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Editors Selection IGR 8-3
Examination methods: Temperature of the AS and bloodflow
Comment by Selim Orgül on:
19488 Evaluation of ocular surface temperature and retrobulbar haemodynamics by infrared thermography and colour Doppler imaging in patients with glaucoma, Galassi F; Giambene B; Corvi A et al., British Journal of Ophthalmology, 2007; 91: 878-881
Temperature measurements of the anterior segment of the eye have been applied in research in several areas, such as tear film abnormalities, photorefractive surgery, and ocular inflammation. Deeming corneal temperature to be a possible surrogate for ocular blood flow measurement, Galassi et al. (955) compared the ocular surface temperature between 32 primary open-angle glaucoma patients (all under treatment with prostaglandin analogues) and 40 age and sex-matched controls. In their group of glaucoma patients, the authors also correlated corneal temperature with colour Doppler imaging parameters, currently probably the most powerful reference standard to validate a relationship with ocular blood flow. In this cross sectional study, ocular surface temperature was measured by means of a thermographic device. Such infrared detectors allow, based on Wien's approximation (describes the spectrum of thermal radiation, frequently called the blackbody function), a very rapid, precise, and reproducible measurement of the surface temperature. Because corneal transmission in the infrared area is strongly reduced, and above a wavelength of 3 mm no longer occurs, it has been postulated that radiation emitted from deeper ocular tissues adds little to the spectrum emitted by the cornea and that the cornea can be regarded as a black body radiator. The authors very cautiously standardized the measurement conditions. Their results disclosed, despite the multiple comparisons, a quite convincing statistical correlation between resisitivity indices in the retrobulbar vasculature and central corneal temperature, confirming earlier results published by others. It is unfortunate that a similar comparison was not investigated in the control group. Indeed, this study also demonstrated that primary open-angle glaucoma patients have, on average, a lower ocular surface temperature, a compelling finding since the temperature was recorded at five anatomical points across a line running horizontally through the centre of the cornea and taken at the eye opening as well as at the 20th second after opening, all ten comparisons showing highly significant differences between the two groups. Although in Doppler ultrasonography, the interpretation of differences in blood flow velocity and resistivity index is difficult and remains speculative, measuring retrobulbar blood flow in the control group would have allowed linking more convincingly the difference in corneal temperature to ocular blood flow dissimilarity. The authors believe that altered ocular surface temperature may be an indirect marker of impaired ONH perfusion in patients with glaucoma. Body surface temperature is certainly influenced by transfer of heat from the circulation and when all heat brought to the body surface is lost to the environment, local blood flow may be assumed to be closely related to local body surface temperature. However, because corneal temperature may very heavily depend on the temperature of the surrounding tissue, it is certainly debatable how much corneal temperature is reflecting hemodynamics at the level of the optic nerve. But, especially in conditions with rather generally altered ocular blood flow, as is suggested for glaucoma, the most outstanding merit of this study is certainly to attract the attention of the readership to a possibly powerful, simple, comfortable, and inexpensive way to assess ocular blood flow.
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