Corneal and retinal temperatures under various ambient conditions: A model and experimental approach

Background: To determine the distribution of temperatures between the cornea and retina under various external conditions, such as illumination and external temperature, using a simplified heat exchange model. Materials and Methods: The human eye is modeled as a water sphere surrounded by different heat sources: choroidal and ciliary body blood flow, tears evaporation, metabolic activity, diffuse retinal illumination and external convection. Corneal and retinal temperatures are derived from this model using finite element theory applied to heat transport. Each of five subjects (28 +/- 10 years old) were placed in three different environments: -20degreesC, 20degreesC and 40degreesC. After 15 mn, their corneal temperatures were measured with an infrared camera. Corneal temperatures are compared with the values obtained from the model, assuming the same blood flow for all environments. Results: 1) Ciliary body and choroid contribute at least 20 times more than retina to the retinal temperature; 2) temperature increase of the retina due to illumination is negligible; 3) a 10% reduction of choroidal blood flow induces a corneal temperature decrease of 0.2degreesC at -20degreesC and <0.1degreesC at 20degreesC and 40degreesC. At normal choroidal blood flow, changes in ambient temperature have a negligible effect on retinal temperature. Measured corneal temperatures agreed with the values from the model: 26.4 +/- 0.9 versus 26.8degreesC, at -20degreesC and 36.2 +/- 0.5 versus 36.7 at 40degreesC. Conclusions: Our simplified eye model predicts in a satisfactory way measured corneal temperatures under very different conditions. Calculations show that changing external temperatures from -20 to + 40degreesC affects the retinal temperature by less than 1.8degreesC.