Studies

Vet Clin North Am Equine Pract. 2001 Apr;17(1):95-113.

Diagnostic Thermography

Turner TA1

AUTHOR INFORMATION
1Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul, Minnesota, USA.

ABSTRACT
Thermography is a practical aid in the clinical evaluation of the equine patient. It is particularly germane to the evaluation of lameness. This modality specifically increases the accuracy of diagnosis. Thermography is the pictorial representation of skin temperature. The technique involves the detection of infrared radiation, which can be directly correlated to blood flow. To be accurate, thermography must be performed in a controlled area free of drafts. The area should be protected from sunlight to avoid erroneous heating of the skin, and the horse's hair length should be uniform. Thermography detects heat before it is perceptible during routine physical examination and thus is useful for the early detection of laminitis, stress fractures, and tendinitis. It offers a noninvasive means of evaluating the blood supply to an injured region and represents one of the only reliable noninvasive means to evaluate blood flow to the foot of the horse. Thermography is also useful for the early identification of stress injuries to the contralateral limb of convalescing orthopedic patients. Thermography is an excellent adjunct to clinical examination as well as being complementary to other imaging techniques such as radiology, ultrasonography, and scintigraphy.

Am J Vet Res. 1980 Aug;41(8):1167-74.

Thermography in the diagnosis of inflammatory processes in the horse

Purohit RC, McCoy MD

ABSTRACT
To evaluate the use of thermography in equine medicine, a three-phase study was conducted. In the first phase, six horses were examined thermographically, before and after exercise, to determine a normal thermal pattern. In the second phase, nine horses with acute and chronic inflammatory processes were examined thermographically. In the third phase, thermography was used to evaluate the effectiveness of anti-inflammatory drugs on chemically induced inflammatory reactions. All normal horses tested had similar infrared emission patterns. There was a high degree of symmetry between right and left and between front (dorsal) to rear (palmar, plantar) in the legs distal to the carpus and the tarsus. The warmer areas of the thermogram tended to follow major vascular structures. The coronary band was the warmest area of the leg. Heat increase due to exercise did not substantially alter the normal thermographic pattern. Use of thermography in clinical cases successfully detected a subluxation of the third lumbar vertebra, a subsolar abscess, alveolar periostitis and abscess, laminitis, serous arthritis of the femoropatellar joint, and tendonitis. Thermography was effective in quantitative and qualitative evaluation of anti-inflammatory compounds in the treatment of chemically induced inflammation.

Vet Clin North Am Equine Pract. 1991 Aug;7(2):311-38.

Thermography as an aid to the clinical lameness evaluation

Turner TA1

AUTHOR INFORMATION
1Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul, Minnesota, USA.

ABSTRACT
Thermography has been shown to be a practical aid in the clinical evaluation of lameness. This modality specifically increases the accuracy of diagnosis. Thermography represents skin temperature, usually pictorially. The techniques involve contacting and noncontacting modalities. Noncontacting thermography, which detects infrared radiation, is the most accurate. In order to be accurate, thermography must be performed in a temperature-controlled, draft-free area. The area should be protected from sunlight to avoid erroneous heating of the skin, and the hair length should be uniform. Thermography detects heat before it is perceptible during routine physical examination; therefore, it is useful for early detection of laminitis, stress fractures, and tendinitis. It offers a noninvasive means of evaluating the blood supply to an injured part and offers one of the only reliable means to evaluate blood flow to the foot of horses with navicular syndrome. Thermography also is useful for the early identification of stress injuries to the contralateral limb of convalescing orthopedic patients. Thermography is an excellent adjunct to clinical and radiographic examination. It is complementary to other imaging techniques such as ultrasonography and scintigraphy.

Vet Clin North Am Equine Pract. 1999 Apr;15(1):161-77, viii.

Thermographic diagnostics in equine back pain

Graf von Schweinitz D1

ABSTRACT
Infrared thermographic imaging (ITI) is the most sensitive objective imaging currently available for the detection of back disease in horses. It is, however, only a physiological study primarily of vasomotor tone overlying other superficial tissue factors. Interpretation requires extreme care in imaging protocol and in understanding the significance of altered sympathetic nervous tone and the sympathetic distribution. Most discussions on back pain have centered on nociception and inflammatory events. ITI provides information and localization for more significant than diagnosing areas of hot spots. Chronic back pain usually involves vasoconstriction at the affected sites and from ITI studies in man, we have an opportunity to appreciate chronic pain phenomena that involves non-inflammatory events. These occur commonly in horses, but are still seldom recognized and treated.

Vet Surg. 2014 Oct;43(7):869-76. doi: 10.1111/j.1532-950X.2014.12239.x. Epub 2014 Jul 8.

Medical infrared imaging (thermography) of type I thoracolumbar disk disease in chondrodystrophic dogs

Grossbard BP1, Loughin CA, Marino DJ, Marino LJ, Sackman J, Umbaugh SE, Solt PS, Afruz J, Leando P, Lesser ML, Akerman M.

AUTHOR INFORMATION
1Department of Surgery, Long Island Veterinary Specialists, Plainview, New York.

ABSTRACT
Objective:
To: (1) determine the success of medical infrared imaging (MII) in identifying dogs with TLIVDD, (2) compare MII localization with magnetic resonance imaging (MRI) results and surgical findings, and (3) determine if the MII pattern returns to that of normal dogs 10 weeks after decompression surgery.

Study Design:
Prospective case series.

Animals:
Chondrodystrophic dogs (n=58) with Type I TLIVDD and 14 chondrodystrophic dogs with no evidence of TLIVDD.

Methods:
Complete neurologic examination, MII, and MRI studies were performed on all dogs. Dogs with type I TLIVDD had decompressive surgery and follow-up MII was performed at 10 weeks. Pattern analysis software was used to differentiate between clinical and control dogs, and statistical analysis using anatomic regions of interest on the dorsal views were used to determine lesion location. Recheck MII results were compared with control and pre-surgical images.

Results:
Computer recognition pattern analysis was 90% successful in differentiating normal dogs from dogs affected by TLIVDD and 97% successful in identifying the abnormal intervertebral disc space in dogs with TLIVDD. Statistical comparisons of the ROI mean temperature were unable to determine the location of the disc herniation. Recheck MII patterns did not normalize and more closely resembled the clinical group.

Conclusions:
MII was 90% successful differentiating between normal dogs and 97% successful in identifying the abnormal intervertebral disc space in dogs with TLIVDD. MII patterns 10 weeks after surgery do not normalize.

J Am Vet Med Assoc. 2006 Dec 15;229(12):1940-4.

Influence of exercise on thermographically determined surface temperatures of thoracic and pelvic limbs in horses

Simon EL1, Gaughan EM, Epp T, Spire M.

AUTHOR INFORMATION
1Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506, USA.

ABSTRACT
Objective:
To determine the amount of time required for surface temperatures of thoracic and pelvic limbs in horses to return to pre-exercise temperatures after high-speed treadmill exercise, as detected via infrared thermographic imaging.

Design:
Prospective study.

Animals:
6 Thoroughbreds.

Procedures:
All horses had been trained on and conditioned to use of a high-speed treadmill. Baseline thermographic images were obtained 3 days prior to exercise (baseline). Horses were exercised on a treadmill at a walk for 5 minutes, a slow trot (3 m/s) for 5 minutes, a trot (5 to 6 m/s) for 5 minutes, and a slow gallop (6 to 8 m/s) for 5 minutes, then back to a trot for 3 minutes, a slow trot for 3 minutes, and a walk for 3 minutes prior to stopping. Thermal images were obtained immediately after stopping exercise (0 minutes) and 5, 15, 45, and 60 minutes and 6 hours after stopping exercise. Ambient temperature surrounding each horse was recorded.

Results:
In all regions, significant differences in surface temperatures were detected between thermograms obtained before exercise and those obtained immediately after, 5 minutes after, and 15 minutes after exercise was stopped. There were no significant differences in surface temperatures between thermograms obtained before exercise and those obtained > or = 45 minutes after exercise was stopped.

Conclusions and Clinical Relevance:
In horses, images generated via infrared thermography are not influenced by exercise-generated heat > or = 45 minutes after exercise is stopped.

Published online Jul 1, 2013. doi: 10.3344/kjp.2013.26.3.219
PMCID: PMC3710935

Infrared Thermography in Pain Medicine

Francis Sahngun Nahm, MD

INTRODUCTION
Since BC 400 when Hippocrates used temperature in a diagnosis by applying mud to a patient's body and speculating that dry areas had disease, temperature has been an important area of interest in medicine. The skin is a very important organ in temperature regulation, and body temperature is controlled by the combined control of the central and autonomic nerve system. Infrared thermography (IRT) detects infrared light emitted by the body to visualize changes in body heat due to abnormalities in the surface blood flow of diseased areas. IRT is not a tool that shows anatomical abnormalities, but is a method that shows physiological changes. It objectively visualizes subjective symptoms, therefore, it is useful in making diagnoses and doing evaluations in the field of pain medicine where a diagnosis is based on subjective complaints of symptoms. The advantages of IRT is that 1)it is non-invasive and painless, 2) it is not harmful to the patient, 3) it is possible to conduct tests in a physiologically natural state, and 4) its testing time is short. The aim of this paper is to introduce the basic mechanism of IRT, significance in interpretation, and clinical utilization.

BASIC MECHANISM
The most important theoretical background of IRT is that the distribution of body heat in a normal body is symmetrical [1]. Therefore, the symmetry of body heat is considered to be the most important element when interpreting IRT images. An infrared camera is used to measure infrared light emitted from the body and displays this on the screen, and pseudocolor mapping is done on the obtained infrared image to facilitate visual interpretation [2]. Therefore, when comparing the distribution of body heat on both sides of the body, the region of interest (ROI) is set to an equal size on each side of the obtained pseudocolor image, and the mean temperature within each ROI is calculated to compare the difference. There are two methods to compare the temperature difference within an ROI of the affected and unaffected sides. The first method is to define a significant difference such as when the asymmetry of temperature deviates from 1-standard deviation of the unaffected side ROI [3], and second is to define the significance such as when the difference in mean temperature of both ROIs is more than the 'reference temperature difference'. The latter method is mainly used in the clinical field.

UTILIZATION OF IRT IN PAIN MEDICINE
After Galileo designed the first thermometer in 1592, infrared light was discovered by William Herschel in 1800, and the first diagnostic IRT was used in diagnosis of breast cancer by Lawson in 1956 [4]. Then, in 1982, the US Food and Drug Administration approved IRT as an adjunctive screening tool of breast cancer, and up to now, there have been many studies regarding the usefulness of IRT in various areas such as complex regional pain syndrome (CRPS) [5-7], postherpetic neuralgia [8], whiplash injury [9,10], inflammatory arthritis [11,12], temporo-mandibular joint disorder [13,14], headache [15,16], and myofascial pain syndrome [17,18]. The diseases where IRT can be used are presented in Table 1. Considering that IRT visualizes physiological and functional abnormalities rather than anatomical abnormalities, there is no doubt that compared to other imaging diagnostic methods, IRT is an effective diagnostic method for diseases difficult to diagnose with CT or MRI, such as CRPS, neuropathic pain, headache, and myofascial pain. In fact, for CRPS, it is known to have higher sensitivity compared to MRI or three phase bone scan [5,19], and it is reported that thermography has higher sensitivity in diagnosis of neuropathic pain compared to the sympathetic skin response test [20]. When deciding an abnormality in specific diseases, there are different views on what the 'reference temperature difference' should be according to researcher, and for CRPS, standards such as 0.6℃ [21] and 1.0℃ [22] are used. Meanwhile, regarding the reliability of IRT, research has been conducted for CRPS [7] and myofascial pain syndrome [17,23], and it was reported that there is high reliability for these diseases. In terms of correlation between pain and temperature difference measured with IRT, it was reported that there was a significant correlation between the severity of pain caused by lumbar disc herniation with the difference in skin temperature [24]. It was also reported that there was a significant correlation between the pressure pain threshold and the temperature difference in myofascial pain syndrome [18]. Recently, the technique, which obtains a dynamic image using a stress loading test as well as static IRT, is widely used. The theoretical basis for this is that normally the temperature change on both sides of the body after stress loading is symmetrical, and the degree of temperature restoration after removing the stress is symmetrical on both sides. Therefore, when restoration of temperature is asymmetrical after removal of stress, it is considered that physiological abnormalities exist. For the stress loading test, cold/warm stress, exercise, pharmacological stress, vibration, and visual stimulation are used as stress, and from these, the cold stress test is used the most. When using cold stress thermography, it is known that sensitivity and specificity is enhanced for diagnosis of CRPS [25-27], but it causes pain for the patient during the cold stress thermography, and a standardized guideline for the stress loading test has not been established.


Table 1
Indications for the Use of Infrared Thermography in the Pain Medicine

POSSIBILITY OF ERROR IN COMPARING THE TEMPERATURE DIFFERENCE OF BOTH SIDES ACCORDING TO THE ROI SETTING
Currently there are no established standards for setting an appropriate ROI. The ROI is set as symmetrical on the pseudocolor image based on the discretion of the examiner taking into consideration the medical history and symptom area of the patient. Therefore, according to the size and shape of the ROI, the temperature difference on both sides can be calculated differently. In addition, the IRT equipment currently used only shows the mean temperature and standard deviation within the fixed ROI, and the actual interpretation of the IRT image only compares the mean temperature of the ROI without considering the size of the ROI. In principle, when comparing two means, statistical difference is determined by considering the mean, standard deviation, and sample size. Thus, when only the mean values are simply compared without considering all these items, there is the possibility of error based on statistical interpretation. Therefore, considering the number of pixels in the fixed ROI (reflecting sample size), and the mean and standard deviation of the temperature in interpreting results can reduce false positives and false negatives, and enable objective interpretation of the results. For this, an ROI of equal size symmetrical for both sides of the body is set, and the t-test can be used taking into consideration the mean temperature, standard deviation, and number of pixels in the ROI, or the pixels on each side can be matched 1:1 to conduct a paired t-test for the temperature difference in each matched pixel [28]. Based on personal opinion, it is difficult to satisfy the assumption that the left and right side of the body are independent; thus, using the paired t-test with matched pixels is thought to be a more valid method statistically. However, there is no testing equipment which provides this kind of function presently. Hence, it is anticipated that an IRT system will be developed to enable such statistical analysis in the future.

DEVELOPMENT OF IRT TECHNOLOGY
Recently, there has been much effort to improve the hardware and software of medical IRT. Developments have been achieved such as enhanced performance of the infrared sensor, improved image quality, real-time image processing, and a multi-channel system. As a result, it is possible to obtain precise images with a thermal resolution of 0.08℃ or lower and a special resolution of 1×1 mm or lower. A 3-dimensional image technique was also developed to show the body heat in a more detailed image compared to the existing 2-dimensional image [29]. In addition, recently a remote diagnosis system was established to decipher images from a long distance away.

CONCLUSION
IRT is a non-invasive and safe diagnostic method which visualizes functional abnormalities and is used effectively in the diagnosis of numerous diseases and in the evaluation of treatment effect. Compared to other imaging diagnostic methods, it shows high diagnostic performance in pain diseases, and even higher sensitivity and specificity is obtained when using the stress loading test. Together with the development in medical technology, it is anticipated that the use of IRT will gradually increase in the field of pain medicine.



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PMCID: PMC340147

The effect of perineural anesthesia on infrared thermographic images of the forelimb digits of normal horses

Layne C. Holmes, Earl M. Gaughan, Denise A. Gorondy, Steve Hogge, and Mark F. Spire

ABSTRACT
Infrared thermography is an imaging modality gaining popularity as a diagnostic aid in the evaluation of equine lameness. Anecdotal reports of skin hyperthermia induced by local anesthesia, detected by thermography, have been made; however, no controlled studies have been reported. The purpose of this study was to examine the effects of perineural anesthesia on infrared thermographic images of the forelimb digits in normal horses. After environmental acclimation, infrared thermographs were made at intervals of 0, 5, 10, 15, 30, and 45 min from administration of mepivacaine hydrochloride or phosphate buffered saline in 6 adult horses with no clinical evidence of abnormality of the forelimb digits. The mean limb surface temperatures were compared by 2-factor ANOVA. Results indicated no significant difference between treatments, time after injection, or an interaction of time and treatment. Infrared thermographic imaging apparently can be performed within 45 min of perineural mepivacaine hydrochloride anesthesia without risk of artifactual changes in limb surface temperature.

INTRODUCTION
Infrared thermographic imaging of horses has been used increasingly in equine practice and has been considered a good tool for detection of lesions with the potential to cause lameness. Recent studies have reported the relationship between the detection of early thermographic changes and the onset of clinical lameness (1,2,3). While these studies are very encouraging, most reports review the clinical utilization of infrared imaging and there is very little documentation describing controlled evaluations of thermography as a diagnostic tool.

Infrared thermography and potential veterinary applications for this imaging technique have been described (2,3,4,5,6,7,8,9,10,11). These reports mostly describe thermographic imaging of spontaneous disease and attempts to correlate images to disease or injury diagnosed by other means. Normal thermographic patterns in the horse have been described (4,12). A study in cattle has revealed the successful utilization of thermography in the detection of localized sepsis in the pinna after contaminated growth stimulant pellets had been administered (13). One report describes the potential for thermography to detect early tissue changes in flexor tendons that may precede the frank tendon fiber tearing that is characteristic of a “bowed” tendon (3). This work may lead to improved diagnostic interpretation of infrared images for tendon evaluations and at other sites. In a study in which the correlation between Thoroughbred racehorse trainers' perceptions of potential problems and veterinary diagnoses supported by thermographic assessment was evaluated, it was determined that the correlation was excellent and that, in most cases, there were increases in heat 2 wk before clinical problems were noted (1). While this work is encouraging, most veterinary descriptions of infrared thermography are based on anecdotal impressions rather than on controlled evaluations of the potential for this modality to be an effective diagnostic tool in equine practice.

Current infrared thermographic instrumentation can detect skin temperature differentials of 0.1°C (12). Thermographic information can be detected and mapped with graphic and numerical data to allow the determination of focal temperature changes between 2 regions within an image (10,14). Inflammation in subcutaneous and deeper tissues can be reflected by local tissue temperature changes of ≥ 1°C (12,14). Human hands and fingers can detect temperature differentials on patient skin of ≥ 2°C (14). It has been claimed that modern infrared cameras are at least 10 times more sensitive than human hands in detecting temperature changes in a patient's skin (12). Instrumentation that can reveal tissue inflammatory change associated with injury or disease would be very beneficial in patient evaluation.

Thermography likely has great potential to assist diagnosis in equine lameness. The assessment of subtle temperature changes associated with inflammation could be very important in the detection of early and clinically relevant inflammation associated with pain and lameness. The ability to describe and relate thermographic data to specific injured or inflamed sites in the equine limb has seldom been documented in the veterinary literature (11,15,16). There is a common agreement among practitioners that this is possible, but complete descriptions are lacking. Other considerations are necessary to completely understand the appropriate use of infrared thermography in examinations for equine lameness. The timing of thermography in relation to the other standard components of a lameness examination needs to be thoroughly delineated. Specifically, the creation of artifacts that may confuse interpretation of infrared images should be understood and avoided if possible. Preparation for and utilization of local anesthetic nerve blocks has been speculated to create thermographic artifacts. It has been suggested that regional nerve anesthesia adversely affects infrared thermography by causing a thermal blush, secondary to vasodilation from sympathetic blockade (14,17). Conversely, another author believes nerve injuries with disturbed sympathetic tone may result in an image that reveals thermal cooling (2).

The purpose of this study was to evaluate the effects of regional nerve anesthesia on thermographic imaging of the digit in the forelimbs of normal horses.

MATERIALS AND METHODS
Horses:
Six adult horses with no abnormalities on physical examination of the fetlock and pastern regions were used in this study. Horses were housed in 12' 3 12' stalls or paddocks for a minimum of 3 h prior to experimentation in order to eliminate the effects of exercise. Each horse was fed to meet National Research Council requirements and each was routinely vaccinated for protection against eastern equine encephalitis, western equine encephalitis, influenza, and tetanus. Each horse was routinely dewormed. The research protocol performed on these horses was submitted to and accepted by the Kansas State University Institutional Animal Care and Use Committee, prior to the initiation of the study.

Preimaging preparation:
Gross debris was removed distal to the proximal aspect of the metacarpus on each forelimb. Each forelimb was cleansed and prepared for routine perineural local anesthetic injection over the palmar digital neurovascular structures at the abaxial sesamoid regions on both medial and lateral aspects of the limb. Chlorhexidine gluconate 4% and isopropyl alcohol 90% were utilized for topical disinfection. A minimum of 60 min was allowed for the skin surface and hair to dry before study injections were performed.

One forelimb was randomly selected on each horse for local anesthetic injection. Mepivacaine hydrochloride (HCl) (3 mL) (Carbocaine; Pharmacia & Upjohn, Kalamazoo, Michigan USA) was injected into the SC tissue plane at the medial and lateral abaxial sesamoid sites over the palmar digital neurovascular tissues, using a 2-cm, 23g needle and a 3-mL syringe. The contralateral limb was injected similarly at the abaxial sesamoid sites, as an injection control, with 3 mL of phosphate buffered saline (PBS). All perineural injections were subsequently tested by pressure application around the coronary band with a blunt instrument to assess that anesthesia was appropriately attained.

Thermal imaging:
All thermographic data and images were collected in a closed, covered, indoor environment following a minimum 3 h period of acclimation to this environment for the horses under study. The horses were restrained with a halter and lead rope. Neither sedatives nor tranquilizers were utilized for restraint, due to the vasodilatory properties of these pharmaceuticals. Imaging was performed in all instances at the same time of day under similar ambient temperature conditions, as recorded and measured by the thermographic camera and analytical software. The horses were fasted for 2 h prior to and during the imaging procedure to avoid postprandial thermal variation. Infrared thermograms of the skin temperatures of the lateral and medial surfaces of the forelimbs at the fetlock and pastern regions, centered at the disinfected abaxial sesamoid regions, were obtained. To establish baseline, thermal images were taken on 3 consecutive days before the injections began. Then, thermal images were collected immediately after local anesthetic and saline injections (time 0) and at the following temporal increments: 5, 10, 15, 30, and 45 min or until baseline measurements were resumed. Images were repeated at 24 h in 3 of the horses. The thermal imaging equipment consisted of a high resolution, short wave (3 to 5 mm), radiometric infrared (IR) camera (PM-280 ThermaCam; Inframetrics, North Billerica, Massachusetts USA). The IR camera is equipped with a 16° field of view lens with images displayed in a focal plane array arrangement of 256 3 256 pixels. The camera is calibrated annually by the manufacturer to maintain precision of 0.2°C per pixel, and tested against a known heat source at 1.0 emissivity to assure accuracy. Images were taken at a distance of 2 m, perpendicular to the lateral and medial surfaces of the study sites. This imaging distance results in a single pixel area of 2.2 mm2 (2 m 3 17.5 mrad 3 16° lens/256 pixels = 2.187 mm/pixel; actual measurement using IR camera on a 1'' object was 25 mm/11 pixels = 2.27 mm) on the limb surface.

Images were stored on high-resolution SVHS videotape for postimaging processing and evaluation with analytical software (ThermaGram Pro 95; Inframetrics). Effective mean limb surface temperature (MLST) for the targeted site was calculated from an approximately 3000-pixel area selected over the pastern and fetlock region of each forelimb image. The analytical software utilizes the mean of the pixels composing the targeted area within each image to determine the surface temperature of the limb. Effective mean temperature for the areas was calculated to allow comparison of changes in MLST, as affected by the time after the injection protocol. Regional surface temperature changes in each limb in relation to the baseline images were noted and recorded.

A minimum washout period of 72 h was allowed for each horse. Then, the limb that received mepivacaine HCl was injected with PBS and the limb that received PBS was injected with local anesthetic. Infrared thermographic imaging, imaging storage, and analysis were similarly repeated.

Statistical analysis:
All results are the mean of 3 replicate thermal images of both medial and lateral views at each time interval after injection. Each forelimb injection was considered a separate treatment for purposes of analysis. Data analysis was completed by using a commercial statistical software package (SAS, version 8; SAS Institute, Cary, North Carolina USA). Residual plots were used to evaluate data for normality and variance before and after logarithmic transformation. Normal distribution and equal variance occurred in the residual plots of logarithmically transformed data. Two-factor ANOVA with repeated measures was used to determine the effects of anesthetic injection, time after injection, and their interactions on MLST. Covariance was evaluated by first-order autoregression. Dunnett's test was applied to compare all groups to the control for separation of time after injection. Significance was established at P , 0.05.

RESULTS
Each horse tolerated the chlorhexidine scrub and alcohol applications, local anesthetic and saline injections, and infrared imaging without complication or resistance. No environmental complications were encountered and ambient temperatures were held steady at 20° to 22°C. Infrared images were obtained successfully for each horse at all time intervals.

The perineural injection of mepivacaine HCl did not create a significant effect on MLST compared with the perineural injection of PBS (P = 0.8527) (Figure 1). The MLST at each time interval up to 45 min postinjection was not significantly different from the MLST at time 0. The MLST at 30 min (P = 0.2100) and 45 min (P = 0.0666) was less than time 0 and the preceding time intervals (P = 0.9086 to 0.9930) (Figure 2). Images obtained at 24 h on 3 horses indicated that the skin temperature was similar to that at the time of the 45-min images, but they were not included in the statistical analyses. The interaction between mepivacaine HCl or PBS and time after injection displayed no significant effect on MLST (P = 0.8665) (Figure 3 and 4).

Figure 1. Mean limb surface temperature (MLST) (°C) as a result of treatment. No significant (P . 0.05) differences were revealed between perineural anesthetic and perineural phosphate buffered saline (PBS) injections at the abaxial sesamoid sites ...

Figure 2. Mean limb surface temperature (MLST) (°C) for all treatments over time. Time after injection had no significant influences on MLST (P . 0.05). Error bars indicate 95% confidence intervals.

DISCUSSION
The results of this study reveal that local perineural anesthesia with mepivacaine HCl had no effect on mean limb surface temperature, as detected by thermographic imaging. The data indicates that the MLST is not significantly altered within 45 min of performing perineural anesthetic injection. The calculated temperature of the images of the digit at 24 h was quite similar to that of the 45-min images, so further 24-hour images were not made on the remaining 3 horses. The values for 24-hour images were not included in the statistical analyses. Other authors have indicated that abaxial sesamoid nerve blocks can result in elevated limb temperature; however, these observations were made on horses being evaluated for lameness and without having had thermographic images taken prior to local anesthetic injection (2,14). Therefore, an alternative interpretation is that the temperature elevation was due to local inflammation that may have preexisted in the affected forelimbs. In discussing areas of increased temperature on thermographic images, Denoix (2) stated, “Normal thermographic variation also must be considered.”

Denoix (2) observed that local tissue cooling might result from injured nerves with decreased sympathetic tone. Conversely, induced and spontaneous Horner's syndrome in the horse has resulted in thermographic images showing unilateral skin temperature elevation in the neck (18). Murray et al (19) utilized thermography as an aid to diagnose neoplasia at the cervicothoracic ganglion. The presenting signs were similar to those of Horner's syndrome, including unilateral hyperthermia of the neck and forelimb. Elevated skin temperature, unconfirmed by thermography, was also observed in adult horses subjected to cervicothoracic ganglion blockade induced by injection of lidocaine HCl (20). Instillation of local anesthetic agents appears to mimic the signs of naturally occurring sympathetic blockade. However, these reported findings all involved more centrally located neural lesions, where the concentration of sympathetic nerve fibers is known to be much greater.

Myelinated type B fibers originate in the thoracolumbar spinal cord, leave the cord with motor fibers, but soon separate from the spinal nerve to enter the chain ganglia of the sympathetic truck (22). These are the preganglionic fibers of the sympathetic nervous system. Under “fight or flight” stimulation, type B nerve fibers initiate peripheral vasoconstriction, which may allow cooling of the skin temperature. Consequently, sympathetic blockade by local anesthetics could allow increased skin temperature (21). The sympathetic nerve fibers that travel peripherally with the spinal nerves are postganglionic nonmyelinated type C fibers (22). These fibers are the sympathetic supply for blood vessels, erector pili muscles, and sweat glands throughout the dermatome supplied by the spinal nerve. Signs of blockade of type C fibers include “pain relief, loss of temperature, sensation (21).” This might indicate that a decrease in surface temperature of the distal part of the forelimb is more likely after sympathetic blockade.

Local anesthetic agents may have variable influence on local vascular activity (21). Less vasodilator activity is produced when using mepivacaine HCl rather than lidocaine HCl, resulting in a prolonged duration of activity for mepivacaine HCl (21). This could help to explain why no effect was observed in the thermographic images obtained in this study, which evaluated the effects of mepivacaine, a commonly used local anesthetic agent in the diagnosis of equine lameness.

treatment of lameness in horses. The ability to detect potential underlying injury and associated inflammation from changes in the skin surface temperature changes in a noninvasive manner is very desirable. To combine thermography with other tools of standard lameness evaluation would improve the utilization and understanding of diagnostic capabilities. It appears from this study that infrared thermographic imaging can be performed after administration of perineural anesthesia with mepivacaine HCl without creating artifactual changes of the limb surface temperature. CVJ

FOOTNOTES
Funding provided by the Food Animal Health and Management Center and the Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas, USA.

Dr. Holmes' current address is Veterinary Medical Teaching Hospital, College of Veterinary Medicine, Kansas State University, 1800 Denison Avenue, Manhattan, Kansas 66506, USA.

Address all correspondence to Dr. Layne C. Holmes: e-mail: ude.usk.tev@semlohl

Reprints will not be available from the author.

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