Transscleral Diode Laser Cyclophotocoagulation (CPC)

Abstract

Traditionally cyclodestructive procedures, achieved either by freezing (cryotherapy) or laser CPC, have been used to treat refractory glaucoma with poor visual potential, presence of extensive scarring from prior surgery, immobile conjunctiva precluding an ab externo approach, or in those at high risk for intraoperative complications (Glaucoma: the requisites in ophthalmology, St. Louis, MO, 2000). Even in eyes with no light perception, the procedure can provide relief from chronic pain, conjunctival injection, and corneal decompensation from elevated IOP. Cyclodestruction is associated with several potential complications including visual loss that may result not necessarily from the procedure itself, but from the underlying disease process including hypotony, macular edema, cataract formation, proliferative diabetic retinopathy, central retinal vein occlusion, neovascular glaucoma and glaucoma progression (Glaucoma: the requisites in ophthalmology, St. Louis, MO, 2000; Curr Opin Ophthalmol 24:102–110, 2013).

The transscleral diode laser delivers a continuous 810 nm beam of energy via a customized delivery tip called the G-Probe (Iridex Corp, CA) (Glaucoma: the requisites in ophthalmology, St. Louis, MO, 2000). The presence of a fiber-optic tip protrusion at the base of the probe is designed to optimally deliver energy to the ciliary body by indenting the sclera 1.2 mm posterior to the limbus. The laser is absorbed by pigment within the ciliary body and coagulates proteins within pigmented epithelial cells reducing aqueous production (Insert to Glaucoma Today, 2012). A clinical judgment needs to be made regarding how much one needs to laser. If the IOP is exceedingly high and needs to be decreased substantially, then more laser applications should be administered. Conversely, if a modest drop in IOP is the goal, then one needs to taper the number of delivered laser applications.

In patients with altered limbal anatomy from prior surgery, pannus formation, megalocornea, or congenital glaucoma, accurate localization of the ciliary body is paramount with transillumination to guide proper placement of the G-Probe (J Ophthalmol 1–16, 2013). Otherwise, there may be no apparent benefit in IOP lowering with laser being delivered to the peripheral cornea. Scleral thinning following standard transscleral diode laser CPC has been reported in a series of patients all under 30 years of age using recommended manufacturer settings (Ophthalmic Surg Lasers Imaging 38:301–306, 2007). This may in part be due to an enhanced heat-induced thinning of juvenile collagen vs. adults (Ophthalmic Surg Lasers Imaging 38:301–306, 2007). Conjunctival and scleral burns can occur on the surface either due to accumulation of debris on the fiber-optic tip or the presence of perilimbal conjunctival pigment absorbing the laser energy (Peri-limbal burns in areas of conjunctival melanosis during cyclophotocoagulation, San Francisco, CA, 2013).

Recently, the same company has introduced MicroPulse Technology (http://www.iridex.com/Products/GlaucomaDevices/CYCLOG6-MicroPulseP3.aspxMicropulseG6) with a novel contact probe whereby a continuous-wave laser beam is delivered in a pulsatile manner interspersed with brief rest periods allowing heat to dissipate (see next chapter). This approach is more tissue sparing and potentially enables glaucoma intervention in earlier phases of the disease with functional vision and not just refractory cases. However, based on personal communication of initial experience with several glaucoma colleagues, the success rates to date have been variable with guarded expectations (Personal e-mail communications from the American Glaucoma Society (AGS) membership via agssociety.net). The operative template below outlines the use of the established transscleral diode laser G-Probe delivery system, which can be performed either in the office or OR setting.