Micropulse transscleral cyclophotocoagulation
Glaucoma, one of the leading causes of irreversible blindness worldwide, is a progressive optic neuropathy. The treatment of refractory glaucoma includes a variety of cyclodestructive procedures. The first cyclodestructive laser procedure was performed by Beckman and colleagues in 1972, and since then various other cyclodestructive procedures have been implemented (see table below).
|Year||Author||Type of Laser|
|1989||Brancato et al.||Neodymium:yttrium-aluminum-garnet (Nd:TAG) laser (1064 nm)|
|1992||Gaasterland et al.||Semiconductor diode laser (810 nm)|
|2010||Tan et al.||Micropulse diode laser|
Traditional transscleral diode cyclophotocoagulation (TSCPC), or continuous wave transscleral diode cyclophotocoagulation (CW-TSCPC), has been widely used since its development in the 1990s. In these procedures, a diode laser targets and destroys the pigmented ciliary body epithelium, thereby decreasing production of aqueous humor. However, laser delivered at a continuous dose frequently results in significant collateral tissue damage, contributing to serious complications, such as uveitis, vision loss, chronic hypotony, choroidal detachment, and more rarely, phthisis bulbi and sympathetic ophthalmia. Micropulse transscleral diode cyclophotocoagulation (MP-TSCPC) has therefore been developed over recent years as an alternative, potentially safer approach to cyclodestruction.
The main difference between MP-TSCPC and traditional TSCPC is that MP-TSCPC delivers a series of repetitive short pulses of energy alternating with rest periods in between pulses. This is thought to allow for a “cooling period” with thermal dissipation between bursts, thus potentially minimizing collateral tissue damage.
MP-TSCPC has primarily been used to lower intraocular pressure in cases of refractory glaucoma. Whereas traditional TSCPC is typically only used in refractory cases with poor visual potential, MP-TSCPC can also be used in seeing eyes with good visual potential.
MP-TSCPC can be performed on eyes that have received prior incisional glaucoma surgery, such as trabeculectomy, tube shunt surgery, excessive pressure-regulating shunt system miniature glaucoma shunt (Alcon, Fort Worth, TX) surgery, or a combination of these. The procedure is well suited for younger individuals (over 45 years of age) and also older individuals (over 65 years of age). No definitive conclusions have been drawn regarding the suitability or efficacy with respect to other demographic factors such as patient ethnicity.
Various studies have demonstrated that MP-TSCPC is both safe and effective in treating a variety of subtypes of glaucoma, including primary open-angle glaucoma (POAG), pseudoexfoliation glaucoma, neovascular glaucoma, chronic angle closure glaucoma, normal tension glaucoma, uveitic glaucoma, and other secondary glaucomas. ,,
Further, MP-TSCPC has increasingly been considered as an early therapy for glaucomatous eyes, or even in glaucoma suspects, as a potential adjunct or alternative to laser trabeculoplasty, or for patients who are poor candidates for incisional glaucoma surgery. MP-TSCPC can also be repeated in eyes without successful outcome after the initial treatment, though there is little available evidence on retreatment outcomes.
MP-TSCPC is typically performed under monitored anesthesia care as well as a retrobulbar or peribulbar block with 3-5 ml of 2% lidocaine. The instrument commonly used for the procedure is the Cyclo G6® Glaucoma Laser System with MicroPulse P3® Glaucoma Device (Iridex Corporation, Mountain View, CA). Over twenty peer-reviewed articles have reported data using this device.
After anesthesia is administered, the 810 nm diode laser probe is placed perpendicularly to the limbus, while maintaining firm contact at all times. The probe is set on the micropulse delivery mode with specified “on” and “off” times. A duty cycle of 31.3%, i.e., an “on” time of 0.5 ms and an “off” time of 1.1 ms per cycle. Although 2000 mW has been the power most commonly used, various studies have reported settings ranging from 1600 mW to 2500 mW.
There has thus far been no standardized protocol for the parameters of this surgical technique, and settings and technique typically vary based on surgeon preference., Five modifiable parameters—total time of treatment and power, area treated, the position of the probe, and velocity of sweeping motion—can influence the clinical outcome of this technique. Typically, a surgeon moves the probe in a continuous sweeping or fast, sliding, or “painting” or slow motion over the upper and/or lower limbus (180 or 360) of the eye, avoiding the 3 and 9 o’clock positions to protect the ciliary neurovascular structures from an injury. Cystic blebs and other areas of thin conjunctiva should be avoided. Total treatment duration has varied between 100 to 360 seconds per session in reported studies, and in many cases is selected based on surgeon preference. There is no current consensus on ideal treatment duration, though a higher duration may be associated with higher likelihood of adverse events.
Preda and colleagues performed the procedure with differing laser treatment times according to the severity of patients’ intraocular pressures:
Group 1: intraocular pressure < 26 mmHg for 80 seconds Group 2: intraocular pressure between 26 and 30 mmHg for 100 seconds Group 3: intraocular pressure between 31 and 49 mmHg for 120 seconds Group 4: intraocular pressure > 50 mmHg for 130 seconds.
Although success rate varied in each group, at least 60% of patients had reduced postoperative intraocular pressure.
At the conclusion of the procedure, a subconjunctival steroid may be administered. If a retrobulbar block was performed, a patch should be applied to the eye postoperatively.
Postoperatively, patients typically receive topical prednisolone acetate 1% with or without a nonsteroidal anti-inflammatory agent such as ketorolac 0.4% or 0.5% for a minimum of one week. A topical cycloplegic medication may be added as well. These medications can then be tapered depending on the level of inflammation and patient comfort. Topical antibiotic medications are typically not necessary for this non-incisional procedure. Glaucoma medications can be decreased, usually one at a time, if the target intraocular pressure is reached postoperatively.
The majority of data indicate that MP-TSCPC has similar efficacy as conventional TSCPC in the treatment of refractory glaucoma, though with more consistent and predictable results. Two main outcome measures that are similar to traditional TSCPC have been reported. They are 1) reduction of the intraocular pressure after completion of the procedure and 2) reduction of the number of preoperative medications. A systematic review of the literature observed that, at 18 months after treatment, 52% of patients treated with MP-TSCPC and 30% of patients treated with CW-TSCPC were successful in maintaining an intraocular pressure between 6—21 mmHg, corresponding to an approximate 30% reduction from their preoperative intraocular pressures. A postoperative reduction in medications were observed at 18 months, although medications were tapered off as early as three months after the procedure.
Eyes with a history of prior incisional glaucoma surgery appear to have a higher probability of successful outcome after MP-TSCPC compared to those without a preexisting aqueous surgical outflow pathway.
Postoperative reduction of intraocular pressure is also related to baseline intraocular pressures, with lower baseline pressures resulting in greater improvement. Furthermore, a study in which MP-TSCPC was performed at varying power settings while holding laser application constant found that higher power settings were correlated with increased reduction of intraocular pressure, with 2000 mW resulting in 30% reduction and 2500 mW resulting in 57%.
Retreatment rate has also been utilized as an outcome measure. A study reported that, at 12 months after MP-TSCPC, 12% of patients with primary open-angle glaucoma, 16% of patients with pseudoexfoliation glaucoma, and 41.2% of patients with secondary glaucoma had to undergo retreatment. Similarly, another study reported a treatment success rate of 73.7% when only considering initial treatment, and as 89.5% after factoring in patients who received retreatment.
There is some evidence that the efficacy of MP-TSCPC may differ depending on patient age. A study comparing the efficacy in adults compared to pediatric patients reported a success rate in adults of 72.22% versus 22.22% in children.
In MP-TSCPC, the pulsatile energy delivery likely prevents the buildup of excessive heat in surrounding structures, which may minimize collateral tissue damage. This mechanism appears to substantially reduce the complication rate as compared to the traditional TSCPC. The majority of studies have reported no significant or lasting complications after the use of MP-TSCPC.
Most commonly, adverse effects are transient and mild and include pain, anterior chamber inflammation, short-term increase in intraocular pressure, and corneal edema. Rare instances of persistent hypotony, choroidal detachment, permanent vision loss, and phthisis bulbi have been reported. Intraoperative and postoperative pain, however, can be of substantial concern with MP-TSPCP. Administration of retrobulbar anesthesia may mitigate this concern in many cases, though some patients have reported persistent pain postoperatively for 24 hours.
In summary, over the last decade, MP-TSCPC has emerged as a viable alternative for traditional continuous wave diode TSCPC procedure in the treatment of refractory glaucoma. MP-TSCPC uses a series of repetitive short pulses of energy alternating with rest periods in between the pulses to allow for thermal dissipation. MP-TSCPC has been performed successfully in various types of glaucoma. Converging evidence suggests that the procedure has a satisfactory outcome in terms of the reduction in intraocular pressure and in reducing preoperative antiglaucoma medications. Further research is needed to elucidate the role of MP-TSCPC as an early glaucoma treatment and in glaucoma suspects.
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