Letter to the Editor: One-Year Outcomes of Micropulse Cyclophototherapy for Primary Open-angle Glaucoma

We read with interest the original study “One-year outcomes of micropulse cyclophototherapy for primary open-angle glaucoma.”1 We would like to congratulate the authors for their work which highlights some important concepts about MicroPulse transscleral cyclophototherapy (MPTCP). Reviewing these concepts will help to put into context the results of Tong et al1 and optimize and standardize MPTCP treatment parameters moving forward.

In Tong et al’s1 study, the authors concluded that the intraocular pressure (IOP)-lowering effect of MPTCP treatment in primary open-angle glaucoma patients was modest and transient. They did not see a significant reduction in medication use and found that additional glaucoma surgery was needed in a number of patients. The authors suggested that while the IOP-lowering effect of MPTCP appears to be transient, it might have a role as a temporizing measure before other glaucoma surgery.

We believe that the authors’ conclusion that the effect of MPTCP in primary open-angle glaucoma patients is modest and transient, is based on the use of subtherapeutic treatment parameters. The laser settings used in this study were 2 W applied over 100 seconds of total treatment time at a set duty cycle of 31.3%. At these settings, the maximum total energy (TE) of 62.6 J (TE is calculated by multiplying power×total treatment duration×the duty cycle) used in this study is at the lowest end of the range reported in the literature.2 In addition, Tong et al1 state that in some instances, up to 50% of the limbal circumference of the eye was left untreated when areas of previous surgery were avoided. The authors also state that the power and duration of the treatments were sometimes decreased as well. Thus, in many patients, the energy delivered was effectively <62.6 J.

The low TE used by Tong et al1 has already been shown in the prior literature to result in a suboptimal nonsustainable IOP-lowering effect.3 Tong et al1 do state that maximal IOP decrease was greater when higher energy was used, and this is supported by the literature, which shows a dose-response relationship for MPTCP.4,5 For example, Marchand et al5 performed a prospective 18-month study using TE between 150.2 and 200.4 J. At 18 months, mean IOP was reduced by 40.1% in the group that received 200.4 J of treatment compared with 30.8% in the group that received 150.2 J of treatment. Treatment absolute success, as defined as IOP between 6 and 21 with a reduction in IOP of 25% with equal or less number of medications, was overall 61.5% at 12 months and 59.6% at 18 months.

When rates of retreatment are examined in the literature, it is clear that lower energy administered during MPTCP is also associated with higher rates of retreatment. Aquino et al6 and Tan et al7 used the exact same amount of energy as Tong et al1 (62.6 J) and had retreatment rates of 48% (Aquino et al6) and 35% (Tan et al7). In contrast, the rates of retreatment were significantly lower in Al Habash and AlAhmadi8 (5.6%) and Yelenskiy et al9 8.6%. Al Habash et al8 used TE of 165.2 J and Yelenskiy et al9 111.6–148.8 J, almost 2–3 times more than Tong et al.1

The final important variable of MPTCP treatment is sweep velocity.2 Tong et al1 mention that the treatment probe was moved in a continuous sliding motion; however, sweep velocity was not described. To illustrate the importance, slowing sweep velocity from 2.8 mm/s (10 passes of 8 s sweeps over 80 s of total treatment time) to 1.4 mm/s (5 passes of 16 s sweeps over 80 s of total treatment time) doubles the treatment fluence.2 Fluence, which is significantly impacted by sweep velocity, is the energy delivered per unit area (Fluence = Energy used × duty cycle × dwell time/area).2

We believe the current best practices discussed here put into perspective the work by Tong et al.1 We also recommend that every new publication have a standardized and clear description of all the MPTCP parameters (power, duty cycle, sweep velocity, and number of sweeps) used to better assess the amount of energy delivered to the eye. In addition, we recommend consideration of the World Glaucoma Association (WGA) guidelines as a systematic approach when evaluating for side effects or potential complications.

Tomas M. Grippo, MD*†
Natalie Brossard Barbosa, MD, MSc‡
Robert Noecker, MD§∥
Valentina Campisi, BSBME*
Ziad Khoueir, MD¶#**
Syril Dorairaj, MD#
*Grippo Glaucoma & Cataract Center
†Hospital Alemán, Buenos Aires, Argentina
‡Department of Ophthalmology and Vision Sciences, Faculty of Medicine, University of Toronto, Toronto, ON, Canada
§Department of Ophthalmology and Visual Science, Yale School of Medicine, New Haven
∥Ophthalmic Consultants of Connecticut Fairfield, CT
¶Glaucoma Department, Beirut Eye and ENT Specialist Hospital
**Ophthalmology Department, Saint-Joseph University, Faculty of Medicine, Beirut Lebanon
#Ophthalmology Department, Mayo Clinic Jacksonville, FL