Duration of Relief from Diagnostic Medial Branch Blocks
Diagnostic medial branch blocks typically provide temporary pain relief lasting approximately 15-20 hours, corresponding to the duration of the local anesthetic used, though some patients may experience extended relief lasting up to 15 weeks per injection. 1, 2
Expected Duration Based on Local Anesthetic Type
The duration of relief from diagnostic medial branch blocks is primarily determined by the pharmacologic properties of the local anesthetic agent used:
- Short-acting anesthetics (lidocaine): Relief typically lasts 2-6 hours, though one case report documented approximately 20 hours of relief 2
- Longer-acting anesthetics (bupivacaine): Relief extends to 6-12 hours based on the drug's half-life 3
The key diagnostic criterion is that pain relief should last for the expected duration of the local anesthetic used—this temporal correlation confirms that the medial branch nerves are the true pain generators. 1, 4
Extended Relief Phenomenon
Some patients experience relief that extends well beyond the expected pharmacologic duration of the local anesthetic:
- Multiple medial branch blocks with local anesthetics alone may provide significant pain relief for up to 44-45 weeks total, with each individual injection providing approximately 15 weeks of relief 1
- This extended therapeutic effect offers a less invasive treatment option before proceeding to radiofrequency ablation 1
Clinical Implications for Proceeding to Ablation
Two separate diagnostic blocks are required before radiofrequency ablation, with each block demonstrating >50-80% pain relief for the duration of the local anesthetic. 1, 4
The requirement for dual confirmatory blocks is critical because:
- A single positive block has insufficient specificity to justify an irreversible denervation procedure 4
- The second block confirms that the initial positive response was not a false positive and that facet-mediated pain is the primary pain generator 4
- False-positive rates for single blocks range from 27-63%, making confirmatory testing essential 5
Important Technical Considerations
The volume of injectate used during diagnostic blocks significantly affects specificity—0.25 mL provides adequate nerve coverage while minimizing false positives from spread to adjacent structures, whereas 0.5 mL volumes consistently spread to adjacent levels and superficial tissues. 5
Using excessive volume (>0.25 mL) may produce adjacent-level nerve blocks in addition to the intended target, decreasing diagnostic specificity and potentially leading to false-positive results that would not predict successful radiofrequency ablation outcomes 5.
Contrast with Radiofrequency Ablation Outcomes
After successful diagnostic blocks, radiofrequency ablation provides substantially longer relief:
- Initial ablation typically provides 6-12 months of relief, with some patients experiencing benefit up to 2 years 6
- Repeat ablations in responders provide similar duration of 6-10 months of relief 2
- At long-term follow-up (median 39 months), 58% of patients maintain ≥50% improvement in function and 53% maintain ≥50% improvement in pain 6