Thermography in Pain Management
The four most commonly used devices to measure heat emanating from body surfaces are electrical thermistors, contact thermography, video-thermography, and infrared “beam” thermography. Video-thermographs produce highly accurate television like pictures of heat patterns in which different temperatures are shown as different colors in a spectrum with reds usually being warmer and blues being cooler. Most vidoethermographs can differentiate between temperatures as little as 0.1 degrees Celsius apart and can image areas ranging from four square millimeters to the entire body.1 A typical videothermograph and its depiction of skin temperatures is shown in Figure 1. Unfortunately, they cost in the range of $50,000 to $75,000. This sensitivity can create false impressions of abnormality as paired areas of the extremities can be up to a degree different from each other under normal circumstances.2
Hand held digital infrared “beam” thermometers such as those used to take temperatures from children’s ears are also highly accurate1 but, of course, measure only one spot on the skin at a time. So, many measurements need to be made to construct a useful view of the important sites. This involves making a grid pattern diagram of the areas of interest and filling it in with consecutive measurements. This method has proven to be simple, quick, and accurate but able to provide all the necessary data needed to detect differences and track changes in skin temperature.1 A typical grid diagram showing both the patient’s pain pattern and heat measurements is illustrated in Figure 2. In compensations for the extra effort involved in filling out the grid temperature diagram, infrared thermometers usually cost less than $100.
Contact thermographs consist of a series of flexible pillow detectors about 46 CM square that are pressed against the area of interest. They use temperature sensitive crystals that change color at different temperatures to form each pixil in an array and form a picture of the area they are pressed against. Each pillow has only a few crystals at each pixil and thus, has a very limited range. This means that several pillows may have to be pressed against the same spot on the skin to record the full range of temperatures. A typical contact thermograph is shown in Figure 3. Sherman, et al1 have compared the accuracy of videothermography, contact thermography, and infrared beam thermography for scanning relative skin temperature. They determined that both videothermography and infrared beam thermography (the hand-held infrared thermometers) are both accurate and reliable while the contact thermograph is much less so. Thermisters are rarely used to measure a pattern of temperatures because they take up to five minutes to stabilize each time they are moved and must be taped to the spot to be measured.3
Surface heat is an indirect measure of blood flow and anything that interferes with the transfer of heat from near surface blood or with the recording process — such as habituation to the environment3 — causes artifacts. Dopler ultrasound is now replacing thermography as it is a more direct method of imaging blood flow.4,5 However, the devices are expensive (in the $50,000 range) and more complex to use accurately than a hand-held digital, infrared thermometer that costs less than one hundred dollars.
Relationship of Thermography to Pain
There is a well recognized, but very poorly understood, relationship between both acute and chronic decreased blood flow and pain. In fact, the report of burning or tingling pain usually leads the diagnostician to look for a source of decreased blood flow. Simply occluding a few peripheral blood vessels with a blood pressure cuff or falling asleep in an awkward position causes pain. Chronically decreased blood flow leads to hypersensitivity of surrounding tissues and can eventually lead to tissue death (e.g., as in reflex sympathetic dystrophy.6) Larsson, et al7 showed that much of the non-trauma related pain in the trapezius region of the upper back is caused by decreased blood flow in the muscle itself and not related to increased muscle tension. When decreased blood flow related to pain returns to normal — either on its own or through some purposeful intervention — the pain generally decreases as well.8 Changes in blood flow that result in pain may occur in the central nervous system as well as in the periphery.9
Numerous studies have shown that reducing blood flow to an extremity results in it becoming cooler.10 Thus, skin temperature and near surface blood flow are highly correlated. Measuring skin temperature is useful in situations when heat emanating from the body is expected to be abnormal: e.g., rheumatoid arthritis,11 Reynaud’s syndrome, burning phantom limb pain,12 phantom body pain,13 reflex sympathetic dystrophy,8,14 stress fractures, and tendonitis.15 The technique is sensitive enough to pick up differences between painful and pain-free areas on the limbs and torsos of people having complete spinal cord injuries who report phantom body pain.10,12 Thermography has been shown to be a valuable way to check changes in blood flow caused by interventions such as sympathetic blocks.16 It is also sensitive enough to detect differences in autonomic functioning resulting in differences in patterns of sweating such as found above and below the level of injury among spinal cord injured people.17 It is not likely to be effective when there is no reason to expect a temperature difference between the painful area and either paired or surrounding tissues as in myofascial tender spots.18 The use of thermography for assessing and tracking numerous disorders through the course of treatment is discussed in detail elsewhere.19
Videothermographs, contact thermographs, infrared thermometers usually measure heat emanating from the surface of the skin, or from thermisters taped to the skin.
Representative Thermography Applications
Thermography has found to be effective in assessing and tracking pain related problems having associated vascular changes. It has also proven to be particularly effective in detecting and tracking changes in burning phantom limb pain and reflex sympathetic dystrophy and is illustrated in the following sections.
Burning Phantom Limb Pain
Consistent, inverse relationships between intensity of phantom limb pain and temperature in the residual limb relative to that of the intact limb have been demonstrated for burning, throbbing, and tingling descriptions of phantom pain but not for any other descriptions.12 It has also been established that (a) for these descriptors of phantom pain there is a day to day relationship between the relative amount of blood flow in the stump and pain intensity and that (b) there is an immediate change in pain when blood flow changes.20 The relationship of burning phantom pain to blood flow in a bilateral amputee is illustrated in Figure 4.