I read the article “Association Between Choroidal Thickness and Metabolic Activity on Positron Emission Tomography in Eyes With Choroidal Melanoma” with great interest. The authors have reported increases in submacular choroidal thickness in patients with metabolically active choroidal melanoma, even in situations of extramacular involvement.

Increase of submacular choroidal thickness in case of extramacular involvement indicates a diffuse response in the choroid. How can choroidal melanoma, being a localized lesion, trigger a diffuse reactivation in the choroid? The authors explained the cause of high thickness in metabolically active melanoma with increase of metabolic requirements. However, the choroid has a very strong blood flow that seems to provide enough blood supply for every situation, even for choroidal melanoma. Blood flow in the choroid is the highest blood flow in the body. The choroid has 10 times higher blood flow than blood flow through the gray matter of the brain. However, this high blood flow does not correspond to metabolic requirements and is believed to have a function of distributing heat that generates around the macula. In cases of choroidal melanoma, temperature in the environment of the lesion is expected to rise because of high metabolic activity. Choroidal blood flow and, thereby, choroidal thickness may increase to distribute heat. In a similar study, submacular choroidal thickness was found to increase in extramacular toxoplasmosis. The hypothesis of metabolic requirement does not work in this situation. However, inflammation in toxoplasmosis also causes temperature rise. Increased vascular permeability also does not seem to be reasonable. Increased permeability may cause subretinal serous fluid accumulation rather than thickening in the choroid. The temperature control hypothesis seems to be applicable to both of these situations. Another finding that supports the temperature control hypothesis is the rising of choroidal thickness after dynamic exercise. Exercise causes a general temperature rise in the body, and choroidal blood flow may increase to distribute heat that is generated as a result of exercise.

As a result, increased submacular choroidal thickness in metabolically active choroidal melanoma may be related to the temperature control role of the choroid. I appreciate this study, which may give us new ideas about choroidal functions. Considering temperature increase may widen the discussion area of the study.