Determination of the intraocular irradiance and potential retinal hazards at various positions in the eye during transscleral equatorial illumination for different applied pressures.

PURPOSE: With diaphanoscopic illumination of the eye, the intensity of light entering its interior depends on the transmission properties of the eyewall. Light that passes through the eyewall can cause damage to the retina. Therefore, in this study, the intraocular irradiances are determined at different positions on the retina, directly behind the illuminated eyewall, the opposite eyewall and near the macula of ex-vivo porcine eyes. These irradiances are examined for their dependence on the pressure applied on the eyewall with the illuminating fiber and for the influence of the pigmentation of the eye. METHODS: In total 221 ex-vivo porcine eyes were investigated. For transscleral illumination an illumination fiber with a diffusing adapter cap is pressed against the equatorial eyewall. The illumination fiber is pressed onto the eye and the pressure is measured in the anterior chamber. Three different pressures are applied, 23, 78 and 132 mmHg. A detection fiber with diffusing fiber tip is inserted into the eye at the desired position. The eyes were divided in groups with high and less pigmentation to investigate the influence of the pigmentation on the intraocular irradiance. RESULTS: The intraocular irradiances Eintra increases for various increasing applied pressures with the illumination fiber on the eyewall and for various positions inside the eye. With this the irradiances weighted with the photochemical and thermal hazard weighting function, EA-R and EVIR-R, also increases. Differences in Eintra, EA-R and EVIR-R could be found for different pigmented eyes as these values are higher for less pigmented eyes than for strong pigmented ones. CONCLUSION: The hazard to the retina during diaphanoscopic illumination of the eye depends on how strong the surgeon presses the illumination fiber on the eyewall. Depending on the applied pressure and the measuring position in the eye, the specified limit for the photochemical hazard to the retina is partly exceeded. The pigmentation of the eye also plays a role. The irradiance in less pigmented eyes appears to be higher than in strongly pigmented eyes. Because of this, the surgeon should be able to adjust the intensity of the light source to the color of the patient's eye.

Intraocular reflectance of the ocular fundus and its impact on increased retinal hazard.

PURPOSE: Inside the eye light can be reflected multiple times due to light-tissue interactions and the spherical geometry of the eye. Due to these optical properties, a defined retinal area is not only illuminated by direct light but also by indirect, reflected light from the inner side of the eyewall. During illumination for ophthalmic surgery, this could lead to an unintended increase in intraocular retinal irradiance, which was already discussed in previous studies but without a detailed consideration of spectral differences and a potential influence of pigmentation. In this study this effect is investigated wavelength-dependent to see if different wavelengths lead to different increase in irradiance, with a special focus on the raise in photochemical and thermal hazard to the retina. It is also examined whether this effect is dependent on the pigmentation of the eye. METHODS: The reflectance properties of either less or highly pigmented porcine eyes are measured in the wavelength range between 350 and 1100nm with an integrating sphere and a spectrometer. With these reflectance spectra the wavelength-dependent Sphere Multiplier M of porcine eyes can be calculated, which represents the increase of radiance due to multiple reflections inside a sphere compared to a planar diffuser of the same size. Based on measurements of the emitted irradiance of ophthalmic illumination fibers the increase in photochemical and thermal retinal hazard due to these multiple reflections is calculated for eyes with small and high amounts of pigmentation. RESULTS: The reflectance of the inner eyewall in the range between 350 and 1100nm is significantly higher for eyes with low pigmentation (between 4.90% and 37.44% reflectance) in comparison to eyes with a high amount of pigmentation (between 4.30% and 28.88% reflectance). The Sphere Multiplier for the inner side of the eyewall (sclera, choroid and retina) ranges between 1.13 and 1.59 and between 1.13 and 1.48 for eyes with low and high pigmentation, respectively, in the range between 350 and 1100nm. The reflectance, as well as the Sphere Multiplier, is strongly wavelength-dependent due to the absorption spectra of melanin and hemoglobin, which are located in the eye. With increasing wavelength, the reflection properties and the Sphere Multiplier also increases. With this, the photochemical retinal hazard of highly pigmented eyes increases by (14.11± 0.09)% and of lightly pigmented eyes by (16.75±0.35)% compared to if the reflection properties are not considered. The thermal retinal hazard increases by (14.30±0.07)% for highly pigmented eyes and by (19.65±0.17)% for low pigmented eyes. CONCLUSION: This study demonstrates that the anatomy and pigmentation of the eye plays an important role for the reflectance properties of the eye and for the photochemical and thermal hazard to the retina.

Higher Risk of Light-Induced Retinal Damage Due to Increase of Intraocular Irradiance by Endoillumination.

INTRODUCTION: All applied illumination systems are validated according to a standard that measures in an experimental setup the direct radiation intensity on a surface in an aqueous solution, not involving an eyeball. Due to various factors, multiple intraocular light-tissue interactions could occur and lead to retinal irradiation intensities that are higher than the irradiation caused by direct illumination. The aim of this work is to investigate the hypothesis that intraocular and technical reference irradiance is different. METHODS: Using an illumination system and a calibrated optical fiber, the irradiance in porcine eyes was measured at the posterior pole (macula) and compared with reference measurements. We compared two endoilluminators (spotlight and wide-angle) at a total of nine porcine eyes with a brown iris and five porcine eyes with a blue iris. RESULTS: The intraocular irradiance was always significantly higher compared to reference measurements (p < 0.001). Between eyes with a blue or brown iris, no significant difference was observed. CONCLUSION: A significantly higher irradiance could be measured compared to a reference measurement with the same illumination setup. The intraocular illumination increased between 30 and 60%, dependent on the distance of the distal end of the light fiber (4-12-mm distance to the retina). This leads to the assumption that the so far allowed "safe" exposure times for illumination systems are overestimated and the potential hazard to the retina is higher.

Effects of substance P and its antagonist L-733060 on long term potentiation in guinea pig hippocampal slices.

The neuropeptide substance P (SP) has been suggested to be involved in several physiological and pathological conditions including learning and memory and the processing of pain. This study investigated for the first time acute effects of SP and the neurokinin-1 (NK-1) receptor antagonist L-733060 on long term potentiation (LTP) in the hippocampus. Electrically evoked fEPSP was tested under the influence of SP in the CA1 region of the guinea pig hippocampus. Concentrations of 1 and 10 microM SP increased fEPSP slopes to 114.3+/-4.5% and 115.8+/-2.7%, respectively. A threshold concentration was found at 0.1 microM SP. The SP-specific NK-1 receptor antagonist L-733060 did not influence fEPSP in a concentration of 1 microM. In experiments with LTP, a significant increase of potentiations after 60 min was seen with 1 microM SP. Even if the initial baseline increase due to SP (1 microM) was subtracted, potentiations were bigger compared to controls. L-733060 (1 microM) suppressed the excitatory effects of 1 microM SP nearly complete and subsequent induced LTP was not increased. In conclusion, SP has excitatory effects in the hippocampus and is able to facilitate LTP via activation of the NK-1 receptor.