Bisphosphonates in Complex Regional Pain syndrome type I: how do they work?

Complex Regional Pain syndrome type I (CRPS-I) is a disease characterised by extreme pain for which no gold-standard treatment exists to date. In recent years a possible role for bisphosphonates in the treatment of CRPS-I has been proposed. These drugs were first used for their effect in decreasing pain in bone diseases in which bisphosphonates act through their antiosteoclastic properties (metastatic disease, Paget disease, myeloma). In CRPS-I, enhanced osteoclastic activity has never clearly been demonstrated and the benefit shown is possibly exerted by different mechanisms of action. In this paper we review other conjectural mechanisms involved in reducing pain intensity and improving clinical signs and functional status in these patients. The results of most studies on this topic show that bisphosphonates may be effective in the early phases of the disease, when scintigraphic bone scan more frequently shows a local radiotracer accumulation that possibly means a high local concentration of the drug. These features probably represent the required conditions by which bisphosphonates might modulate various inflammatory mediators that are upregulated in CRPS-I. Patients in whom a scintiscan is often negative (long-standing disease or a primarily cold disease) could be less responsive to this treatment. With these limitations, bisphosphonates appear to present a therapeutic strategy that has been proven to reliably offer benefits in patients with CRPS-I.