Diagnostic blocks for chronic pain

Many conditions associated with chronic pain have no detectable morphological correlate. Consequently, the source of pain cannot be established by clinical examination or medical imaging. However, for some such conditions, the source of pain can be established using diagnostic blocks. The aim of this paper is to review the available evidence concerning the validity and utility of diagnostic blocks, and to identify areas where research is needed. Diagnostic blocks for cervical and lumbar zygapophysial joint pain have been extensively studied. Single blocks are associated with about 30% false-positive responses. Patients can report relief of pain for reasons other than the effect of a local anaesthetic injected during a diagnostic block, e.g. as the result of placebo effect. Therefore, in order to be valid, diagnostic blocks must be controlled in each patient. Many practitioners find limitations in the clinical applicability of placebo-controlled blocks. Comparative blocks (comparison lidocaine-bupivacaine for each block within each patient) have been investigated as alternatives to placebo-controlled blocks. A positive response requires short-lasting relief when lidocaine is used, and long-lasting relief when bupivacaine is used. The validity of comparative blocks is high when the disease under investigation is common. This is the case for zygapophysial joint pain after whiplash injury. However, the validity of comparative blocks strongly decreases with decreasing prevalence of the condition. This is the case for lumbar zygapophysial joint pain in young subjects: in these patients, the expected false-positive rate with comparative blocks is unacceptably high. Diagnostic blocks for cervical and lumbar zygapophysial joint have therapeutic utility. When positive, radiofrequency denervation is expected to produce substantial pain relief in 60-80% of patients. For all other types of blocks, very little research has been conducted. The few studies that have been published did not use controlled blocks. This may have produced a high rate of false-positive responses. Some data on spinal nerve root blocks suggest that these procedures may be valid for the diagnosis of radicular pain and are perhaps predictive for the success of surgery. The validity of diagnostic sympathetic blocks and their prognostic value in relation to outcomes of sympathectomy are unclear. There is lack of data on the validity of diagnostic intra-articular blocks. Discogenic pain is typically diagnosed by provocative discography, but this procedure remains controversial. Intradiscal and sinuvertebral nerve blocks with local anaesthetics are possible alternatives to provocation discography. At present, the sparse data available on these procedures do not allow an estimation of their validity. In conclusion, nerve blocks have an important potential role in the management of chronic pain. These procedures are not suitable to identify the pathology that is the cause of the pain (e.g. inflammatory, neuropathic, etc.). However, they can reveal the anatomical source of pain, thereby allowing the development of targeted treatments. Unfortunately, there is currently very little research on the validity and prognostic value of blocks. The potential usefulness of this practice remains therefore largely unexplored.