THERMAL VARIATIONS OF THE HUMAN EYE BY Joseph M. Dixon, MD AND (BY INVITATION) Lisa Blackwood, RMA INTRODUCTION HUMAN CORE BODY TEMPERATURE REMAINS ALMOST CONSTANT, BUT THE temperature ofperipheral body structures fluctuates with environmental changes, behavior, and metabolism.' Previous studies2-3 of the temperature of the eye have been done in animals. Huber,5 without recording the number of cases tested, used a thermistor and found the surface of the center of the human cornea to be several degrees Celsius below general body temperature. Duke-Elder6 states that the avascularity of the cornea must be largely responsible for this. Mishima and Maurice3 studied evaporation ofthe tear film from the eye of the rabbit and found that the anterior oily layer of the tear film is protective of evaporation and is not removed by blinking. Benjamin and Hill7 found an increasing corneal oxygen demand with increasing tem- perature. The purpose of this study is to document the thermal variations of the human eye and its adnexa in health and disease and in comparison with standard oral clinical measurements. MATERLALS AND METHODS Oral, corneal and upper and lower fornix temperatures in patients making routine office visits were recorded. Oral and fornix temperatures were measured with a digital fever thermometer (Becton Dickinson model 403000) (Fig 1). Corneal temperature measurements were made using a 2100 tele-thermometer (Yellow Springs Instrument Co Inc) (Fig 2) and a 3.4-mm diameter micro thermistor probe that does not require topical anesthesia (Fig 3). Informed consent was obtained from all patients. Simultaneous testing of the two digital thermometers and the thermistor probe used in this study, performed on a sample offluid, gave readings of 99.7°F and 99.6°F for the digital thermometers and 99.6°F for the probe. Standard deviations were calculated according to Wheeler.8 TR. AM. OPHTH. Soc. vol. LXXXIX, 1991 184 Dixon FIGURE 1 Measurement of fornix temperature. FIGURE 2 Instrument used to measure corneal temperature. Thermal Variations of the Human Eye 185 FIGURE 3 Probe used to measure corneal temperature. Twenty-five patients were measured in each of the first six categories listed below. Eighteen patients with unilateral eye disease were measured in the seventh category. All measurements were made indoors at room temperatures. The categories were as follows: 1. Mouth and lower fornix of one eye 2. Lower fornix and upper fornix of one eye 3. Lower fornix and central cornea of one eye (without anesthesia) 4. Lower fornix of one eye before and after instillation of mydriatic drops (phenylephrine hydrochloride 10% or tropicamide 1%) 5. Lower fornix before and after patching of one eye for 30 to 50 minutes 6. Mouth, lower fornix, surface of contact lens, and cornea under a contact lens of one eye. Lens was removed and probe was immediately placed on the cornea without blinking or anesthesia, and reading was obtained in a few seconds. 7. Lower fornix of both eyes. RESULTS Test results obtained for each ofthe seven categories measured are shown in Tables I through VII. The average increase of mouth temperature 186 Dixon TABLE I: DIFFERENCES IN TEMPERATURE RECORDED BETWEEN MOUTH AND LOWER FORNIX IN 25 PATIENTS* Greatest difference 3.70F Smallest difference 1.30F Mean 2.20F Average mouth temperature 97.90F Average temperature lower fornix 95.30F *Average increase of mouth temperature above lower fornix temperature was 2.20F Standard deviation calculated for mouth 0.544, and for lower fornix 0.845. TABLE II: DIFFERENCES IN TEMPERATURE RECORDED BETWEEN UPPER FORNIX AND LOWER FORNIX IN 25 PATIENTS* Greatest difference 1.4°F Smallest difference .0°F Mean 0.3°F Average temperature upper fornix 96.16°F Average temperature lower fornix 95.80F *Average increase of upper fornix tempera- ture above lower fornix temperature was 0.30F. Standard deviation calculated for upper fornix 0.811, and for lower fornix 0.950. TABLE III: DIFFERENCES OF TEMPERATURE RECORDED BETWEEN LOWER FORNIX AND CENTRAL CORNEA IN 25 PATIENTS WITHOUT ANESTHESIA* Greatest difference 4.0°F Smallest difference 1.3°F Mean 2.70F Average corneal temperature 92.70F Average lower fornix temperature 95.70F *Average increase oflower fornix temperature above corneal temperature was 2.770F Stan- dard deviation calculated for lower fornix 0.744, and for central cornea 0.949. Thermal Variations of the Human Eye 187 TABLE IV: LOWER FORNIX TEMPERATURE IN ONE EYE OF 25 PATIENTS BEFORE AND AFTER INSTILLATION OF DROPS* BEFORE DROPS AFTER DROPS ('F)t ('F)t Highest reading 97.0 97.3 Lowest reading 93.3 94.7 Mean 95.5 95.7 *Average increase of lower fornix temperature after drops was 0.21°F; there were no decreases. Greatest temperature differences was 1.8°F Standard devia- tion calculated before drops 0.921, and after drops 0.873. tEither phenylephrine hydrochloride 10% or tropi- camide 1%. TABLE V: LOWER FORNIX TEMPERATURE IN ONE EYE OF 25 PATIENTS AFTER PATCHING FOR 30 TO 50 MINUTES* BEFORE AFTER PATCHING ('F) PATCHING ('F) Highest reading 97.6 98.6 Lowest reading 93.5 93.8 Mean 95.2 96.18 *Patched eyes increased an average temperature of 0.92°F Standard deviation calculated before patching 0.968, and after patching 1.01. TABLE VI: TEMPERATURES OF MOUTH, LOWER FORNIX, SURFACE OF CONTACT LENS, AND CORNEA UNDER CONTACT LENS OF ONE EYE OF 25 PATIENTS WHO HABITUALLY WEAR CONTACT LENSES* MEAN HIGHEST LOWEST CHANGE READING ('F) READING ('F) MEAN ('F) GRADIENT Mouth 98.9 97.0 98.1 Lower fornix with contact lens 96.8 93.3 95.0 -3.1 Cornea under contact lens 98.9 93.0 92.0 -3.0 Surface of contact lens 92.8 89.9 91.1 -0.9 *There was no significant difference in decrease of corneal temperature under rigid or soft lenses. Standard deviation calculated for mouth 0.521, for lower fornix with contact lens 0.778, for cornea under contact lens 0.787, for surface of contact lens 0.819. 188 Dixon TABLE VII: COMPARATIVE MEASUREMENTS IN LOWER FORNIX OF EACH EYE WITH UNILATERAL EYE ABNORMALITIES* TEMPERATURE NO. OF CASES INCREASE ('F) Iritis 1 1.3 Conjunctivitis 6 1.1, 2.7, 2.2, 1.1, 1.8, 1.5 Superficial punctate keratitis 1 0.6 Subconjunctival hemorrhage 1 0.4 Ptosis 2 0.6, 0.3 Trauma with aqueous flare 2 0.7, 0.6 Granulomatous uveitis 1 0.6 Corneal foreign body with rust 1 1.7 ring Chalazion of upper lid 2 1.6, 1.1 Chronic blepharitis and conjunc- 1 2.0 tivitis *Each case showed an increase in temperature for affected eye. above lower fornix temperature was 2.2°F. The average increase of upper fornix temperature above lower fornix temperature was 0.3°F. The aver- age increase of lower fornix temperature above corneal temperature was 2.7°F. The average increase of lower fornix temperature after instillation of drops was 0.21°F, with no decreases; the greatest difference was 1. 8°F The average temperature increase after patching was 0.92°F. There was no significant difference in the decrease of corneal temperature under rigid or soft contact lenses. In the 18 patients with unilateral disease, all affected eyes showed an increase in temperature. DISCUSSION Temperature gradients between the avascular cornea, the vascular ocular adnexa, and oral temperature are normal and are expected. Multiple factors are responsible for these gradients. The cooling effect of a contact lens on the cornea, as demonstrated in this study, was not expected. The average corneal temperature with a contact lens was 92.0°F, and the average corneal temperature without a contact lens was 92.7°F To verify this finding, 12 measurements were made on the mouths and corneas of healthy individuals in the morning before a contact lens was applied and were repeated later in the day after a contact lens had been worn. The corneal temperature was consistently decreased under a contact lens, regardless ofwhether it was a soft hydrophilic lens or a rigid lens, and this was not parallel with changes in mouth temperature. The anterior oily layer ofthe tear film over the cornea, which is protective ofevaporation as Thermal Variations of the Human Eye 189 Temp OF 100 97.9 98 96 ~~~96.16 95.8 92.7 9 91.1 90 ~~~ ~ ~ ~ ~~~~~9. Mouth Upper Lower Surface Cornea Surface Fornix Fornix of Under of Cornea Contact Contact Lens Lens FIGURE 4 Thermal variations of human eye. studied by Mishima and Maurice,3 may be disturbed over the surface of a contact lens, and increased evaporation may be responsible for lowering the temperature. Mean mouth temperature in category 1 was 97.90F, and mean mouth temperature in category 6 was 98.1°E This 0.20 variation is not signifi- cant. The slight variation before and after instillation of mydriatic or cyclo- plegic drops was only 0.210F, which is not significant. The increase of almost one degree with patching may be clinically significant in wound healing as described by Friedenwald and Buschke,9 who found more rap- id epithelial proliferation with increasing temperature and slowing prolif- eration with decreasing temperature. The increase of upper fornix tem- perature 0. 3°F above lower fornix temperature represents a normal phys- iologic variation. This study documents these physiologic gradients (Fig 4). Further studies are indicated in the arctic, the tropics, and the desert as well as in febrile patients. We therefore expect conditions that increase ocular temperature, such as patching and various diseases, to increase the oxygen demand of the cornea. Other conditions that decrease ocular surface temperature, such 190Dxo190 Dixon as wearing of contact lenses, would therefore decrease oxygen demand of the cornea, and probably of adjacent tissues as well.