Discussion

This study has taken a unique approach by measuring luminosity using a lux meter rather than solely relying on light source settings. Additionally, this study has considered an extensive list of variables, which have previously not been considered together in a succinct experimental study.

We have demonstrated that thermal energy transfer from operating endoscopes is greatest when the light intensity is high and when the endoscope is touching the surface. Furthermore, new endoscopes appear to generate more heat in comparison to older endoscopes, and, in contrast to some other studies, LED light sources generate the greatest temperature and luminosity.

The thermal energy generated by operating endoscopes has been linked to the ignition of drapes and damage to skin,^{Reference Sandhu, Turner and Pozo13,Reference Bellina and Haas14} and altering of the workings of the inner ear.^{Reference Kozin, Lehmann, Carter, Hight, Cohen and Nakajima4,Reference Kahana, Rosenblith and Galambos6–Reference Bottrill, Perrault and Poe8,Reference Das, Mitra, Agarwal and Sengupta11} However, the maximal temperature achieved has varied greatly amongst the different papers studied. For example, Hensman et al. noted temperatures as high as 225°C within 10 minutes of switching on the light source.^{Reference Hensman, Hanna, Drew, Moseley and Cuschieri15} Conversely, Kozin et al. demonstrated temperatures of 46°C at 0.5–1 mm from the tip of the endoscope.^{Reference Kozin, Lehmann, Carter, Hight, Cohen and Nakajima4} Our study therefore aimed to determine what factors led to these variances in temperature.

The effects of high temperatures on body tissues has been assessed in several studies. Guinea pig models, exposed to high temperatures produced by a xenon light source for 5 minutes, showed lower distortion product otoacoustic emissions and an increased auditory brainstem response threshold.^{Reference Aksoy, Dogan, Ozturan, Eren, Veyseller and Gedik7} Similarly, Kahana et al. noted changes to multiple aspects of cochlear microphonics in the hamster cochlea when temperatures rose only to 39°C.^{Reference Kahana, Rosenblith and Galambos6} Vestibular dysfunction was reported by Bottrill et al. when a temperature rise in the ear caused caloric stimulation.^{Reference Bottrill, Perrault and Poe8} Various studies have specifically investigated the propensity for tissue damage and damage to the inner workings of the ear associated with the thermal energy generated by endoscopes used in ENT.^{Reference Kozin, Lehmann, Carter, Hight, Cohen and Nakajima4,Reference Aksoy, Dogan, Ozturan, Eren, Veyseller and Gedik7,Reference Bottrill, Perrault and Poe8,Reference Tomazic, Hammer, Gerstenberger, Koele and Stammberger10,Reference MacKeith, Frampton and Pothier16–Reference Lewis, Levin and Sommer18} However, thus far, only one paper has reported thermal-related injury in a patient undergoing otology surgery.^{Reference Das, Mitra, Agarwal and Sengupta11} Das et al. demonstrated audiometric change in higher frequencies, which was greater with xenon light sources. Vomiting, vertigo and tinnitus were also noted in patients exposed to either xenon or LED light sources.^{Reference Das, Mitra, Agarwal and Sengupta11}

While the maximal temperature recorded in the study, from an exposed light cable, was 174.3°C, the maximal temperature at the tip of the endoscope, in any experimental condition, was 44.1°C. Furthermore, temperatures recorded at lower light intensities were significantly decreased. This trend has been demonstrated in previous studies. Tomazic et al. reported temperatures as high as 91.4°C at 100 per cent light intensity with a xenon light source, which decreased to 44.3°C at 33 per cent light intensity.^{Reference Tomazic, Hammer, Gerstenberger, Koele and Stammberger10} Ito et al. measured the safety of heat generated by endoscopes with xenon and LED light sources in a 3D model of a human temporal bone.^{Reference Ito, Kubota, Takagi, Watanabe, Futai and Furukawa5} It was noted that a xenon light source generated more thermal energy at 100 per cent light intensity than at 30 per cent intensity.^{Reference Ito, Kubota, Takagi, Watanabe, Futai and Furukawa5} Importantly, the use of a lower light intensity intra-operatively is feasible: McCallum et al. demonstrated no detriment to image quality as a result of the very low light levels used, as low as 10 per cent, which was in turn associated with lower heat generation.^{Reference McCallum, McColl and Iyer12}

This study also demonstrated a significant increase in temperature recordings when the endoscope tip was in contact with the thermometer. This highlights the importance of avoiding resting the endoscope tip on operative surfaces, particularly in areas that may be sensitive to thermal damage. Other factors studied included: endoscope diameter, angled endoscopes versus 0-degree endoscopes, and whether the endoscope was new or used. Larger-diameter endoscopes, angled endoscopes and new endoscopes appeared to generate greater heat; however, none of these factors achieved significance. While some studies have demonstrated higher thermal energy production from endoscopes with larger diameters,^{Reference Aksoy, Dogan, Ozturan, Eren, Veyseller and Gedik7,Reference MacKeith, Frampton and Pothier16} because of the minimal temperature difference in larger-diameter endoscopes measured in this study, the authors would not advocate this as a reason for selecting smaller endoscopes.

Our study investigated the maximum temperatures reached at 0, 30, 60 and 180 seconds after switching on different light sources at 100 per cent intensity. Previous studies have investigated temperatures reached up to 30 minutes after the light source was switched on, although a number of studies showed that maximum temperatures were reached within 60 seconds of switching on the light source.^{Reference Kozin, Lehmann, Carter, Hight, Cohen and Nakajima4,Reference Aksoy, Dogan, Ozturan, Eren, Veyseller and Gedik7,Reference Tomazic, Hammer, Gerstenberger, Koele and Stammberger10,Reference MacKeith, Frampton and Pothier16} This appeared to be the case in our study, with temperature change over time demonstrating a logarithmic pattern of increase, with the greatest temperature rise within the first 60 seconds (Figure 4). These findings suggest that operative pauses, while rapidly cooling the endoscope,^{Reference Kozin, Lehmann, Carter, Hight, Cohen and Nakajima4} are unlikely to be of benefit overall to the patient because of the rapid return to close to maximal temperatures. One additional method to avoid excessive heat exposure within the ear is frequent aspiration and irrigation, and this could be adopted intra-operatively.^{Reference Ozturan, Dogan, Eren and Aksoy19}

While there were attempts to optimise the experimental design, this study does have some limitations. It was only possible to assess 12 endoscopes, as these were the only otological endoscopes available for study in the department. This led to a small sample size, which could have affected statistical outcomes. The authors argue, however, that if the influence of a particular factor on intra-operative temperatures would only be significant with a large sample size, its relevance clinically would be minimal. In addition, the plastic tubing and matt black surface will not have accurately simulated the middle-ear environment. However, it was not feasible to use animal or cadaveric tissue with the luminosity meter or in the operating theatre environment; furthermore, it would not have been possible to test as many endoscopes without this simulation.