Normal thermal patterns in cornea and periorbital skin

Thermal mapping the eye: A critical review of advances in infrared imaging for disease detection

Infrared thermography (IRT) has become more accessible due to technological advancements, making thermal cameras more affordable. Infrared thermal cameras capture the infrared rays emitted by objects and convert it into temperature representations. IRT has emerged as a promising and non-invasive approach for examining the human eye. Ocular surface temperature assessment based on IRT is vital for the diagnosis and monitoring of various eye conditions like dry eye, diabetic retinopathy, glaucoma, allergic conjunctivitis, and inflammatory diseases. A collective sum of 192 articles was sourced from various databases, and through adherence to the PRISMA guidelines, 29 articles were ultimately chosen for systematic analysis. This systematic review article seeks to provide readers with a thorough understanding of IRT's applications, advantages, limitations, and recent developments in the context of eye examinations. It covers various aspects of IRT-based eye analysis, including image acquisition, processing techniques, ocular surface temperature measurement, three different approaches to identifying abnormalities, and different evaluation metrics used. Our review also delves into recent advancements, particularly the integration of machine learning and deep learning algorithms into IRT-based eye examinations. Our systematic review not only sheds light on the current state of research but also outlines promising future prospects for the integration of infrared thermography in advancing eye health diagnostics and care.

Relationship between ocular surface temperature and 0.1% cyclosporine a in dry eye syndrome with meibomian gland dysfunction

To determine the relationship between ocular surface temperature (OST) and 0.1% cyclosporine A in patients with dry eye syndrome and meibomian gland dysfunction (MGD). This study retrospectively analyzed 35 eyes from 18 patients with dry eye disease (DED) and MGD, who were divided into two groups. Group 1 was treated with artificial tears, and eyelid margin scrubs without anti-inflammatory eye drops, while group 2 received the same treatment as group 1 along with 0.1% cyclosporine A. The ocular surface disease index (OSDI), tear meniscus height (TMH), noninvasive tear breakup time (NIBUT), lipid layer thickness (LLT), meibum quality score (MQS), and OST were measured at baseline and 1 month later. Nineteen and 16 eyes were included in groups 1 and 2, respectively. Both groups showed a significant decrease in OSDI and OST; however, the decrease was more significant in group 2. No other significant differences in TMH, NIBUT, and LLT were observed; however, MQS significantly differed in group 2. This study found that 0.1% CsA administration can relieve symptoms in patients with DED and MGD although there were no definite keratitis clues, such as epithelial erosion. In addition, the conjunctival temperature showed a correlation with symptom improvement.

Applications of Infrared Thermography in Ophthalmology

Body temperature is one of the key vital signs for determining a disease’s severity, as it reflects the thermal energy generated by an individual’s metabolism. Since the first study on the relationship between body temperature and diseases by Carl Reinhold August Wunderlich at the end of the 19th century, various forms of thermometers have been developed to measure body temperature. Traditionally, methods for measuring temperature can be invasive, semi-invasive, and non-invasive. In recent years, great technological advances have reduced the cost of thermographic cameras, which allowed extending their use. Thermal cameras capture the infrared radiation of the electromagnetic spectrum and process the images to represent the temperature of the object under study through a range of colors, where each color and its hue indicate a previously established temperature. Currently, cameras have a sensitivity that allows them to detect changes in temperature as small as 0.01 °C. Along with its use in other areas of medicine, thermography has been used at the ocular level for more than 50 years. In healthy subjects, the literature reports that the average corneal temperature ranges from 32.9 to 36 °C. One of the possible sources of variability in normal values is age, and other possible sources of variation are gender and external temperature. In addition to the evaluation of healthy subjects, thermography has been used to evaluate its usefulness in various eye diseases, such as Graves’ orbitopathy, and tear duct obstruction for orbital diseases. The ocular surface is the most studied area. Ocular surface temperature is influenced by multiple conditions, one of the most studied being dry eye; other diseases studied include allergic conjunctivitis and pterygium as well as systemic diseases such as carotid artery stenosis. Among the corneal diseases studied are keratoconus, infectious keratitis, corneal graft rejection, the use of scleral or soft contact lenses, and the response to refractive or cataract surgery. Other diseases where thermographic features have been reported are glaucoma, diabetic retinopathy, age-related macular degeneration, retinal vascular occlusions, intraocular tumors as well as scleritis, and other inflammatory eye diseases.

Ocular Surface Temperature: Characterization in a Large Cohort of Healthy Human Eyes and Correlations to Systemic Cardiovascular Risk Factors

Purpose: To characterize ocular surface temperature (OST) in healthy eyes and its association with systemic risk factors of cardiovascular and ischemic heart disease. Methods: This prospective cross-sectional study included consenting subjects who were examined at the Institute for Medical Screening in Sheba Medical Center. A Therm-App™ thermal imaging camera (Opgal LTD, Israel) was used for OST acquisition, and the mean OST of the medial canthal, lateral canthal, and central cornea regions were measured. Room and body temperatures were also recorded. Past medical and ocular history as well as data from various clinical examinations performed at the same visit were obtained. Results: Thermographic images were obtained from 186 subjects, 150 of which were included in the final analysis. OST was significantly higher in the medial canthal, central cornea, and lateral canthal regions in people with a history of ischemic heart disease (p = 0.02, p = 0.02, and p = 0.03, respectively). There were no significant OST differences (ANOVA test) associated with the presence of hypertension, diabetes mellitus, or active smoking status. Conclusions: OST correlated positively with the presence of ischemic heart disease. This correlation, its pathophysiological base, and its clinical application warrants further investigation.

Ocular surface temperature measurement in diabetic retinopathy

Infrared thermography provides functional imaging by picturing the temperature pattern of the region imaged. The temperature correlates to the blood flow pattern and is used in the diagnosis of diseases like breast cancer, peripheral vascular disorders, diabetic neuropathy and fever screening. In the present study, the usage of ocular thermography for diagnosis of diabetic retinopathy is explored. Ocular thermograms using infrared imaging camera were obtained for normal subjects (80 volunteers - 40 males and 40 females) age groups 21-30, 31-40, 41-50 and 51-60 years, non-proliferative diabetic retinopathy (NPDR) patients (50 volunteers -25 males and 25 females) and proliferative diabetic retinopathy (PDR) patients (20 volunteers -10 males and 10 females) belonging to age group of 51-60 years. The temperature at various points of interest (POIs) and horizontal temperature profiles were studied. Ocular surface temperature (OST) and effect of eye dilation on OST was studied for control, age matched NPDR and PDR. Statistical analyses were carried out to find the significance of correlation between OST of controls and NPDR and PDR. The global minimum temperature on the ocular surface for controls (21-60 years) was found to be at cornea which is about 34.79 ± 0.68 °C, and maximum at the inner canthus viz. 36.08 ± 0.62 °C. Dilation studies showed an average increase of 0.82 ± 0.13 °C in cornea and 0.75 ± 0.14 °C in conjunctiva and limbus (p < 0.001). The temperature of cornea is around 33.22 ± 0.12 °C and 32.64 ± 0.12 °C for NPDR and PDR patients respectively, in the age group of 51-60 years. OST of NPDR patients was 0.60 ± 0.15 °C lesser than that of age matched normal eyes (p < 0.001) at cornea and limbus regions and 0.71 ± 0.20 °C at inner canthus. The OST of PDR patients was lesser than age matched controls by 1.18 ± 0.12 °C at cornea, 0.9 ± 0.13 °C at inner canthus and 1.0 ± 0.14 °C at other POIs. During dilation studies a positive variation of 0.61 ± 0.12 °C in cornea and 0.48 ± 0.13 °C in conjunctiva and limbus was observed (p < 0.001) in NPDR eyes. Similarly an average increase of 0.62 ± 0.11 °C in cornea and an average increase of 0.47 ± 0.15 °C in conjunctiva and limbus were observed (p < 0.001) in PDR eyes. The OST of NPDR and PDR patients was less compared with age matched counterparts in both pre and post dilation studies. Dilation of eye showed increase in OST for both controls and diabetic retinopathy patients. The degree of increase is less compared with controls. The variation in OST observed during pre and post dilatation studies of diabetic retinopathy patients is a functional marker of pathology, and can be used as a parameter for diagnosis.

Ocular surface cooling rate associated with tear film characteristics and the maximum interblink period

The surface of the human eye is covered with a protective tear film that refreshes with each blink. Natural blinking occurs involuntarily, but one can also voluntarily blink or refrain from blinking. The maximum time one can refrain from blinking until the onset of discomfort is the maximum interblink period (MIBP). During the interblink period the tear film evaporates and thins from the ocular surface. Infrared thermography provides a non-invasive measure of the ocular surface temperature (OST). Due to evaporation, ocular surface cooling (OSC) generally occurs when the eyes are open and exposed to the environment. The purpose of our study was to investigate the effect of OSC rate on the MIBP, and to investigate the association of the MIBP with tear film characteristics in subjects who do and do not exhibit OSC. The MIBP was measured simultaneously with OST over time. Non-invasive tear breakup time, tear meniscus height, tear lipid layer thickness, and Schirmer I test strip wetted lengths were measured on a day prior to the thermography visit. Subjects were divided into cooling and non-cooling groups based on OSC rate, and demographic and tear film characteristics were tested for inter-group differences. A faster OSC rate was associated with an exponentially shorter duration of the MIBP overall and within the cooling group alone. Faster non-invasive tear breakup time was significantly associated with a shorter MIBP in both groups. These results suggest that tear film evaporation initiates a pathway that results in the onset of ocular discomfort and the stimulus to blinking. The presence of a subset of subjects with no or minimal OSC who nevertheless have a short MIBP indicates that evaporative cooling is not the only mechanism responsible for the onset of ocular discomfort.

Ocular surface temperature differences in glaucoma

PURPOSE

Accumulating evidence suggests that neuroinflammation and immune response are part of the sequence of pathological events leading to optic nerve damage in glaucoma. Changes in tissue temperature due to inflammation can be measured by thermographic imaging. We investigated the ocular surface temperature (OST) profile of glaucomatous eyes to better understand the pathophysiology of these conditions.

METHODS

Subjects diagnosed with glaucoma (primary open angle glaucoma [POAG] or pseudo exfoliation glaucoma [PXFG]) treated at the Sam Rothberg Glaucoma Center (11/2019-11/2020.) were recruited. Healthy subjects with no ocular disease served as controls. The Therm-App thermal imaging camera was used for OST acquisition. Room and body temperatures were recorded, and the mean temperatures of the medial cantus, lateral cantus, and cornea were calculated with image processing software.

RESULTS

Thermographic images were obtained from 52 subjects (52 eyes: 25 POAG and 27 PXFG) and 66 controls (66 eyes). Eyes with glaucoma had a significantly higher OST compared to controls (mean 0.9 ± 0.3°C, p < 0.005). The difference between the two groups remained significant after adjustment for age, sex, intraocular pressure (IOP) and room and body temperatures. Lens status and topical IOP-lowering medication did not significantly affect OST. A subgroup analysis revealed that the OST was higher among eyes with POAG compared to eyes with PXFG, but not significantly.

CONCLUSIONS

Differences in the OST between glaucomatous and normal eyes strengthens current thinking that inflammation affects the pathophysiology of glaucoma. Longitudinal studies are warranted to establish the prognostic value of thermographic evaluations in these patients.

Impact of Ocular Surface Temperature on Tear Characteristics: Current Insights

Infrared (IR) thermographic assessment of ocular surface temperature (OST) is gaining interest as an adjuvant method to evaluate the ocular surface. It is a quick, non-invasive test that causes minimal, if any, discomfort to patients. The purpose of this article was to summarize research on how OST relates to tear film parameters and dry eye disease (DED). PubMed, Google Scholar, and Scopus searches for specific terms were carried out and eligible articles reviewed. OST of the central cornea is ~34-35°C when measured as a single time-point (typically right after a blink). Dynamically, OST values decrease over time at a rate of ~ -0.01 °C/s in healthy eyes. Single time-point OST values are impacted by temperature, with positive correlations noted with both ambient (1°C↓ results in ~0.16°C↓ in OST) and body (1°C↑ results in ~0.98°C↑ in OST) temperature. Single time-point OST values are also impacted by tear parameters, with negative correlations noted with tear break-up time (TBUT; r=-0.61) and positive correlations with lipid layer thickness (~r=0.50). Dynamically, the rate of OST cooling over the interblink period correlates with various tear parameters including Schirmer's test scores (r=-0.39), tear meniscus height (r=-0.52) and the rate of tear film break-up (r=-0.74). These data imply that OST decreases more rapidly in individuals with greater tear production, larger tear volumes, and shorter tear break-up times (faster rates of tear film break-up). There are discrepancies in relationships between OST and DED across studies, which is not surprising given that DED encompasses a number of different phenotypic presentations. However, most studies found that OST decreases at a more rapid rate in DED vs. control groups. As such, cooling rate may have utility as a screening tool in DED in combination with established point-of-care tests.

Innovative corneal active storage machine for long-term eye banking

Optimal ex vivo corneal storage in eye banks is crucial to increase both the number of corneas suitable for graft and their intrinsic quality, mainly the number of viable endothelial cells, which dictates graft survival in recipients. With both passive storage methods used worldwide (short-term cold storage in the United States, long-term organ culture in Europe), significant endothelial cell loss is inevitable. Here we show that, with an active storage machine, also called a bioreactor, which restores 2 fundamental physiological parameters, intraocular pressure and medium renewal, endothelial cell survival is improved by 23% compared with organ culture after 4 weeks' storage. Also observed in the bioreactor is a 4-fold higher expression of Na⁺ /K⁺ ATPase, which supports one of the major endothelial cell pumping functions. In addition, corneas remain thin and transparent, so they are suitable for surgery at any time. This new active eye banking method may help to reduce the severe global scarcity of donor corneas.

Physiological and behavioral reactivity to social exclusion: a functional infrared thermal imaging study in patients with psoriasis

Recent studies have shown that sympathetic nervous system (SNS) activity can be heavily impacted not only by basic threats to survival but also by threats to social bonds. In this study we explored the behavioral and physiological consequences of social exclusion/inclusion in patients with psoriasis, a disease frequently associated with the experience of being ostracized and with deficient emotion regulation skills. We employed a virtual ball-tossing game (Cyberball) to induce the experience of social exclusion/inclusion. We then used a Trust Game to measure the effects of this social modulation on trust. During Cyberball, infrared thermal imaging was used to record participants' facial temperature and thus obtain an online measure of SNS activation. Behavioral data showed that social exclusion shifted participants' trust toward unfamiliar players who had not previously excluded them. Physiological data indicated that in control participants, social exclusion triggered higher SNS activation than inclusion. No such effect was found in patients with psoriasis, whose SNS activity was the same during inclusion as it was during exclusion, suggesting that they benefit less from inclusive experiences than control participants. In addition, in patients but not in controls, higher SNS activation during social exclusion was linked to higher monetary investment toward unfamiliar players, a result in keeping with the social reconnection hypothesis, according to which emotions triggered by social rejection can be regulated by investing in new social interactions. We also found that an increase in periorbital temperature is accompanied by a decrease in happiness ratings after social exclusion was experienced during the Cyberball game. NEW & NOTEWORTHY Previous research on emotional processes in psoriasis has mainly employed self-report measures. In this study we used thermal imaging to obtain an online measure of the sympathetic nervous system (SNS) activity during social exclusion and tested how this experience influenced subsequent trust. We found that being included was a less positive experience for patients compared with controls and that SNS activity during exclusion had a stronger influence on subsequent trust in patients than in controls.

Cornea Thermography: Optimal Evaluation of the Outcome and the Resulting Reproducibility

Purpose

The aim of the study was to establish a standardized quantitative evaluation of corneal temperature (CT) that includes anchoring reference points in the topography and minimization of artifacts. We further investigated the distribution and the short- and long-term reproducibility of the CT values, as well as the influence of the core temperatures.

Methods

The CT values in both eyes of 40 healthy subjects were measured through thermography. These examinations took place over the course of four visits within 2 consecutive weeks. At each visit, the CTs were measured twice in both eyes with intervals of 15 minutes between measurements.

Results

CT values were not significantly different between the right and left eyes and their distribution was nearly normal. The CTs increased slightly when measured twice over the 15-minute intervals (short-term reproducibility) but remained stable over a period of 2 weeks (long-term reproducibility). In addition, the CT values depended on the core temperatures.

Conclusions

Ocular surface thermography is a fast and noninvasive examination. The methods of optimized and standardized evaluation of the CT values facilitate comparisons and follow-ups.

Translational Relevance

Thermography can be used clinically and scientifically only if both the measurement and its evaluation are efficient and standardized and if the outcomes are highly reproducible.

Static and Dynamic Measurement of Ocular Surface Temperature in Dry Eyes

Purpose. To study ocular surface temperature (OST) in dry eyes by static and dynamic measures. Methods. OST was recorded on 62 dry eyes and 63 age- and sex-matched controls. Static measures were study of absolute OST at t = 0, 5, and 10 s after eye opening. Dynamic measures were study of mean change and net change in OST over 10 s of sustained eye opening. Ten OST indices studied were temperatures of the geometric center of the cornea (GCC), extreme temporal (T1) and nasal conjunctiva (T4), midtemporal (CT) and nasal conjunctiva (CN), temporal (LT) and nasal (LN) limbus, and mean (MOST), maximum (Max T), and minimum (Min T) temperatures of the region of interest. Results. For static measures, dry eyes recorded significantly lower GCC, MOST, Min T, Max T, T4, CT, LT, LN, and CN. For dynamic measures, dry eyes had significantly steeper regression line of mean change (corresponding to greater net change) for Max T 5 s onward and T4 at 3 s onward. Conclusions. Both static and dynamic measures of the OST were valuable and can be used as clinical tool to assess dry eye.

Ocular Surface Cooling Corresponds to Tear Film Thinning and Breakup

PURPOSE

To investigate the relationship between ocular surface temperature (OST) and tear film thinning and breakup.

METHODS

Simultaneous imaging of OST and fluorescein tear thinning and breakup (FTBU) was performed on 20 subjects. Subjects were asked to open their eyes and refrain from blinking for as long as they could during testing. Ocular surface temperature was measured using an infrared thermographic camera (FLIR A655sc) and rates of ocular surface cooling (OSC) were analyzed using commercially available software. A method was developed to quantify the rate of FTBU formation using image-processing software.

RESULTS

Areas of FTBU and regions of OSC were observed to be colocalized, with localized cooling preceding the formation of FTBU. The rates of OSC and FTBU formation were positively correlated (r = 0.74). A second-order polynomial model accurately describes the physiological relationship between the area of FTBU and OST (p < 0.001). A linear approximation provides a more clinically interpretable rate of FTBU formation with decreasing OST (p < 0.001), while still retaining high R.

CONCLUSIONS

The results suggest a direct relationship between FTBU formation and OSC. That cooling of the ocular surface precedes FTBU formation implies a process of evaporation contributing to tear film thinning and breakup. Our study suggests that measuring the OSC rate could be an indirect assessment of tear evaporation and could contribute to the management of evaporative dry eye.

Measurement of dynamic ocular surface temperature in healthy subjects using a new thermography device

PURPOSE

To investigate the routine use and applicability of a new ocular thermography device (TG 1000; Tomey Corp, Nagoya, Japan) in healthy individuals.

METHODS

Sixty eyes of 30 healthy subjects were included in this prospective study. Intraobserver reproducibility was tested with an ocular surface-oriented infrared radiation thermographic device in a non-contact manner. Using a standard examination protocol, the ocular surface temperature was assessed by dynamic thermal imaging over a time period of 10 s. The procedure was repeated three times during a single session by one examiner. Intraclass correlation coefficients (ICC) were calculated.

RESULTS

Ocular surface temperature measurements were highly reproducible. The mean ocular surface temperature was 34.02°C ± 0.22. The ICC was 0.947%, 0.949%, and 0.955% for minimum, maximum, and mean temperatures, respectively.

CONCLUSION

Ocular surface temperature measurements made using the Tomey TG 1000 in healthy subjects showed excellent intraobserver reproducibility. This novel non-invasive technique offers new options for increased understanding of the physiology of the ocular surface.

OCULAR TEMPERATURE DISTRIBUTION: A MATHEMATICAL PERSPECTIVE

Mathematical modeling has proven to be a viable alternative for investigating the temperature distribution inside the human eye. This is due to its ability to overcome the limitations infrared (IR) thermography; the leading method in ocular temperature measurement. A wide range of mathematical studies on the ocular temperature distribution during various conditions have been published in the literature. In this paper, we carry out an in-depth review of the various mathematical models of the eye that have been developed in the past. Various problems and the implications from the mathematical predictions of these studies are discussed. The future directions of studies in ocular temperature distribution are deliberated.

Accuracy and sensitivity of the dynamic ocular thermography and inter-subjects ocular surface temperature (OST) in Chinese young adults

PURPOSE

To investigate the accuracy and sensitivity of the Thermo-Tracer TH9100MV, NEC San-ei in monitoring the temperature of the anterior eye surface and to establish an inter-subjects ocular surface temperature (OST) norm in Chinese young adults.

METHODS

A blackbody with fixed temperature was used to study accuracy and sensitivity of the thermal camera used (Thermo-Tracer TH9100MV, NEC San-ei). Sixty healthy Chinese young adults aged 17-21 (mean age 19) were recruited from the Singapore Polytechnic Optometry Centre. A room with standard room temperature and humidity was used. Subjects were adapted to the room conditions for 20min before measurement. OST was then measured within 4-5s after a blink and with their upper lid lifted by a cotton bud, using infrared thermo-camera, in a non-invasive manner.

RESULTS

All future Thermo-Tracer readings could be converted to 'true' temperature via the regression equation derived y=1.0884x - 3.4026 (r(2)=0.9985), where y is the 'true' temperature and x is reading by Thermo-Tracer. Sensitivity of the machine for all 16 temperatures studied was found to be 91.3%. Mean OST for the population studied was 34.70+/-0.50 degrees C. Mean OST at geometrical centre cornea (GCC) was 34.39+/-0.47 degrees C and inter-ocular temperature difference was approximately 0.20+/-0.15 degrees C. Both the horizontal and vertical OST profile was established. No significant differences between right/left eyes and between genders were found at 95% CI.

CONCLUSIONS

Findings on the accuracy can be used as a reference by researchers who are using the same infrared thermal camera. It is proven that the machine has high sensitivity in determining the temperature of anterior ocular surface. Methodology used in this study can also be applied to the determination of the accuracy and sensitivity of the dynamic range for other infrared thermal cameras. Inter-subjects OST norm in Chinese young adults aged 17-21 was established.

Ocular Surface Temperature

PURPOSE

To review the evolution in ocular temperature measurement during the last century and examine the advantages and applications of the latest noncontact techniques. The characteristics and source of ocular surface temperature are also discussed.

METHODS

The literature was reviewed with regard to progress in human thermometry techniques, the parallel development in ocular temperature measurement, the current use of infrared imaging, and the applications of ocular thermography.

RESULTS

It is widely acknowledged that the ability to measure ocular temperature accurately will increase the understanding of ocular physiology. There is a characteristic thermal profile across the anterior eye, in which the central area appears coolest. Ocular surface temperature is affected by many factors, including inflammation. In thermometry of the human eye, contact techniques have largely been superseded by infrared imaging, providing a noninvasive and potentially more accurate method of temperature measurement. Ocular thermography requires high resolution and frame rate: features found in the latest generation of cameras. Applications have included dry eye, contact lens wear, corneal sensitivity, and refractive surgery.

CONCLUSIONS

Interest in the temperature of the eye spans almost 130 years. It has been an area of research largely driven by prevailing technology. Current instrumentation offers the potential to measure ocular surface temperature with more accuracy, resolution, and speed than previously possible. The use of dynamic ocular thermography offers great opportunities for monitoring the temperature of the anterior eye.

Ocular lipid deposition and hyperlipoproteinaemia

In all species there are potential ocular manifestations when circulating lipoproteins are raised and these may be transient or permanent Many factors, both systemic and local, influence lipid influx and accumulation (progression) and lipid mobilisation and efflux (regression). In both humans and animals some types of lipid deposition will regress if the local and systemic factors involved in pathogenesis can be modified. There are inescapable parallels with the same phenomena in other tissues.Three types of corneal lipid deposition have been linked with hyperlipoproteinaemia. In corneal arcus, lipid is deposited preferentially in the warmest part of the cornea initially and, in people, the lipid remains almost exclusively extracellular. In animals, corneal arcus is associated with initial extracellular lipid deposition followed by the appearance of intracellular lipid and vascularisation, so that established corneal arcus tends to become more typical of lipid keratopathy. In humans, hyperlipoproteinaemia may be an associated systemic factor and early onset corneal arcus is a recognised feature of certain primary hyperlipoproteinaemias and their secondary phenotypes. In dogs, corneal arcus is always associated with hyperlipoproteinaemia. Corneal vascularisation is a ubiquitous feature of lipid keratopathy in all species and both necrotic fibroblasts and foam cells are common in progressive lesions. The extent and position of lipid deposition and the evolution of lipid keratopathy can be related to local ocular disease and circulating lipids and lipoproteins. Many aspects of the pathogenesis of lipid keratopathy are similar to those of atherogenesis. Hyperlipoproteinaemia, especially hypercholesterolaemia is the commonest systemic abnormality. In crystalline stromal dystrophy (Schnyder's crystalline stromal dystrophy) of the cornea there is no inflammatory element and no vascularisation. The dystrophy is associated with accumulation of lipid within the corneal fibroblasts, but typical foam cells are absent, the crystalline opacity involves the coolest part of the cornea, correlates with local fibroblast death, and is always bilateral. Hyperlipoproteinaemia, may be present, but this is not universally so.The objective of this paper is to evaluate the factors that may influence ocular involvement in hyperlipoproteinaemia. A comparative approach, utilising information available from studies of both ocular and non-ocular tissues, aids elucidation of the complex pathogenesis.

The role of tear physiology in ocular surface temperature

PURPOSE

To determine whether the more rapid cooling of the tear film in dry eyes is related to other tear film parameters, a battery of tear physiology tests was performed on dry eye patients and control subjects.

METHODS

Tear evaporation rate was measured with a modified Servomed (vapour pressure) evaporimeter and ocular temperature with an NEC San-ei 6T62 Thermo Tracer in 9 patients diagnosed as having dry eye and in 13 healthy control subjects. Variability in temperature across the ocular surface was described by the temperature variation factor (TVF). Lipid layer structure and tear film stability were assessed with the Keeler Tearscope and tear osmolality was measured by freezing point depression nanolitre osmometry.

RESULTS

The data were explored by principal component analysis. The subjects with and without dry eye could be separated into two distinct groups entirely on the basis of their tear physiology. Dry eye patients exhibited higher tear evaporation rates, osmolalities and TVF, lower tear film stabilities and poorer-quality lipid layers than the control subjects. A significant linear relationship was found to exist between tear evaporation rate and TVF for all subjects (R2 = 0.242, p = 0.024).

CONCLUSIONS

Rapid cooling of the tear film in dry eyes appears to be related to the reduced stability of the tears and the increased rate of evaporation. The higher latent heat of vaporisation, associated with the increased evaporation in dry eyes, may account for the increased rate of cooling of the tear film in this condition.

Infrared thermography of the tear film in dry eye

Infrared ocular thermograms were recorded for a group of 36 dry eye patients and for 27 age- and sex-matched controls. Mean ocular surface temperature was greater in the dry eye group (32.38 +/- 0.69 degrees C) compared with the control group (31.94 +/- 0.54 degrees C; p < 0.01). In addition, there was a greater variation of temperatures across the ocular surface in the dry eye group, illustrated by the difference in temperature between the limbus and the centre of the cornea (0.64 +/- 0.20 degrees C in dry eye patients compared with 0.41 +/- 0.20 degrees C in the control group; p < 0.001). This parameter was also shown to be greater in dry eye patients who displayed either a fast tear break-up time or a poor Schirmer's test result. Infrared thermography is a non-invasive and objective technique that may prove a useful research tool for study of the tear film, its deficiencies and its various treatment modalities.

Determinants of corneal temperature

Images