Challenges in cancer pain management--bone pain.

Whilst not strictly a neuropathic injury, cancer-induced bone pain (CIBP) is a unique state with features of neuropathy and inflammation. Recent work has demonstrated that osteoclasts damage peripheral nerves (peptidergic C fibres and SNS) within trabeculated bone leading to deafferentation. In addition, glia cell activation and neuronal hyperexcitability within the dorsal horn, are all similar to a neuropathy. Gabapentin and carbamazepine (both anti-convulsants that modulate neuropathy) are effective at attenuating dorsal horn neuronal excitability and normalizing pain-like behaviours in a rat model of CIBP. However alterations in neuroreceptors in the dorsal horn do not mimic neuropathy, rather only dynorphin is upregulated, glia cells are active and hypertrophic and c-fos expression is increased post-noxious behavioural stimulus. CIBP perhaps illustrates best the complexity of cancer pains. Rarely are they purely neuropathic, inflammatory, ischaemic or visceral but rather a combination. Management is multimodal with radiotherapy, analgesics (opioids, NSAIDs), bisphosphonates, radioisotopes and tumouricidal therapies. The difficulty with opioids relates to efficacy on spontaneous pain at rest and movement-related pain. Potential adjuvants to standard analgesic therapies for CIBP are being explored in clinical trials and include inhibitors of glutamate release.