Cyclodestructive Techniques

Mechanism of Action

Several mechanisms may underlie intraocular pressure reduction following cyclodestruction. The common features of destruction and loss of ciliary epithelium imply reduced aqueous production.

Cyclocryotherapy (CCT) refers to ciliary body damage by freezing. The cryolesion is produced by transscleral application of a −80°C probe aimed at targeting the ciliary processes. Rapid cooling results in intracellular ice crystals, which on slow thawing lead to larger crystals; these are highly destructive to the ciliary body epithelium. The precise mechanism of injury is not completely understood, although obliteration of microcirculation induces ischemia within the frozen tissue. Histological studies of CCT in human and primate eyes show destruction of epithelial and capillary components of ciliary processes with scar replacement, and breakdown of the blood–aqueous barrier. The non-pigmented epithelium is destroyed initially. The collateral damage to the adjoining trabecular meshwork may impede the aqueous outflow and this, with regeneration of ciliary epithelium, reduces some of the effect in the longer term, leading to frequent need for retreatment. The treatment is also thought to cause loss of function of sensory nerves to the cornea, allowing some patients with painful eyes to become comfortable in spite of persistent high pressures.

Cyclophotocoagulation (CP) results in ciliary body damage by the use of laser energy. The first attempts at the use of laser energy were made with ruby laser followed by transscleral application of the Nd : YAG laser in the free-running mode and the diode laser (810 nm) causing thermal tissue injury to the ciliary body. The diode wavelength has greater melanin absorption than the Nd : YAG laser. Histological studies in cadaveric eyes confirm ciliary process destruction with the Nd : YAG laser. Transscleral lesions produced by the diode laser are similar to those produced by the Nd : YAG continuous mode, with blanching at the gross level, and coagulative necrosis microscopically, but relatively more effect on the ciliary muscle.

Under direct visualization, endoscopic diode laser applications produce an active whitening and shrinkage of the ciliary processes with eventual conversion into fibrous scar tissue.

Lin et al. attempted to quantify the evolution of vascular changes following CP; their results demonstrate that both transscleral cyclophotocoagulation (TSCP, sometimes referred to as cyclodiode) and endoscopic cyclophotocoagulation (ECP) are associated with an acute occlusive vasculopathy, but that with the endoscopic modality the chronic underperfusion (as measured by endoscopic fluorescein angiography of treated ciliary processes in rabbit eyes) is less than with the transscleral route.

The effects of CP on aqueous secretion are multifactorial. It is widely accepted that a major mechanism of aqueous suppression after CP is coagulative necrotic damage to the secretory ciliary epithelium consequent upon laser energy uptake by the pigmented ciliary epithelium. Further effects are due to ischemia; in both TSCP and ECP, some vascular damage occurs due to propagation of laser energy from the ciliary epithelium to nearby vessels in the ciliary processes or from tissue disruptions (‘pops’), although these are uncommon in ECP. Tissue disruption may cause collateral damage that compounds the effects of vascular occlusion (which may occur at the level of large or smaller vessels) and variations in clinical efficacy may be in part explained by differences in damage, regeneration, and reperfusion of the ciliary process.

Although it has been previously assumed that the effects of cyclophotocoagulation are on inflow, there is evidence that TSCP may also act as an outflow procedure. This might be explained by damage to the pars plana of the ciliary body that renders it ‘leaky’, promoting an increase in nonconventional outflow through the uveoscleral pathways, in a manner similar to that seen with the ocular hypotensive action of prostaglandin analogs. Furthermore, it is well reported that aggressive forms of ciliary ablation can cause cyclodialysis clefts, and it is possible that some of the effects of more gentle forms of ciliary body treatment are also due to small ciliary clefts.

Indications

There are still large regional and geographic variations in the use of diode lasers, and the perceived indications for cyclophotocoagulation vary according to a number of factors, including availability of resources, patient compliance, and physician preference.

One framework for indications, based upon the Ophthalmic Technology Assessment of the American Academy of Ophthalmology, is detailed in Box 122-1 . The indications for ECP differ from those of the external cycloablative procedures because it is an intraocular procedure. As such, unlike transscleral cyclophotocoagulation, it requires increased instrumentation and sterile technique, and carries risks, such as intraocular hemorrhage and endophthalmitis.

An area of controversy is the use of cyclophotocoagulation as either the primary treatment (before medications) or as the primary surgical treatment (after medications) for glaucoma. Preliminary results from one small study in Ghana raised the possibility that TSCP might be useful in areas without ready access to medications or other forms of surgery. However, intraocular pressure (IOP) lowering was modest, the follow-up was short, some eyes had an increase in IOP after treatment, and some vision loss was observed. Other longer and larger trials are indicated to determine whether there is a role for cyclophotocoagulation as primary treatment in certain settings.

Preoperative Considerations

Transscleral cyclophotocoagulation is an extraocular procedure. Mild transient uveitis is quite common following TSCP and some practitioners use preoperative topical corticosteroids or nonsteroidal anti-inflammatory medications to minimize this response. Systemic medications that predispose to bleeding (such as antiplatelet agents or anticoagulants) are not usually discontinued prior to treatment, but use of these drugs can be associated with per- or postoperative intraocular bleeding if surgery induces tissue disruption (pops).

Excessive ocular surface pigmentation (including conjunctival and scleral pigmentation) is associated with increased risk of surface laser absorption ( Fig. 122-2 ); such uptake of laser energy can cause partial- or full-thickness burns to the ocular surface and also reduces laser transmission to the ciliary body. It is therefore best to avoid TSCP in areas of ocular surface pigmentation.

Normal glaucoma medications, both topical and systemic, are taken up to and including the day of surgery and then titrated according to the postoperative IOP response.

Anesthetic Considerations

Most forms of cyclodestructive procedure are performed under regional anesthesia. TSCP requires effective periocular infiltration of local anesthesia (LA). Most practitioners use a peribulbar block (see ‘ Spotlight 1 ’ below) to provide adequate LA. Sub-Tenon's LA may also provide a good combination between regional and truly local anesthesia. Some claim that subconjunctival anesthesia is adequate, presumably due to truly local effect on ciliary body innervation. Suboptimal local anesthesia can be supplemented with intravenous sedation. A potential disadvantage of both sub-Tenon's and subconjunctival anesthesia is the chemosis and/or subconjunctival bleeding that can occur with these techniques. Such bleeding may make it difficult to achieve the conjunctival and scleral compression required for effective passage of laser energy through the ocular surface. General anesthesia (GA) is preferred by some practitioners, especially for bilateral treatments, treatment in children, and for treatment in patients who for whatever reason are unsuitable for infiltration anesthesia.

Operative Techniques and Potential Modifications