Three-Phase Bone Scan

In cases of acute to subacute CRPS, the three-phase bone scan (TPBS) typically shows an increased radiotracer uptake in the affected limb in all three phases. In cases of chronic CRPS, the uptake is reduced in all three phases. When nerve trauma precedes the development of CRPS (CRPS type 2), TPBS performed within the first 5 months of symptom onset may show an increased uptake in the mineralization (third) phase in joints not affected by the initial trauma.

However, the sensitivity and specificity for TPBS in the diagnosis of CRPS is low; studies have found that when CRPS is diagnosed using the Budapest Criteria, the sensitivity of TPBS is 40%–55.1% and the specificity is 76.6%–93.5%. Therefore, a negative TPBS does not exclude the diagnosis of CRPS.

TPBS can be helpful, however, in ruling out malignancy, metabolic bone disease, fracture, heterotopic ossification, or infection in patients with limb pain, edema, and/or erythema.

Skin Temperature Measurement (Thermometry and Thermography)

Skin temperature is an indicator of skin blood flow. Skin temperature should be measured at several corresponding points on the affected and contralateral limb and on several different occasions using an infrared thermometer. Alternately, skin temperature can be assessed via infrared thermography (IRT). The standard protocol for infrared thermography includes acclimating the patient in a room warmed to 23°C and relative humidity 50% for 20 min without clothing. IRT images are then obtained using a computer-assisted infrared camera that detects infrared energy emitted from the surface of the body.

A limb temperature difference of >1°C between the affected and unaffected limb is significant. Temperature in the affected limb can be increased or decreased compared with the unaffected side. Note that skin temperature is dynamic; therefore, the presence of symmetric thermometric or thermographic findings does not exclude the diagnosis of CRPS. Gulevich et al. found a 93% sensitivity and 89% specificity of infrared thermography for the diagnosis of CRPS type 1; however, it should be noted that this study was performed before the creation of the Budapest Criteria.

Imaging

MRI or CT is useful only in assessing for other pathology, not for confirming the diagnosis of CRPS. Plain X-rays may show patchy osteopenia of the affected extremity. If one hand is involved, both hands can be imaged on the same radiograph (side-by-side comparison film) to assess for loss of bone density. The sensitivity of plain X-ray in CRPS is less than 30%.

Sympathetic Nerve Block

Historically, immediate and transient relief from pain and dysesthesia after sympathetic nerve block (stellate ganglion block or lumbar sympathetic block) was considered to be supportive of a diagnosis of CRPS. However, the role of the sympathetic nervous system in CRPS has, more recently, been called into question. The vasomotor and sudomotor dysfunction commonly seen in acute CRPS (edema, increased temperature of the affected limb) seems to be multifactorial, because of both increased levels of circulating neuropeptides (substance P, calcitonin gene related peptide) causing “neurogenic inflammation” and pain, and alterations in the sympathetic nervous system. There is evidence that central inhibition of cutaneous sympathetic vasoconstrictor neurons in CRPS causes increased limb edema and warmth, as well as pain via sensitization of nociceptors to catecholamines (sympathetic-afferent coupling). In addition, pain in CRPS is likely due, at least in part, to central sensitization. Because pain in CRPS may or may not be sympathetically mediated, response to a sympathetic nerve block, although sometimes a helpful part of the workup, cannot be used independently to substantiate or exclude the diagnosis of CRPS.