Infrared thermography is a noninvasive, non-stressful way of detecting vascular, neoplastic and inflammatory pathologies. Researchers recently measured the ocular surface temperature (OST) of patients with AMD and DR, since both conditions result from abnormal vascular changes likely related to low-grade inflammation and vasculopathy. They found that these conditions demonstrate marked temperature increases compared with those of healthy controls.

The retrospective study included 133 patients (260 eyes) diagnosed with DR (n=97 eyes) or AMD (n=163 eyes) and a control group of 48 patients (55 eyes) without ocular surface disease. The researchers used a thermal imaging camera to measure OST and calculated temperatures for the medial canthus, lateral canthus and cornea.

On thermographic image analysis, the researchers noted major differences in OST between AMD and DR patients. OST was higher among patients with AMD and lowest among those with DR. In a subgroup analysis, eyes with DME had similar OST to those with AMD but significantly higher OST than DR eyes without DME. The researchers believe this finding indicates a greater inflammatory role in DME pathogenesis, compared with a perfusion deficit, as well as possible active leakage from retinal blood vessels causing a compensated increase in blood flow that increases OST of eyes with DME. “Alternatively, laser photocoagulation performed on PDR patients might have damaged their retinal blood vessels, causing reduced blood flow and thus a lower OST,” they noted.

The researchers stated that OST has been previously demonstrated to indirectly represent ocular hemodynamics. “Ischemic conditions are a well-known cause of decreased tissue temperature as a result of poor tissue perfusion and subsequent vasoconstriction of the peripheral blood vessels,” they explained. “Thermography has been used to demonstrate the associations between ischemic conditions and decreased OST.”

“Although the pathology of diabetes occurs in the posterior segment, we now found that it has an effect on the anterior segment as well, with significant reduction on OST,” the researchers said. Their findings are supported by previous studies that have found eyes with NPDR demonstrate lower OST values compared with controls. “This suggests OST values can be used as an indirect marker for ocular blood flow in DR.”

Temperature findings in the present study also support the idea that inflammation plays a role in AMD activity. “Unlike ischemia, inflammation is a pathophysiological process that consists of regional vasodilation and hyperperfusion resulting in hyperthermia,” they noted. “The associations between known inflammatory processes and surface temperature have been demonstrated in several studies. Our literature research yielded no evidence for an association between AMD and changes in corneal vascularization, dry eye syndrome or other ocular surface pathology which might explain the observed increased OST measurements among AMD patients. Cataract surgery was more prevalent in the AMD group, but our analysis didn’t find any evidence of it having a significant effect on OST.”

The researchers believe thermography may be a useful diagnostic tool for distinguishing between ischemic and inflammatory conditions. “Although posterior segment ischemia and/or inflammation may contribute to the pathophysiology of DR and AMD, it may actually be multifactorial,” they said. “The proposed mechanisms suggested in this study are speculative, and further research is needed to better understand the underlying causes for the apparent correlation between posterior eye diseases and ocular hemodynamics.”

Naidorf H, Landau Prat D, Moisseiev J, et al. Ocular surface temperature differences in retinal vascular diseases. Retina. 2021. [Epub ahead of print].