• S M Lord, L Barnsley, and N Bogduk.
• Cervical Spine Research Unit, University of Newcastle, Callaghan, Australia.
• Clin J Pain. 1995 Sep 1;11(3):208-13.

BackgroundThe development of target-specific local anesthetic blocks has enabled pain physicians to explore the anatomical source of chronic spinal pain. However, such blocks rely on subjective responses and may be subject to the placebo effect. Comparative local anesthetic blocks have been advocated as a means of identifying true-positive cases and excluding placebo responders. This paradigm employs two local anesthetics with different durations of action; only patients who obtain reproducible relief and correctly identify the longer-acting agent are considered positive.ObjectiveOur objective was to evaluate the reliability of comparative blocks of the medial branches of the cervical dorsal rami in the diagnosis of cervical zygapophysial joint pain.DesignWe compared comparative blocks and the criterion-standard of randomized, double-blind, placebo-controlled blocks.SettingThe study was conducted at a tertiary referral center.PatientsWe studied the first 50 consecutive patients referred for assessment of chronic neck pain (> 3 months' duration) after a motor vehicle accident, who completed a series of placebo-controlled blocks after an initial positive response. Patients were 41 +/- 11 years (mean +/- SD) old with a male/female ratio of 1:2.MethodsPatients underwent three blocks using three different agents-lignocaine, bupivacaine, and normal saline--administered on separate occasions, in random order and under double-blind conditions. The diagnostic decision based on comparative blocks alone was compared with that based on placebo-controlled blocks.ResultsComparative blocks were found to have a specificity of 88%, but only marginal sensitivity (54%). Although comparative blocks result in few false-positive diagnoses, their liability is that they result in a high proportion of false-negative diagnoses. Expanding the comparative blocks diagnostic criteria to include all patients with reproducible relief, irrespective of duration, increases sensitivity to 100% but lowers specificity to 65%.ConclusionsWhether physicians use comparative or placebo-controlled blocks depends upon the implications of their results. If innocuous therapy will be prescribed, comparative blocks might suffice. However, when diagnostic certainty is critical, such as in a medicolegal context or when surgical intervention is contemplated, placebo-controlled blocks are recommended.

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