Cyclodestructive Procedures in Glaucoma

Introduction

Cyclodestructive procedures reduce intraocular pressure (IOP) by decreasing aqueous production through destruction of the ciliary epithelium. Historically these procedures have been reserved for eyes with glaucoma refractory to medical, laser, and surgical treatment because destruction of the ciliary body can be difficult to titrate and can cause significant damage to adjacent structures and the blood–aqueous barrier. Newer technologies have improved selective targeting of the ciliary epithelium, reducing the dreaded side effects of phthisis bulbi, vision loss, and unpredictable IOP reduction. Micropulse application of the diode laser has recently emerged as an additional treatment option to mitigate these risks.

Historical Review

Selective destruction of the ciliary body with nonpenetrating diathermy was described by Weve in 1933, and penetrating diathermy was described by Vogt in 1936. Destruction of the ciliary body by freezing or cryotherapy was introduced in 1950 by Biette and found to be less destructive and more predictable compared with cyclodiathermy. However, IOP reduction was inconsistent, and complications were common.

Cyclophotocoagulation was made possible with the introduction of the xenon arc photocoagulator in 1961 and the ruby laser in 1971. Transpupillary laser delivery was hampered by limited visualization of the ciliary processes, so transscleral delivery was developed. In 1981, Fankhauser described a longer pulse duration thermal ablation using a neodymium:yttrium–aluminum–garnet (Nd:YAG 1064-nm) laser system to perform transscleral cyclophotocoagulation (TCP). The availability of the instrument facilitated wide clinical use, but the subsequent introduction of a solid-state diode (810-nm) laser equipped with a disposable probe increased economy and accessibility. More recently, endoscopic diode cyclophotocoagulation (ECP) has allowed direct visualization and targeting of the ciliary epithelium.

Preoperative Evaluation and Diagnostic Approach

Cyclodestructive procedures are traditionally reserved for eyes with poor visual potential (<20/400), eyes in which incisional procedures have failed, eyes in which filtering surgery has a high failure rate (e.g., extensive conjunctival scarring, neovascular glaucoma, aphakic and pseudophakic glaucoma, and glaucoma associated with silicone oil), and eyes of patients who have a medical contraindication to filtration surgery.

The risks of cyclodestruction include inflammation, pain, chronic hypotony, macular edema, vitreous hemorrhage, and phthisis. Because of these risks, proper technique and patient selection are critical. Modified treatment parameters have resulted in fewer side effects and a willingness to use the procedure in eyes with better vision. In ECP, cyclophotocoagulation is guided by visual endpoints that limit overtreatment and damage to nontargeted tissue. Indeed, the role of ECP has expanded and is now used in nonrefractory glaucoma, in pediatric glaucomas, in combination with cataract extraction (CE), and in cases where glaucoma filtering surgery carries higher risks.

Mechanism of Action

TCP lowers IOP by destroying ciliary epithelium and the associated vasculature, leading to decreased aqueous humor production. The 810-nm diode laser has lower scleral transmission than the previously used continuous-wave Nd:YAG laser (1064 nm) but greater absorption by melanin, allowing the use of 50% less energy than Nd:YAG to achieve the same therapeutic effect.

Histologically both diode and Nd:YAG TCP produce fragmentation and detachment of the ciliary epithelium with simultaneous destruction of the ciliary body vasculature. At least three mechanisms are thought to be important in decreasing IOP: (1) inflammation, which is prominent in the first week or so after treatment; (2) decreased aqueous production through ablation of the pars plicata epithelium and associated vasculature; and (3) increased uveoscleral outflow resulting from laser delivery to the region of the pars plana.

ECP diode laser treatment appears to limit coagulative necrosis to the ciliary epithelium and stroma of the ciliary processes, decreasing aqueous production only at the level of the ciliary body epithelium.

Alternatives to Cyclodestruction

The alternatives to cyclodestruction include trabeculectomy or drainage device implantation. TCP is advantageous because it is quick, nonincisional, and can be performed in the office. It is generally reserved for eyes with poor visual potential because of the risk of chronic inflammation and decreased visual acuity. ECP must be performed in the operating room and does not effect a rapid decline in IOP but is less likely to result in pain, inflammation, and overtreatment compared with TCP.