha,Dr.SANTOSH KUMAR THAKUR

ABSTRACT :

PURPOSE: The main aim of the present study was to assess the efficacy of selective laser trabeculoplasty (SLT) treatment in patients suffering from pigmentary glaucoma (PG) as compared to primary open angle glaucoma (POAG).Changes in tear film cytokine levels were also assessed pre and post laser.

METHODS : 10 newly diagnosed patients each of PG and POAG underwent inferior 180˚ SLT. Intraocular pressure changes and pre and post laser cytokine level changes were compared between the 2 groups. Followup period was 1 year.

RESULTS : There was statistically insignificant difference in amount of IOP reduction till first 7 months in the two groups; after which 60 % of patients in PG group required repeat superior 180˚ SLT followed by comparable IOP reduction in two groups at 12th month of followup. Post laser rise in tear film cytokine IL-1 and IL-8 (3rd day) were more in PG group than POAG group and result was statistically significant.

KEYWORDS :

Argon laser trabeculoplasty (ALT), Selective laser trabeculoplasty (SLT), laser trabeculoplasty (LTP), Primary open angle glaucoma (POAG), Pigmentory glaucoma (PG), Intraocular pressure (IOP). 

INTRODUCTION

Laser trabeculoplasty is a widely used and commonly accepted procedure for lowering IOP in eyes with medically uncontrolled open-angle glaucoma. During a last-decade SLT, a 532-nm Q-switched frequency-doubled Nd- YAG laser has been successfully employed in the treatment of open-angle glaucoma. SLT can be used in cases of uncontrolled IOP as a replacement for medical therapy and even as first-line therapy. SLT can be used to treat patients with POAG, pseudoexfoliation glaucoma, pigmentary glaucoma, normal-tension glaucoma, Ocular hypertension, juvenile glaucoma, pseudophakic and aphakic glaucoma. SLT is easy to perform and well tolerated by patients and offers the advantages of ALT while using much less energy with less apparent collateral damage to the TM. It seems to have equivalent efficacy to topical latanoprost and to be an effective and safe glaucoma treatment without compliance risks or systemic side effects.

Potential laser-based treatments for pigmentary glaucoma include argon laser trabeculoplasty and laser peripheral iridotomy (LPI). One study showed that argon laser trabeculoplasty lowered IOP similarly in patients with pigmentary or primary open-angle glaucoma and that the change in IOP was inversely related to the age of the patient.Selective laser trabeculoplasty (SLT) on the other hand may be relatively contraindicated in eyes with pigmentary glaucoma, because the the trabecular meshwork is heavily pigmented, increasing the risk of significant post-SLT elevations in IOP.

The main aim of the present study was to assess the efficacy of selective laser trabeculoplasty (SLT) treatment in patients suffering from pigmentary glaucoma (PG) as compared to primary open angle glaucoma (POAG).Changes in tear film cytokine levels were also assessed pre and post laser. 10 newly diagnosed patients each of PG and POAG underwent inferior 180˚ SLT. Intraocular pressure changes and pre and post laser cytokine level changes were compared between the 2 groups. Followup period was 1 year. There was statistically insignificant difference in amount of IOP reduction till first 7 months in the two groups; after which 60 % of patients in PG group required repeat superior 180˚ SLT followed by comparable IOP reduction in two groups at 12th month of followup. Post laser rise in tear film cytokine IL-1 and IL-8 (3rd day) were more in PG group than POAG group and result was statistically significant. This suggests more of a biochemical mechanism of action in pigmentory glaucoma.

Some studies show lower efficacy of SLT in PG compared to POAG while some show similar efficacy . this study showed similar efficacy of SLT in PG and POAG.

DISCUSSION

Selective laser trabeculoplasty is a safe and effective treatment for glaucoma, with greater cost effectiveness than its pharmacological and surgical alternatives. While the effectiveness of these therapies is well established, the mechanism by which laser treatment reduces IOP remains uncertain. Understanding the mechanism of laser trabeculoplasty (LTP) is not only essential to maximizing efficacy, minimizing side-effects and gaining insights for developing and refining glaucoma treatments, but also to understanding the underlying pathophysiology of the disease.

In 1995, Latina and Park proposed selective laser trabeculoplasty (SLT)1, a LTP glaucoma treatment using a green (532 nm), Q-switched, frequency doubled Nd:YAG laser. The laser’s 400-μm spot size covers the entire width of the TM, facilitating aiming and making SLT less skill-intensive.

Selective photothermolysis (SP) – a method of confining laser-induced thermal damage to selected pigmented targets within an irradiated tissue – is the basis for SLT. It was first described by Anderson and Parrish in 1983.2 SP has two principle requirements: (i) the desired target must have an intracellular chromophore with greater optical absorption at the laser wavelength than its surrounding tissue and (ii) laser duration must not exceed the time required for thermal diffusion into this tissue.

This duration is referred to as thermal relaxation time. All forms of LTP fulfil the first requirement of SP, as melanin contained within pigmented cells of the TM is the chromophore primarily responsible for absorbing laser energy. However, the ALT treatment duration (0.1 s) is much longer than the thermal relaxation time of melanin (1 μs); this allows heat, initially restricted to pigmented cells, to dissipate into and damage the surrounding TM tissue.3-7

The mechanical theory suggests that the laser electromagnetic energy is converted to thermal energy when it contracts the TM. Tissue contraction and scar formation result in mechanical stretching of the surrounding untreated regions of the meshwork , facilitating flow into Schlemm’s canal with subsequent reduction in IOP.

The celluar theory is based on stimulation and increased cell division and repopulation of the trabecular meshwork. An increase in DNA replication and cell division following argon laser treatment have been demonstrated.8

The biochemical theory suggests a release of chemical mediators after laser treatment that increase aqueous outflow facility. LTP has been shown to increase macrophage recruitment at the site of treatment, resulting in remodeling of the extracellular matrix and increased outflow facility.9-10

CONCLUSION

SLT can be used to treat patients with POAG, pseudoexfoliation glaucoma, pigmentary glaucoma, normal-tension glaucoma, Ocular hypertension, juvenile glaucoma, pseudophakic and aphakic glaucoma. SLT has the advantage of being a repeatable procedure and prevents collateral damage. SLT can be used as an effective procedure in PG with controlled power settings to prevent postlaser IOP spikes and has more of a biochemical mechanism of action. 

REFERENCES

  1. Latina MA, Park C. Selective targeting of trabecular meshwork cells: in vitro studies of pulsed and CW laser interactions. Exp Eye Res 1995; 60: 359–71.
  2. Anderson RR, Parrish J. Selective photothermolysis: precise microsurgery by selective absorption of the pulsed radiation. Science 1983; 220: 524–6.
  3. Rodrigues MM, Spaeth GL, Donohoo P. Electron microscopy of argon laser therapy in phakic openangle glaucoma. Ophthalmology 1982; 89: 198–210.
  4. Kramer TR, Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmology 2001; 108: 773–9.
  5. Cvenkel B, Hvala A, Drnovsek-Olup B, Gale N. Acute ultrastructural changes of the trabecular meshwork after selective laser trabeculoplasty and low power argon laser trabeculoplasty. Lasers Surg Med 2003; 33: 204–8.
  6. Melamed S, Pei J, Epstein DL. Short term effect of argon laser trabeculoplasty in monkeys. Arch Ophthalmol 1985; 103: 1546–52.
  7. Rasmussen CA, Kaufman PL. Primate glaucoma models. J Glaucoma 2005; 14: 311–4.
  8. Acott TS, Samples JR, Bradley JM, et al. Trabecular repopulation by anterior trabecular meshwork cells after laser trabeculoplasty. Am J Ophthalmol 1989; 107: 1-6
  9. Parshley DE, Bradley JM, Samples JR, et al. Early changes in matrix metalloproteinases and inhibitors after in vitro laser treatment to the trabecular meshwork. Curr Eye Res 1995; 14: 537-544.
  10. Parshley DE, Bradley JM, Fisk A, et al. Laser trabeculoplasty induces stomyelsin expression by trabecular juxtacanalicular cells. Invest Ophthalmol Vis Sci 1996; 37:795-804

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