Diagnostic nerve blocks in chronic pain.

Diagnostic blocks are used to obtain information about the source of a patient's pain. As such they differ in principle and in practice from regional anaesthetic blocks. In order to be valid, diagnostic blocks must be precise and target-specific. They must be controlled in order to exclude false-positive responses. Sympathetic blocks have traditionally been performed without pharmacological controls, but studies have shown that the features of complex regional pain syndromes can be relieved equally well when normal saline is administered as when local anaesthetic is used. This warns that sympathetic blocks must be controlled in each and every case lest false conclusions be drawn about the response. Medial branch blocks of the lumbar and of the cervical dorsal rami have been extensively investigated in order to establish their validity, diagnostic utility and therapeutic utility. They provide an example and benchmark for how diagnostic blocks can and should be validated.