CHRONIC PAIN: COMPLEX REGIONAL PAIN SYNDROME: MAKING THE DIAGNOSIS

There is no general agreement on how to make the diagnosis of complex regional pain syndrome (CRPS) and no single test that represents a "gold standard." Experts generally agree that the medical experience of the doctor and the clinical history are the most important factors in making a correct diagnosis, especially because the patient's symptoms and physical findings can vary greatly over time and certain tests may or may not be "positive" at any stage of the disease. For these reasons, doctors often will disagree about whether a patient has CRPS. This may present problems for patients in accessing proper care, including getting authorization for tests and treatment from insurance carriers.

A doctor should always suspect CRPS when a patient's localized pain that is associated with an injury or illness begins to spread and seems to be out of proportion to the original medical problem. The appearance of autonomic symptoms as discussed above may signal the appearance of CRPS.

Following are the most common diagnostic tests used to aid in the diagnosis of CRPS.

· Response to sympathetic blockade. In the past, physicians believed that a positive response (reduced pain) to a sympathetic block was necessary to make the diagnosis of CRPS. This is no longer considered to be the case.

· Tests of sudomotor functioning. During any stage of CRPS, patients may have abnormalities in sweat gland function (called sudomotor function). Patients may have either excessive or reduced sweating. Special laboratories are able to test resting sweat output (RSO), thermoregulatory sweating (TST) and quantitative sudomotor axon reflex testing (QSART). These tests are helpful if they are positive, but not if they are negative. Sudomotor functioning tests are difficult to conduct and available in only a few parts of the country.

· Three-phase bone scans. Once the only diagnostic tool available to doctors, this test has proven to be of limited usefulness. It becomes positive in only approximately 50 percent of CRPS patients (usually in later stages of the illness) and is, therefore, not particularly useful in making the diagnosis in earlier stages.

· Nerve conduction testing (NCV) and electromyography (EMG). Despite the fact that EMG/nerve conduction testing is very common, there are actually very few studies about their usefulness in patients with CRPS. Studies that do exist show that there are nerve conduction abnormalities in almost half of CRPS patients, but the abnormalities tend to be mild. Some researchers have suggested that the abnormalities may be due to swelling (called edema) or changes in blood supply to affected limbs, which then affects nerve functioning. A specialized test of nerve conduction is called somatosensory evoked potentials (SEP). Like other nerve conduction tests, there may be borderline abnormal findings in CRPS patients and the tests may be misinterpreted. SEP recording is not recommended as a routine method to diagnosis of CRPS-I. Based on these mild or borderline abnormalities, some test readers (called electromyographers) may make the mistake of saying that patients have "peripheral neuropathy," which is a different disease process than CRPS. The exception to this is when a patient does have a definite nerve injury associated with CRPS (CRPS-II).

Nerve conduction testing uses skin electrodes placed on the surface of the skin and usually is not painful. Electromyography, on the other hand, uses needles that are placed within muscle tissue and is painful. EMG recordings generally are not helpful in CRPS patients and may worsen CRPS. Experts generally agree that EMG recordings have no diagnostic value in CRPS.

· Quantitative sensory testing (QST). QST may be a useful method for a physician to confirm the clinical findings of abnormalities in sensation. The problem with this test, however, is that it is not specific for the disease CRPS. It may help, however, to confirm the doctor's findings, particularly when multiple physicians have recorded a wide variety of findings on testing sensation. These tests are available in only a few parts of the country.

· Sympathetic skin response (SSR). These tests may help confirm the doctor's impression that there are "sympathetic" abnormalities in function; however, it is a very specialized type of test done in only a few laboratories that conduct other electrical testing such as electromyography and nerve conduction. So far, it is of unproven value and not generally recommended for making the diagnosis of CRPS.

· Other radiological imaging studies. Plain X-rays and MRI scans may occasionally be useful, particularly in later stages of the disease. MRI can demonstrate the presence of soft tissue changes such as swelling or skin thickening. Plain X-rays may show demineralization of bone in later stages of CRPS. For these reasons, imaging tests can be useful in later stages of CRPS but not as a screening tool earlier on.

Infrared Thermography**

Infrared thermography is a diagnostic imaging procedure that records body surface temperature by detecting the heat (infrared radiation) emitted from the surface of the skin. An infrared thermogram essentially is a "heat map" of the surface of the skin. This heat map accurately records changes in skin blood flow. By evaluating alterations of the surface skin temperatures, a physician is able to indirectly evaluate the neurological status of the autonomic nervous system. This information may be very helpful, as the autonomic nervous system is intimately involved in both CRPS type I, CRPS type II and other painful conditions that can mimic CRPS.

In healthy individuals, there are very slight differences in skin temperature from one extremity to the other; however, patients with CRPS may have temperature differences greater than 1 degree Celsius between the affected and unaffected extremity.1,2 Subsequent research has shown that a greater than 1.5 degrees Celsius computer generated side-to-side temperature difference is helpful in telling if a patient's condition is post-traumatic (that is, a normal somatoautonomic reflex that follows a trauma) or an autonomic dysfunction that is present in CRPS.3

Patients undergoing infrared thermographic testing must follow certain instructions before testing, including special restrictions regarding the use of nicotine and caffeine, skin lotions, physical therapy the day before the test, and other diagnostic procedures prior to testing. Patients may be required to discontinue certain pain medications and sympathetic blocks. After a patient arrives at a thermographic laboratory, he or she is equilibrated in a 16 to 20 degrees Celsius draft free steady-state room wearing a loose fitting cotton hospital gown for approximately 20 minutes. The infrared study includes obtaining infrared images of the affected limbs and the normal limbs as well as imaging other parts of the body including the face, upper back and lower back. After capturing the baseline images, some labs will require the patient to undergo cold-water autonomic functional stress testing to evaluate the function of the autonomic nervous system peripheral vasoconstrictor reflex. This is performed by placing a patient's normal limb in a cold-water bath (approximately 20 degrees Celsius) for five minutes while collecting images. In a normal-intact functioning autonomic nervous system, a patient's painful extremity will become colder. Warming of the painful extremity indicates a disruption of the body's normal thermal regulatory vasoconstrictor function present in patients with CRPS.4 This also is known as "vasomotor instability."

Research has shown that patients undergoing infrared thermographic testing in a controlled cold environment (less than 20 degrees Celsius) who demonstrate computerized side-to-side temperatures greater than 1 degree Celsius and show abnormal response to cold water functional stress testing have a high sensitivity (patients who have the disease) and specificity (patients who do not have the disease) in the diagnosis of CRPS.4,5 Not all patients with CRPS, however, demonstrate the "vasomotor instability" changes just discussed, particularly in later stages of the disease. The results of the thermogram, therefore, must be interpreted as part of the disease history by the clinician.

**Written by Timothy D. Conwell, D.C., Denver, Colo.

Editor's Note

Dr. Conwell, a leading expert in thermography, nicely shows the great promise that this diagnostic tool holds for the evaluation of CRPS. Unfortunately, there are few laboratories in the United States that have the capability to perform the study as described and thermography is therefore underappreciated by the medical community. — R. Stieg, M.D., MPH

References

1. Uematsu S, Edwin DH, Jankel WR, Kozikowski J, Trattner M. Quantification of thermal asymmetry, part 1: normal values and reproduce stability. J Neurosurg 1988; 69:552-5.

2. Low PA. Laboratory evaluation of autonomic function. In: Low PA (Ed.). Clinical Autonomic Disorders. Boston: Little, Brown and Company, 1993, pp 169-192.

3. Birklein F, Kunzel W, Sieweke N, Despite clinical similarities there are significant differences between acute limb trauma and complex regional pain syndrome I (CRPS-I). Pain 2001; 93:165-171.

4. Gulevich SJ, Conwell TD, Lane J, et al. Stress Infrared Telethermography is useful in the diagnosis of complex regional pain syndrome, type I (formerly reflex sympathetic dystrophy). Clin J Pain 1997; 13:50-59.

5. Wasner G, Schattschneider J, Baron R. Skin temperature side differences: a diagnostic tool for CRPS? Pain 2002; 98:19-26.