Laser scatter reaching the posterior pole during laser cyclophotocoagulation

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Purpose. The reported incidence of visual reduction with transscleral Nd:YAG cyclophotocoagulation (CPC) ranges between one-third and two-thirds of treated patients. One mechanism for visual reduction we propose is that retinal light toxicity from scattered laser light might reach the posterior pole. We addressed this theory by measuring the percentage of laser energy reaching the posterior pole in human autopsy eyes treated with Nd:YAG or diode transscleral CPC. Methods. Three different laser modalities were employed in this study: the Nd:YAG laser was used in both the free-running, thermal mode (Microruptor II, LASAG ((MR2)) and the continuous-wave mode (Microrupter III, LASAG ((MR3)); the Oculite SLx semiconductor diode laser was also used. Two pairs of human autopsy eyes were obtained and a 7 mm full-thickness button was excised from the posterior pole. Each eye was mounted on a glass cover slide. The MR2 was used with a duration of 20 msec and an energy level between 5 and 7 J. The MR3 was used with a duration of 1 and 2 sec and a power of between 1.6 and 3.7 W. The diode was used with a 2 sec duration pulse and power levels between 1750 and 3000 mW. Two different laser modalities were applied to one hemifield of each eye and the amount of energy reaching the back of the eye was recorded with a laser Molectron pryometer. Analysis of percentage of input levels reaching the posterior pole among the three lasers was performed using a student t-test. Results. The mean average energy measured at the posterior pole for all energy levels of the MR2 was 191 mJ or 3.99% of input energy. The mean average power for all power levels of the MR3 was 127mW or 3.50% of the original laser power. The mean average power for all power levels of the diode laser was 116 mW or 4.7% of the input power. A statistically significantly higher percentage of input laser was measured at the posterior pole with the diode laser as compared to the MR2 (p=.01) and the MR3 at 1 sec duration (p=.0002). Conclusion. During a typical clinical treatment session, each patient may receive from 18 to 36 exposures with 3 to 5% of the energy reaching the posterior pole with each application of CPC. Clinical correlation is necessary to determine the significance of these findings as related to decreased visual acuity after CPC.

Duke Authors

Cited Authors

  • Herndon, LW; Trevisani, MG; Imami, N; Straub, D; Allingham, RR; Shields, MB

Published Date

  • February 15, 1996

Published In

Volume / Issue

  • 37 / 3

International Standard Serial Number (ISSN)

  • 0146-0404

Citation Source

  • Scopus