Jen-Hong Tan et al.

8.4. Applications to Ocular Studies

OST is volatile and easily affected by factors other than changes in ocular physiology and pathology. The rise in environment temperature increases OST,44,45 and an increase in air flow decreases OST.46 Therefore, during data collection uniform temperature, air flow and humidity are maintained in most studies to minimize discrepancies in the final outcome.

Generally, investigations in thermal imaging by IR on OST can be classified into three categories: studies of ocular physiologies, studies of ocular diseases, and surgery of nonocular diseases. Previous research has indicated that the presence of ocular diseases may significantly alter surface temperature. Many investigators have determined that a large interocular difference in OST may indicate the presence of ocular diseases.25

8.4.1. On Ocular Physiologies

The IR thermal imager was introduced to the field of ophthalmology by Mapstone, who first utilized the Bofor IR camera system to obtain ocular thermogram and delineated thermographic patterns of normal, ischemic and hyperemic eyes.29 Wachtmeister performed a similar study, and both normal and diseased eyes were investigated.30 The affected eye was said to be warmer compared to normal counterpart in both anterior and posterior diseases.

In addition, there were researchers looking into the detection of ocular diseases based on thermal asymmetry.47 They concluded that ocular diseases were present if the interocular differences in temperature were at least 0.5◦C. But the finding was illustrated to be not correct.47 Symmetry itself was determined to be insufficient to discriminate a diseased eye from a normal one.47

The introduction of a wide-field color-coded thermal imager to the field in the late 1980s has enabled research on OST with greater ease. Efron et al. employed the imager to determine the variation in temperature across the anterior surface.33 The temperature apex was determined to be slightly inferior to the geometric center of cornea, and was surrounded by ellipsoidal isotherms.33 Limbus was 0.45◦C warmer than compared to the geometric center of cornea.33

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The Study of Ocular Surface Temperature by Infrared Thermography

In another work, the limbus was found to be 0.23◦C 0.43◦C warmer than that of the geometric center of cornea.34 These authors recruited only Chinese origin subjects. Compared to Caucasian eyes, the tear volume and tear stability was found to be lower in Chinese eyes.48−51 Their findings showed a lower temperature at limbus in brown eyes than in blue eyes.34

The above investigation also determined the horizontal and vertical OST profile. They concurred their findings on the horizontal profile with Efron et al., and further illustrated that the vertical profile resembles a “spoonshape.”34 It was proposed that, throughout a subjects’ life, OST reduces by 0.01◦C per year.52 It was found that the interocular difference in OST was less than 0.62◦C for 95% normal subjects.25

8.4.2. On Ocular Diseases and Surgery

Examination on exophthalmos was one of the earliest investigations into ocular diseases carried out using thermal imager by IR. Diseases such as dry eye and glaucoma were the focus in the later research. At first, OST of the dry eye was found to be high,38 though it cools and evaporates faster,53,54 the conjunctival hyperemia associated with it outweighs the cooling effect, leading an increase in surface temperature38 and it correlates with another finding.15 However, their findings were not correlated with the findings of Craig et al.70 They showed that the temperature of corneal center in dry eyes was found lower. Besides, IR thermal imaging was also used to study the effect of acupuncture on dry eye.55

OST was determined to be correlated with ocular blood flow. Recently, Galassi et al. studied the OST of the patients who had primary open-angle glaucoma (POAG) and normal group using IR thermography.46 The correlations between parameters such as OST, intraocular pressure, and retrobulbar hemodynamics were investigated together with color doppler imaging (CDI). The effect of retrobulbar hemodynamics on OST was highlighted in their experimental results.36 In another study, the OST of central retinal vein occlusion (CRVO) patient was determined to be higher in comparison with normal subjects, and OST in ischemic CRVO eyes was lower than nonischemic ones.56

Besides the correlation between ocular blood flow and OST, the effects of two glaucoma surgeries were studied.37 The temperature of corneal surface

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