Cyclophotocoagulation

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CHAPTER 45 Cyclophotocoagulation

Introduction

Surgical procedures to lower intraocular pressure (IOP) either improve aqueous outflow or reduce production. In the former, laser trabeculoplasty, goniotomy, trabeculotomy, and filtering surgery are usually effective. However, not all eyes respond well to outflow procedures, even with the adjunctive use of anti-metabolites. Thus techniques that reduce aqueous inflow are also needed. Cyclophotocoagulation uses laser energy to lower IOP by selective destruction of the ciliary body. It can be achieved in three different ways1. The first is the transpupillary technique in which laser photocoagulation is applied directly to visible ciliary processes. As this technique is possible only in aniridic patients or when broad peripheral anterior synechiae cause anterior iris displacement, it has limited indications; unpredictable results minimize practical clinical usage. The second approach is trans-scleral, in which energy is transmitted through the sclera to the ciliary body. Lastly, in the endoscopic technique, an endolaser probe is inserted into the eye with direct photocoagulation of the visualized ciliary processes.

Trans-scleral cyclophotocoagulation

History

In 1961 Weekers et al. used light energy for cyclodestruction2. With trans-scleral applications of the xenon arc, they photocoagulated the ciliary body in rabbit and human eyes, thus lowering IOP. In 1969 Smith et al. and Vucicevic et al. reported the first clinical applications of laser energy for trans-scleral cyclophotocoagulation3,4. In 1985 Wilensky et al. and Beckman and co-workers demonstrated the utility of the Nd:YAG laser for trans-scleral cycloablation5,6. In 1987 Brancato et al. suggested the Nd:YAG laser for contact application through an optic fiber, leading to wide acceptance7. In 1990 diode laser was first introduced for trans-scleral cycloablation, and in 1992 the first clinical trials of diode laser trans-scleral cyclophotocoagulation (TCP) were reported: Hennis et al. used the non-contact technique8, and Carassa et al. and Gaasterland et al. used the contact technique9,10; they achieved good IOP reduction. Therefore, contact trans-scleral cyclophotocoagulation with diode laser (also named cyclodiode) has become the technique of choice for cyclodestruction.

Fundamental principles

Thanks to its easy application and non-invasiveness the trans-scleral approach for cyclophotocoagulation is the most widely used. A laser beam transmitted through the overlying conjunctiva and sclera ablates the ciliary body (Fig. 45.1). Laser energy can be transmitted indirectly through the air (non-contact trans-scleral cyclophotocoagulation, NCTCP), or directly through a fiberoptic placed in contact with the eyeball (contact trans-scleral cyclophotocoagulation, CTCP). Energy absorbed by melanin in the ciliary processes induces thermal coagulation and disrupts the ciliary epithelium and its vessels, as demonstrated in animal and human eyes. The ciliary body disruption is similar with both the contact and non-contact techniques. Nevertheless, the longer exposure time in CTCP results in more thermal damage to the stroma with coagulative necrosis of the ciliary non-pigmented and pigmented epithelia, as opposed to the blister formation seen with NCTCP11,12. There is no clear evidence of damage to the sclera, although areas of compression and hypercellularity have been found histologically. The effect on tissue is similar whether using the Nd:YAG or the diode laser, although the latter causes more damage to the ciliary pigmented structures. The long exposure time combined with the shorter wavelength of the diode laser results in deep coagulation necrosis of the pigmented epithelium, wide disorganization of the collagen in the stroma, and intravascular coagulation in the ciliary vessels. Therefore the diode laser needs less energy to produce ciliary body thermal coagulation (1.2 J versus 4 J for the Nd:YAG laser)13,14.

Preoperative assessment

All preoperative medications are continued. The lids are held open manually or with a wire speculum. In the non-contact technique a specific contact lens can be used to facilitate the procedure15. After administering anesthesia, the patient is placed in an upright position at the laser slit lamp for the non-contact operation and in supine position on the operating table for the contact technique.

Operation technique

Trans-scleral application

The Nd:YAG laser (1064 nm) and the diode laser (810 nm) are both suitable for trans-scleral applications. Two trans-scleral approaches have been used in cycloablation: non-contact and contact. The diode laser has several advantages, including better absorption by melanin, good portability, and lower cost. Either continuous-wave or pulsed laser systems are available. Continuous-wave laser allows long and sustained energy delivery while a pulsed laser system transmits light energy at short pre-set time intervals. Trans-scleral photocoagulation is a ‘blind’ procedure because the ablative energy is directed toward an invisible target whose position can only be estimated from data obtained experimentally in human autopsy eyes. Exact beam focusing for NCTCP was thus suggested as being 1–1.5 mm posterior to the limbus16, and the optimal location for the center of the probe in CTCP was 1.5–2 mm posterior to the corneo-limbal junction17. Localization of the ciliary body by transillumination should be performed in every eye before surgery. Ultrasound biomicroscopy can help to locate the exact ciliary body position (Fig. 45.2).

In NCTCP, laser energy is transmitted through the air from a slit-lamp delivery system. The procedure can be carried out using a pulsed Nd:YAG laser such as the Microruptor II, a continuous-wave model such as the Microruptor III (H.S. Meridian, Inc., Mason, OH), or a diode laser such as the DC3000 (Nidek, Inc., Palo Alto, CA), the Microlase (Keeler Instruments, Broomall, PA), or the Oculight SLX (Iris Medical, Mountain View, CA) (Fig. 45.3). The laser beam needs to be focused inside the ciliary body. This is done with the Nd:YAG laser by focusing the HeNe beam on the conjunctiva overlying the ciliary body and selecting the maximum offset18, thus providing a 3.6 mm posterior separation between the HeNe and the therapeutic beam and with the diode laser by defocusing the beam 1 mm toward the ciliary body. The Nd:YAG laser should be set at 4–8 J energy, while the diode laser should be set at 1500–2000 mW power output, 1 second time duration (thus producing 1.5–2 J spots), and 100–400 µm spot size (Table 45.1). The eye is kept in primary position because no significant differences were found between applying laser energy parallel to the visual axis and perpendicular to the sclera18. The beam is focused on the conjunctiva 1–2 mm behind the limbus. The distance is measured with a caliper and a marking pencil, or the slit beam of the biomicroscope. With the latter, the laser spots are placed in the middle of a 3 mm long slit beam, projected perpendicular to the limbus. Thirty to forty applications, evenly spaced over 360°, are placed in a single session, usually sparing the 3 o’clock and 9 o’clock positions to avoid damage to the long posterior ciliary nerves.

Using the diode laser, each spot should be defocused 1 mm toward the ciliary body. It is mandatory to keep a diagram of the treatment, specifically noting the laser settings used.

The contact technique has several advantages over the non-contact technique, which explains its wider clinical use. Reduced light backscattering and greater scleral transmission mean significantly less energy is needed. Arbitrary beam defocusing is avoided. The longer exposure time results in a predominantly coagulative necrosis of the ciliary processes as opposed to the blister formation found NCTCP. The use of specifically designed, hand-held probes improves precision by allowing easy spot location and by avoiding eye movement. The operation is done in the supine position so patients under general anesthesia can be easily treated. CTCP is carried out using either an Nd:YAG laser or a diode laser. The Nd:YAG lasers are the Microruptor III (H.S. Meridian, Inc., Mason, OH), and the Emerald (Crystal Focus, Viewpoint, Miami, FL; Fig. 45.4). The diode lasers are the Oculight SLX or the IQ810 (Iridex, Mountain View, CA; Fig. 45.5

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