Classifying Various Audible “Pops” during Cyclophotocoagulation: A Case Report

Dear Editor,

Cyclophotocoagulation (CPC) plays an important role in the control of intraocular pressure (IOP), particularly in eyes with compromised conjunctival conditions in which bleb-forming surgery is limited [1,2]. Because the mechanism of CPC involves destruction of the ciliary body, the effectiveness of laser application has traditionally been assessed based on the presence of audible “pops” [1,2]. Recently, to reduce complications associated with high-energy CPC, slow-coagulation CPC (SC-CPC) has been introduced [3–5]. SC-CPC utilizes lower laser power (typically 1,250 mW) with a longer duration (4,000 msec) compared with conventional settings (usually 2,000 mW and 2,000 msec). This technique is characterized by avoiding laser energy levels that induce audible “pops.” However, in clinical practice, diverse patterns of these sounds are encountered during SC-CPC. Furthermore, in the absence of audible “pops,” it is difficult to ascertain whether laser energy is being adequately delivered to the target tissue during the procedure. Therefore, rather than adhering to the conventional view that all audible “pops” should be avoided, it may be more advantageous to utilize their specific acoustic characteristics as a clinical tool to optimize energy delivery and improve surgical outcomes.

In the case presented in Supplementary Video 1, SC-CPC was performed on an 82-year-old male patient with secondary glaucoma. His IOP was 40 mmHg despite the use of three IOP-lowering medications, and his visual acuity was no light perception. Notably, even with identical laser settings (1,250 mW, 4,000 msec), various patterns of audible “pops” were observed within the same quadrant. Written informed consent for publication of the research details and clinical images was obtained from the patient.

This case demonstrates that even within the same eye under identical laser settings, the effectiveness of SC-CPC may vary according to (1) spatial or functional variations of the ciliary body; (2) regional differences in the biomechanical properties of the sclera, Tenon’s capsule, and conjunctiva; or (3) probe alignment. However, identification of these anatomical or biomechanical factors before or during the procedure is limited. Therefore, the presence and characteristics of audible “pops” during laser application may represent a useful practical indicator for real-time energy titration.

To date, there has been limited classification of audible “pops” during SC-CPC. Based on my clinical experience, these popping sounds can be classified according to two parameters as follows: (1) onset latency—categorized as early (≤2,000 msec after laser initiation) or late (>2,000 msec), and (2) perceived acoustic intensity—graded as minimal, moderate, and pronounced. Using this classification, audible “pops” can be subdivided into various types (Fig. 1).

Fig. 1

Representative waveforms of distinct patterns of audible “pops” presented in chronological order in the Supplementary Video 1: late moderate (left arrow), late pronounced (middle arrow), and early minimal (right arrow). Vertical bars indicated by black arrows denote the acoustic signal recorded over a 4,000-msec laser application period, during which audible “pops” appear as brief, transient peaks. Waveform height reflects relative acoustic intensity, with higher amplitudes indicating louder sounds. The horizontal axis represents time, and the vertical axis shows the normalized amplitude (−1 to +1); positive and negative values denote opposite signal polarities relative to the zero baseline.

Theoretically, an earlier onset of audible “pops” may reflect either rapid energy absorption by the ciliary body, particularly at high power settings with pronounced intensity or inadvertent energy delivery to superficial tissues resulting from inadequate probe–eye contact or improper probe alignment (i.e., too anteriorly positioned probe) with minimal intensity. Therefore, early minimal “pops” should prompt a reevaluation of probe contact and alignment. If early “pops” exhibit moderate to pronounced intensity, a reduction in laser power should also be considered, as these sounds reflect excessive energy absorption.

Regarding the intensity, audible “pops” with a greater intensity may reflect greater energy absorption. Despite an intensity-based classification perceived during the operation is limited to its subjective nature, this classification may enable more precise titration of laser energy. For example, when pronounced audible “pops” are observed, the laser power should be reduced regardless of their onset latency. In contrast, if late minimal or moderate audible “pops” are heard, the setting may be sustained or subtly adjusted based on the clinical status of each eye. Clinical experience suggests that such late minimal-to-moderate “pops” are not typically associated with severe complications, justifying their role as acceptable indicators of treatment efficacy. When no audible “pops” are detected throughout the procedure, it remains at the surgeon’s discretion whether to conclude the treatment or to apply additional applications at higher power levels to elicit at least a few late minimal or moderate “pops.”

This concept of classifying various audible “pops” may redefine their clinical significance, shifting the paradigm from simple avoidance to a nuanced distinction between “acceptable pops” and “avoidable pops.” While this classification remains at a conceptual stage, further studies evaluating surgical outcomes based on the specific characteristics of audible “pops,” alongside objective photoacoustic signal analysis, are warranted to validate these findings.