Article

Dogma vs Data: Audible Pops During Trans-scleral Cyclophotocoagulation

How many and how often?

Coagulative interventions targeting reduction in intraocular pressure (IOP) via destruction of the ciliary body have long been an interesting facet of the glaucoma treatment paradigm. Laser trans-scleral cyclophotocoagulation (TSCPC) was introduced in 1972, effectively replacing older modalities, such as cyclocryotherapy and penetrating cyclodiathermy.1

Since its introduction, TSCPC has occupied an important place in the treatment of refractory glaucoma in patients who are not responsive or not amenable to surgical intervention. The need to preserve ocular architecture and minimize damage to adjacent non-pigmented tissue while halting the progression of glaucoma has driven continual evolution in the TSCPC treatment paradigm over the past 50 years.2

Key developments have centered on variations in type of laser (ruby, argon, Nd:YAG, diode), technique (contact vs noncontact), delivery mode (micropulse vs continuous wave), and laser parameters (fixed power vs “pop” titrated). The cumulative impact of these sustaining developments has transformed the role of TSCPC in the treatment paradigm from a last-resort intervention reserved for eyes with poor visual potential into a promising primary or secondary surgical intervention in select patients with good vision.3,4

The evidence behind these developments, however, is grossly inconsistent. While there are observable trends in how TSCPC is performed, case series form the bulk of the literature, and there is a notable scarcity of comparative studies providing evidence for treatment protocols.5,6 Energy delivery protocols in TSCPC represent a crucial controversy in this regard. Energy delivery is the key to successful coagulation of the ciliary body, with higher energy levels being associated with increased success in IOP lowering.7 It has elsewhere been suggested, however, that increased energy delivery is associated with the most serious complications, which involve collateral destruction of nonpigmented structures. These complications include hypotony and phthisis, which have been shown to be related to the total amount of energy delivered.8 Optimizing laser parameters is therefore of great interest in augmenting the IOP-lowering effects of TSCPC while minimizing complications. Despite this interest, high-quality comparative evidence is lacking.

The current best-practice iteration of TSCPC utilizes a semiconductor diode laser with an 810-nm wavelength that is strongly absorbed by melanin in the pigmented epithelium ciliary body. Direct delivery of energy is achieved via the use of a contact G-probe placed 1.2 mm from the corneoscleral limbus. Duration is set to 2 seconds with an initial power setting of 1,750 mW. Power is steadily increased in 250 mW increments to a maximum of 2,500 mW until an audible pop is heard. This pop represents a microexplosion of the ciliary body, and it is at this point that power is reduced by 250 mW below the level causing a pop. Treatment is continued at this power level for 6 spots per quadrant, with sparing of the 3-o’clock and 9-o’clock positions.5

There has been controversy about the use of this audible pop in titrating power settings. Some practitioners opt to operate at the level causing the pops, while others prefer the aforementioned approach. Still others have rejected pop-titration altogether, following a fixed-dose protocol. Slow coagulation has recently been advocated as an alternative nontitrated approach that causes less postoperative inflammation.9 Another current iteration of coagulation, micropulse TSCPC, employs an on-off cycled laser without power modulation using a clinically evident end point such as a pop.10

Studies have shown a clear histologic significance of the pop as the rapid cavitation of the ciliary body, while successful TSCPC relies on ablation of the ciliary pigmented epithelium.11,12 The microexplosion of the ciliary body is therefore not an essential part of the TSCPC treatment mechanism, with increased rates of pops being associated with higher rates of postoperative iridocyclitis.13 It does, however, have a clear benefit when used as a cue to modulate intraoperative energy delivery. A recent retrospective chart review of TSCPC treatment parameters noted that “no study has compared variations in the parameters of delivery of [TSCPC] in terms of success and complications within 1 case series.”14 This review concluded that optimizing power per audible cues achieves higher success than fixed-dose TSCPC and no difference in complications.14

Another recent study was the first prospective, case-control study to compare pop titration to fixed-energy delivery. This study found no significant difference in IOP lowering but did observe a significantly higher rate of hypotony following fixed-energy delivery. The authors concluded that pop titration prevents excess energy delivery and thereby avoids hypotony.15

Trans-scleral cyclophotocoagulation has long relied on pop titration without a strong consensus on the specifics and without supportive, high-quality evidence. The current literature shows clear benefit in using the pop as a cue in modulating treatment below the level achieving this tissue microexplosion and also suggests that the number of pops should be minimized to minimize complications. Comparative studies in this area, however, are few, and further studies should be conducted to elucidate the specifics of pop titration in TSCPC. GP

References

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