Thermal therapies have long been used in ophthalmology, but traditional retinal laser treatments can cause irreversible damage because of imprecise temperature control. The ability to cause mild, sublethal hyperthermia may stimulate protective cellular pathways like autophagy reactions and heart-shock without harming retinal tissue. Therefore, accurate real-time retinal temperature monitoring is important. This study presents a novel focal electroretinography (fERG) based thermal dosimetry method, which enables local temperature estimation during the laser-induced hyperthermia.
This study investigated the molecular and biological effects of non-damaging laser exposure on the retinal pigment epithelium (RPE)/choroid of pigs in combination with temperature-controlled laser delivery and molecular analysis. It also established a therapeutic temperature window that induces protective cellular response (HSP70 production and autophagy activation) without any tissue damage.
This study was conducted on 11 healthy male domestic pigs (12 to 16 weeks old) under general anesthesia. A custom-built temperature-controlled laser system integrated in a slit lamp biomicroscope delivered four co-aligned light beams: an 810 nm treatment laser beam for the localizing heating, a 520 nm stimulus beam for fERG recording, and peripheral and central background beams for the uniform light adaptation. Laser spots of 5 mm and 3.4 mm in diameter were applied to the fundus. The fERG signal was recorded using DTL fiber and surface electrodes.
Retinal temperature elevation was estimated from changes in the fERG response kinetics by using a correlation-based algorithm that compared pre-exposure and exposure reactions. The temperature sensitivity coefficient (0.037 °C⁻¹) was derived from pig ERGs at different body temperatures. A calibration protocol with three mild exposures (<5 °C increase) established the local thermal responsivity (°C/mW) before each treatment. It determined the laser power needed for each present target temperature (43 to 50 °C).
Each laser exposure ERG. Seventy-two treatments were performed. Post-treatment evaluation involved fundus imaging, molecular analysis of harvested RPE/ choroid and neural retina, and fluorescein angiography. This included ROS quantification (DCFH-DA assay), immunofluorescence (HSP70, actin, TUNEL), and qPCR for 91 selected genes and immunoblotting for autophagy, heat shock, VEGF-related proteins, and apoptosis. Statistical analysis was performed by using probit modeling, ANOVA with post hoc corrections, and t-tests.
fERG monitoring provided real-time local retinal temperature estimates with an accuracy of about 0.6 °C. This enables individualized power control. Temperature increased linearly with the laser power up to mild heating, but nonlinear effects appeared at high powers. Thermal responsivity scaled proportionally with the laser spot size and pigmentation, which confirms reproducible temperature control.
Biological thresholds were identified. HSP70 expression elevated significantly at 48.5 °C (ED50), and visible lesions appeared at 47.7 to 48.0 °C. Treatments under 46 °C caused no morphological damage. RPE/choroid showed upregulation of heat shock genes like HSP70, HSF1, HSP90, and autophagy makers like ULK2, ATG5, LC3B, and LAMP2 at 44 to 46.5°C. Protein analysis confirmed increased HSP90 and HSP70, and elevated LC3B-II with decreased p62, consistent with activated autophagic flux. The ROS assay showed no major oxidative stress elevation. Antioxidant enzymes like SOD1 and GPX1 were upregulated. TUNEL and caspase-8 analyses showed no apoptosis under 48°C, which confirms the safety of non-damaging exposure.
This study showed that fERG-based retinal thermal dosimetry allows for accurate local temperature control during the laser treatment. It effectively addresses individual differences in pigmentation and heat absorption. It detects a therapeutic temperature window that activates protective cellular mechanisms that avoid structural damage, heat shock proteins support protein stability, and prevent aggregation in this range. Autophagy supports cellular maintenance. Temperatures above 47 °C can diminish therapeutic benefits and increase the risk of thermal injury.
Reference: Amirkavei M, Kaikkonen O, Turunen T, et al. Non-damaging laser treatment with electroretinography-based thermal dosimetry activates hormetic heat response in pig retinal pigment epithelium. Nat Commun. 2025;16:9533. doi:10.1038/s41467-025-64095-6
