Interventional Diagnostic Procedures
Core concepts
Diagnostic blocks answer a yes/no question about a specific structure. Understand medial branch blocks (facet-mediated pain), selective nerve root blocks, sympathetic blocks, and provocative/differential blocks. The value of a block depends on technique, volume, and control injections.
Interpretation principles
A positive response requires concordant, substantial, time-limited relief of the patient's typical pain. False positives arise from systemic drug effect, placebo, and spread to adjacent structures; false negatives from missed targets, intravascular injection, or inadequate volume. Controlled (comparative) blocks improve specificity for procedures like radiofrequency selection.
Traps
- Treating any relief as diagnostic without a threshold or duration criterion.
- Ignoring contrast spread when it contradicts the intended target.
- Using a block to confirm a diagnosis already contradicted by exam.
- Forgetting that local anesthetic systemic toxicity is possible even in small-volume blocks.
Worked rule of thumb
Define the success criterion before you inject (for example, at least 50% relief of the index pain lasting at least the expected anesthetic duration) and document it. A block whose result you cannot interpret should not drive a destructive procedure.
Safety
Aspirate, use incremental injection, monitor for toxicity, and have lipid emulsion and resuscitation available.