Pouzot-Nevoret C, Barthélemy A, Goy-Thollot I, Boselli E, Cambournac M, et al. Infrared thermography: a rapid and accurate technique to detect feline aortic thromboembolism. J Feline Med Surg. 2017 Sep 1;:1098612X17732485. PubMed PMID: 28948905.
Thermographic image of a cat with an aortic thromboembolism. Hind limbs and tail are visibly cooler than the remainder of the cat
Feline aortic thromboembolism (FATE) is a devastating consequence of heart disease in cats characterized by sudden onset pain, paralysis, and loss of blood flow to a limb because of a blood clot. Classically, blood clots form in the left atrium of the heart and travel down the aorta to lodge in the aortic bifurcation, where it splits to supply the legs. This results in decreased or absent bloodflow in both hind legs. FATE may also occur in a single limb, or may manifest as strokes, pulmonary thromboemobli, or other types of blood clot.
Diagnosis of FATE is often through a constellation of signs including the presentation, presence of cardiac disease, cold limb temperature, differences in lactate or glucose between central veins and affected limbs, blood pressure assessment, or definitively by ultrasonic visualization of the blood clot. Many of these tests are subjective, require operator skill, or can be inaccurate.
Infrared thermography is a technology that allows visualization of the external temperature of an animal or object with a specialized camera. It is non-invasive and requires minimal operator training. Medical uses of thermography have included evaluating joints for inflammation, screening for spinal disease, and evaluating potential cancers.
The purpose of this study was to determine if infrared thermography can be used in cats to make an accurate and non-invasive diagnosis of feline aortic thromboembolism.
Cats presenting to a referral hospital for acute onset hind end lameness were enrolled in the study. Cats were excluded if they had any contaminants on their skin that could make the assessment of temperature difficult (ie urine, alcohol), or if they had obvious external injuries. Cats were stratified into two groups: FATE cats diagnosed with bilateral hind limb thromboembolism with Doppler ultrasonography; and control cats diagnosed with a non-ischemic process using radiographs, CT, MRI, or CSF analysis.
A thermal camera was used to collect images of the cats in lateral recumbency. A region of interest (ROI) on each hind leg was highlighted and compared to an equal sized ROI on a forelimb. Whole cat images with a high-rainbow color gradient corresponding to temperature were also collected. Temperature values were analyzed and compared, and images were visually inspected.
10 cats were included in the FATE group; 8 males and 2 females with a median age of 7 years. 6 cats were in the control group; 5 males and one female with a median age of 3 years. All 6 control cats had pelvic fractures, one had meningitis, and two had vertebral fractures.
It was determined that the best predictor of FATE was the difference in temperature between the left front and left hind legs, for which a cut-off of 2.4 Celsius had a sensitivity of 90% and a specificity of 100%. Comparing right limbs with a cut-off of 2.4 had a sensitivity of 80% and specificity of 100%. They also found that direct visualization of the thermal images was usually sufficient to diagnose FATE.
A major drawback to this study is that the cases selected appear to largely be classic presentations where diagnosis may also have been reached by other methods. While it is an excellent proof of concept, further work should be done to determine if thermography assists in making a diagnosis in less definitive cases (ie single limb or resolving thrombi) and if it is superior to existing methods in terms of reliability, speed, cost, and other factors.
Overall, this paper presents a convincing argument for a medical use of infrared thermography in cats with suspected FATE, and may open the path to future research in determining the feasibility of integrating thermography with other diagnostic tools. (MRK)