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 Table of Contents  
REVIEW ARTICLE
Year : 2022  |  Volume : 60  |  Issue : 3  |  Page : 222-226

A review on technology and different probes in transscleral cyclophotocoagulation


1 Department of Ophthalmology, M. N. Eye Hospital, Chennai, Tamil Nadu, India
2 Department of Ophthalmology, Swamy Eye Clinic, Chennai, Tamil Nadu, India

Date of Submission04-May-2022
Date of Decision26-May-2022
Date of Acceptance06-Jun-2022
Date of Web Publication26-Sep-2022

Correspondence Address:
M Salu
M. N. Eye Hospital, 781, T. H. Road, Tondiarpet, Chennai, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tjosr.tjosr_45_22

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  Abstract 


Glaucoma is a chronic progressive optic neuropathy, characterised by retinal ganglion cell loss leading to optic nerve head changes and visual field defects. The control of intraocular pressure remains a key determinant factor in the management of glaucoma. Cyclodestructive procedures target the epithelium of ciliary processes, thereby reduces the aqueous formation. Cyclodestructive procedures have advanced from initial cyclodiathermy to micropulse diode laser cyclophotocoagulation (CPC). The various probes used in transscleral CPC (TSCPC) include G-probe delivery device and G-probe illuminate delivery device for continuous wave TSCPC and micropulse P3 probe for micropulse diode laser TSCPC. This review article provides details on design of probes and their technology.

Keywords: Cyclophotocoagulation, micropulse, G-probe


How to cite this article:
Salu M, Ariga M, Nivean PD, Nivean M. A review on technology and different probes in transscleral cyclophotocoagulation. TNOA J Ophthalmic Sci Res 2022;60:222-6

How to cite this URL:
Salu M, Ariga M, Nivean PD, Nivean M. A review on technology and different probes in transscleral cyclophotocoagulation. TNOA J Ophthalmic Sci Res [serial online] 2022 [cited 2022 Nov 27];60:222-6. Available from: https://www.tnoajosr.com/text.asp?2022/60/3/222/357114




  Introduction Top


Glaucoma is a chronic progressive optic neuropathy, characterised by retinal ganglion cell loss leading to optic nerve head changes and visual field defects.[1] Glaucoma is the second leading cause of blindness in the world.[2] The control of intraocular pressure remains a key determinant factor in the management of glaucoma. This can be achieved by either lowering of aqueous humour production or enhancing aqueous outflow. Treatment options for lowering of intraocular pressure include medical therapy, filtration procedures, glaucoma drainage devices (GDD) and laser trabeculoplasty.[3],[4],[5] Transscleral diode laser cyclophotocoagulation provides an effective solution for treating refractory glaucoma. It is emerging as a primary treatment option for glaucoma in complicated situations.

Cyclodestructive procedures have gained importance in treating refractory glaucoma. Cyclodestructive procedures target the epithelium of ciliary processes, thereby reduce the aqueous formation. Cyclodestructive procedures have advanced from initial cyclodiathermy to micropulse transscleral cyclophotocoagulation. The advancements in cyclodestructive procedures led to more focussed energy and targeted destruction of the ciliary process with minimal collateral damage to the adjacent non-pigmented tissue. These techniques provide lowering of intraocular pressure and halt the progression of glaucoma. The laser cyclophotocoagulation (CPC) has been described in three modes of application: transpupillary CPC (TPCPC), transscleral CPC (TSCPC) and endoscopic CPC (ECPC). TPCPC involves the transmission of 488-nm argon laser beam through the pupil to induce photocoagulation of the ciliary process. TSCPC entails the application of laser beam to the overlying sclera which is absorbed by melanin in the ciliary process, leading to selective thermal coagulation of ciliary body. Neodymium-doped Yttrium Argon Garnet laser (Nd: YAG laser) (1064 nm) and semiconductor laser (810 nm) are used either by contact or non-contact techniques. ECPC is a cilioablative procedure where under the endoscopic guidance, the ciliary processes are photocoagulated.[6] TCPC and ECPC are a useful modality of treatment in treating refractory glaucoma with potential complications. ECPCs have relatively lower complications than TCPC.[7]


  Continuous Wave Transscleral Cyclophotocoagulation Top


A 810-nm near-infrared diode laser applied transsclerally selectively targets the pigmented ciliary epithelium resulting in coagulative necrosis of the ciliary epithelium and stroma and thereby lowering of aqueous production. The procedure is done using Cyclo G6 Glaucoma Laser system [Figure 1] with either G-probe delivery device [Figure 2] or G-probe illuminate delivery device [Figure 3] (duty cycle: 100%). There are two approaches of performing the procedure-standard technique and slow coagulation technique. The standard technique involves a starting power 1750 mW for a 2-s duration, delivering laser energy of 3.5 J per application. The slow coagulation approach involves 1250 mW for 4.0–4.5 s, delivering 5.0–5.6 J of laser energy per application for eyes with dark brown iris and 1500 mW applied for 3.5–4.0 s duration, delivering 5.25–6.00 J of energy for eyes with other degrees of iris pigmentation.[8] Complications like persistent ocular inflammation, hypotony, and phthisis bulbi have been reported. CW-TSCPC is mainly reserved for patients with refractory glaucoma and for palliation of painful eyes that have poor visual prognosis. The procedure can be done in an operating room or outpatient department or in conjunction with procedures like cataract surgery.
Figure 1: Cyclo G6 laser system

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Figure 2: G-probe delivery device

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Figure 3: G-probe illuminate delivery device

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  Micropulse Diode Laser Cyclophotocoagulation Top


MP-TSCPC provides an efficacious solution to control IOP. It is the most recent form of transscleral diode CPC. The procedure involves laser delivery in bursts with an 'on time' and 'off time'. The laser beam is delivered in series of repetitive short pulses with pauses between them, thus allowing the tissues to cool down between pulses, thereby alleviating the cumulative thermal damage. The exact mechanism of action of micropulse TSCPC remains unclear. It is hypothesised that MP-TSCPC enhances the existing uveoscleral outflow without photocoagulating the target tissue and thereby lowers IOP.[9] The procedure is applicable in an operating room or in an outpatient department.

There are various probes used in TSCPC [Table 1]. These include G-probe delivery device, G-probe illuminate delivery device and micropulse P3 (MP3) probe. G-probe and G-probe illuminate delivery devices are used for continuous wave TSCPC. MP3 probe [Figure 4] is used for micropulse diode laser TSCPC.
Figure 4: Micropule P3 probe

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Table 1: Characteristics of probe

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G-probe delivery device

The laser delivery quartz fibre-optic probe was 600 μm in diameter. The planar polished tip of G-probe protrudes 0.7 mm from a handpiece. The footplate of G-probe positioned on the limbus and held parallel to the visual axis of eye.[10] The characteristic wedged tip design of G-probe facilitates the precise placement and movement of the probe around the circumference of the limbus. The footplate of the probe is firmly placed between the anterior border and middle of the limbus. Laser applications are done over 270~ (three quadrants) around the limbal region, sparing the temporal quadrant. Six to seven laser applications are done per quadrant. Laser power settings vary depending in the iris colour ranging from 1250 to 1500 mW delivering 5.00–5.25 J of laser energy. The adjacent burns are spaced one-half of the width of the footplate. Shelf-life of the probe is about 3 years. The procedure is carried out until an audible pop (due to the uveal tissue microexpulsion) occurs. Single use of probe is recommended.[11] Gaasterland et al.[12] used the G-probe with the semiconductor diode laser system with laser settings of maximum power output of 2.5 W and maximum duration of 9.9 s. In recent days, this G-probe is used with the Cyclo G6 system for continuous wave diode laser CPC that helps in achieving a long-term, effective IOP reduction in refractory glaucoma. Aquino et al.[13] conducted the study with the laser settings of 1.5–2 W that delivers 60–112 J of energy. Eyes were treated with 20–28 burns. The laser was applied with exposure time of 2 s per burn. Bhola et al.[14] described the complications like pupillary distortion, scleral thinning and posterior staphyloma following TSCPC based on the histopathological examination of enucleated eyes [Table 2].
Table 2: Studies on CW-TSCPC and MP-TSCPC

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G-probe illuminate delivery device

Like the G-probe, the wedged tip design favours the proper placement of probe on the limbus. This device has an advantage of transillumination property. On placement of probe on the limbus, the anterior margin of ciliary body is highlighted. The footplate of the probe is firmly placed between the anterior border and middle of the limbus.

Laser applications are done over 270~ (3 quadrants) around the limbal region, sparing the temporal quadrant. Six to seven laser applications are done per quadrant. Laser power settings vary depending in the iris colour ranging from 1250 to 1500 mW delivering 5.00–5.25 J of laser energy. The adjacent burns are spaced one-half of the width of the footplate. Shelf-life of G-probe illuminate delivery device is 3 years. It is a single-use product.[15]

Micropulse MP3 probe

Micropulse MP3 probe houses a fibre-optic cable. The fibre-optic tip of MP3 probe is 600μm in diameter and protrudes 0.4 mm from the tip. One side of the probe tip is notched, it is placed facing the limbus and the other side is flat, positioned facing the eyelids. The probe is placed 3 mm posterior to the sclera. The MP3 handpiece with the Iridex Cyclo G6 (IRIDEX Laser Systems) operates at a standardised preset power of 2000 mW and a duty cycle (time during which the laser is delivering energy) of 31.33% (micropulse 'ON' time of 0.5 ms and micropulse 'OFF' time of 1.1 ms).

Two arcs of treatment are applied comprising superior hemisphere from 9.30 to 2.30 clock position and inferior hemisphere from 3.30 to 8.30 clock position (80 s superiorly and 80 s inferiorly). The motion of probe should avoid the 3 o'clock and 9 o'clock meridians; henceforth, the risk of damage to anterior ciliary vessels and nerves is avoided. The areas of scleral thinning, sites of failed filtering blebs and GDD are avoided. The laser probe's fibre-optic tip was applied with steady pressure in a continuous sweeping motion to and fro over the globe. The laser is delivered in a series of repetitive short pulses with pauses between them. The OFF period limits the accumulation of caloric energy in the adjacent tissues of pigmented epithelium. Thus, the procedure enables a subthreshold damage to the pigmented ciliary epithelium, thereby alleviating the cumulative thermal damage.[16],[17],[18],[19]

Studies on CW-TSCPC and MP-TSCPC are discussed in [Table 2]. Ariga et al.[20] reported that a significant intraocular pressure reduction (P < 0.001) was obtained after MP-TSCPC in refractory glaucoma in Indian population. At 3 m, 89.1% had surgical success. Studies have shown that the effects of micropulse diode laser CPC vary with the laser power settings that leads to a considerable variation of IOP reduction. Sanchez et al.[21] described the effects of MP-TSCPC in a dose-dependent manner and reported that with same power of 2 W and duration of laser delivery 320 s and duty cycle of 31.3% and variations in energy levels. The authors studied that the low energy levels <100 J yielded lower IOP reduction with no potential complications and shorter survival rate and moderate energy levels 112–150 J obtained 35% IOP reduction in 15 months and the high energy levels >200 J produced a significant IOP reduction but with greater complication rate. Kaba et al.[22] analysed that the mean IOP reduction of 31.5% and 17.8% was observed in patients treated with laser power >2500 mW and <2500 mW, respectively.

Lun et al.[23] studied that the repeated usage of MP3 probe increases the laser output. The authors postulated that the repeated surface abrasion of the probe tip, as it moves in a circumferential manner on the conjunctiva, enhances the laser delivery. Moreover, repeated use of the MP3 probe leads to formation of a water column surrounding the fibre-optic probe leading to reduced energy dispersion.

The probe can be moved in continuous sweeping over the superior and inferior hemifields of the globe either as fast sliding or slow sweeping motion. Emanuel et al.[24] reported that it was a nascent nature of procedure and there was no standardisation of sweep technique. Some surgeons prefer fast-sweeping motion of about 10 s back and forth over 180°, and others used a slow-sweeping motion of about 1 min over 180°. Wong et al.[25] analysed MP3 Plus therapy to be effective in patients with refractory glaucoma particularly neovascular glaucoma. The protocol of MP3 Plus includes two phases. (a) First phase: 810 nm diode laser was used at 2 W power with a duty cycle of 31.3%. The probe was applied with continuous sliding motion, delivering the laser for duration of 100 s. (b) Second phase: an additional laser was delivered at 1.5–2 W for 2 s per shot with a duty cycle of 41.5%. The laser was applied in 12–16 shots over the superior and inferior peri-limbal region. They hypothesised that prolonged inflammation or chronic fibrovascular changes might result in the formation of a hydrophobic layer on the pars plana inhibiting the effects of MP-TSCPC and this modified approach with additional pulses dissipates this layer and improves the uveoscleral aqueous outflow.

Garcia GA et al.[26] observed that the complications following MP-TSCPC were more if the treatment duration is > 180 s compared to lesser laser duration. Decrease in BCVA from baseline (7.8%), hypotony (1.7%), prolonged AC (0.9%), choroidal effusion (0.9%), corneal abrasion (0.9%) and cystoid macular oedema (0.9%) were reported following greater exposure time of micropulse laser. Asian race and phakic status were observed with increased frequency of postoperative mydriasis succeeding MP-TSCPC by Radhakrishnan et al.[27] in their multi-centred retrospective study on MP-TSCPC.

Aquino et al.[13] demonstrated that complications like prolonged anterior chamber inflammation, hypotony and phthisis bulbi were found to be more following continuous wave CPC than micropulse TSCPC.


  Conclusion Top


Both continuous wave and micropulse transscleral cylophotocoagulation aid in the treatment of refractory glaucoma. Both the procedures are non-invasive, safe and a useful option to control IOP in patients who are alone, not able to follow-up, with limited visual potential. The extended duration of treatment has been associated with the greater complication rate. However, micropulse TSCPC can be considered as a safe alternative to continuous wave CPC in the management of refractory glaucoma with better surgical outcomes and minimal complications.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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[PUBMED]  [Full text]  
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Iridex.com. 2022. G-Probe Laser Probe, Glaucoma Drainage Device, TSCPC Probe, Glaucoma Surgery. Available from: https://iridex.com/Products/GlaucomaDevices/G-Probe.aspx. [Last accessed on 2022 Mar 12].  Back to cited text no. 11
    
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Bhola RM, Prasad S, McCormick AG, Rennie IG, Talbot JF, Parsons MA. Pupillary distortion and staphyloma following transscleral contact diode laser cyclophotocoagulation: A clinicopathological study of three patients. Eye (Lond) 2001;15:453-7.  Back to cited text no. 14
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

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