This study investigated temperature changes in the anterior chamber of a model eye caused by a surgical microscope and illuminated chopper. The result of this study showed that as the intensity of the light source of the surgical microscope was increased, the anterior chamber temperature was increased. The anterior chamber temperature was increased by 1.4 ℃ when using the surgical microscope at level 6 (illuminance of 32500 lux) and was increased by 1.0, 0.5, and 0.2 ℃ at level 5 (24900 lux), 4 (16490 lux), and 3 (10050 lux), respectively, for 10 min. In comparison with the surgical microscope placed 20 cm away, the illuminated chopper positioned in the anterior chamber and in direct contact with the aqueous humor was assumed to have a greater thermal effect. However, the illuminated chopper at 99% light intensity (14893 lux) had an illuminance similar to that of the surgical microscope at level 4 and 3 and increased the anterior chamber temperature by 0.2° C, which was equal to the increase in the temperature caused by the surgical microscope at level 3. Even with the illuminated chopper positioned in the anterior chamber, the thermal effect of the illuminated chopper appeared to be similar to that of a microscope with similar illuminance. In our institute where this experiment was conducted, the brightness of the surgical microscope is set to a value between 5 and 6 when performing external or intraocular surgery. In addition, the brightness of the light source of the illuminated chopper is set to 40–50% during cataract surgery. Therefore, it seems that surgery using a surgical microscope rather than an illuminated chopper may significantly increase the anterior chamber temperature over time.
Previous studies have shown that thermal energy produced by the phaco tip during phacoemulsification could increase the anterior chamber temperature and result in corneal tissue burn and damage.[10, 11] Incision size, ultrasound power, tip design, and aspiration flow rate could affect the corneal incision site temperature.[12] Specifically, irrigation and aspiration flow have been reported to prevent an increase in the incision site temperature.[13] There was no irrigation and aspiration flow in this experimental setting. Therefore, if there is adequate irrigation or aspiration flow, the anterior chamber temperature increase caused by the surgical microscope may be negligible. However, in the case of external eye surgery without irrigation and aspiration flow, although the anterior temperature increase is only 1 ~ 2 ℃, an increase in the intraocular temperature caused by the surgical microscope may occur. In addition, as the temperature continued to increase over time in this experiment, it is thought that the longer the operation time, the greater the anterior chamber temperature increase when using the surgical microscope.
Glaucoma is a progressive optic neuropathy with the loss of retinal ganglion cells.[14] Exposure of retinal ganglion cells to high temperatures can induce reactive oxygen species production, resulting in cell stress with apoptosis.[15, 16] Therefore, increased ocular temperature can be a risk factor for the progression and development of glaucoma.[16–18] In addition, a higher environmental temperature seems to increase the prevalence of open-angle glaucoma.[18] Although irreversible retinal damage only occurs when the temperature around the retina has increased by more than 10 ℃,[2] especially in patients with glaucoma, the potential adverse effect of temperature increase during surgery under a surgical microscope should be considered.
There are some limitations in this study. First, this study was conducted using model eyes filled with BSS. Therefore, the results of this study may not be applicable to human eyes. Second, although the mechanism of ocular damage by light exposure includes three major forms (photothermal, photomechanical, and photochemical),[19–21] this study only evaluated temperature changes caused by the surgical microscope and illuminated chopper for a given exposure time. Therefore, it is necessary to evaluate the effect of the anterior temperature increase on ocular tissues when using a surgical microscope in vivo.