Clinical StudiesClinical predictors of screening lumbar zygapophyseal joint blocks: development of clinical prediction rules
Introduction
It is estimated that 15–40% of chronic low back pain patients have pain arising from the lumbar zygapophyseal joints (ZJ) with increasing prevalence in older age groups [1], [2]. Based on complete relief of pain from controlled diagnostic blocks, the prevalence of isolated ZJ-mediated back pain is estimated to be as low as 4% [3]. Near-total pain ablation after controlled injections into the joints or at their nerve supply is a widely accepted reference standard technique for diagnosis of ZJ pain [4], [5]. Both techniques are believed to produce equivalent diagnostic results [6], although recent opinion favors medial branch blocks [4], [7]. Earlier studies have used 50% reduction in pain [8] after intra-articular injections in a placebo-controlled design, 75% reduction [9] after single intra-articular blocks, 75% [10] using double medial branch blocks [10], 80% reduction [11] using double medial branch blocks, verbal numeric scale [12] using single intra-articular blocks, a Likert scale of pain relief [13] using single intra-articular blocks, or a Likert scale for the screening block and 50% or more reduction in pain after confirmatory block [14] using either intra-articular or medial branch blocks. Single uncontrolled blocks carry a high false positive rate between 25% [15] and 38% [1], and can only be used as an initial screening procedure to rule out patients with non-ZJ sources of pain. Previous studies have indicated that history and physical examination findings cannot predict results from diagnostic ZJ blocks [3], [10], [16], [17], but these studies have used less stringent pain reduction standards as references standards than is currently recommended [3], [10], [17], [18]. The question remains “Can clinical variables predict the outcome of ZJ blocks when complete or near-complete ablation of pain is used as the reference standard?”
The low prevalence of isolated ZJ-mediated back pain implies the need for clinical rules to identify patients unlikely to respond to an initial screening ZJ block. Patients with a low probability of a positive anesthetic response need not be subjected to the screening block, and the tissue origin of pain should be sought elsewhere.
Patients with ZJ pain confirmed by controlled blocks do not experience pain in the spinal midline [3]. The extension rotation (ER) test has been found to have 100% sensitivity and 12% specificity in relation to double ZJ blocks [14], but paradoxically, Revel et al. reported that “no pain with the ER test” was associated with a positive response to a screening ZJ block [9]. These authors found that two clinical strategies using five of seven clinical variables may be valuable as a screening test for ZJ blocks [9] and had sensitivities of 92–100% and specificities of 66–80%. These rules and midline pain have been recommended for inclusion in an algorithmic approach to the diagnosis of chronic low back pain [7]. The results of Revel et al. were not replicated in a recent study recording 11% sensitivity and 91% specificity [19], or in a study using double ZJ blocks as the reference standard [10]. These studies did not evaluate the predictive power of clinical variables against the stringent reference standard of immediate total or near-total reduction of pain.
A recent study found that “No pain rising from sitting” was associated with an 80% or more reduction in pain intensity after ZJ blocks (p=.008) and no patients experiencing centralization or the opposite pain behavior, peripheralization, responded to single ZJ blocks [20]. Centralization (CP) is the progressive reduction and eventual abolition of referred pain or movement of referred pain towards the spinal midline during a specific examination of standardized repeated movement testing (the McKenzie evaluation) [21], [22]. CP has been found to be highly specific to positive pain provocation during discography, but not very sensitive [23].
The aim of this analysis was to evaluate potentially valuable predictor variables against the different reference standards, including near-total pain ablation, to see if there were any clinical prediction rules that may assist clinicians in selecting patients for ZJ block procedures.
Section snippets
Design
As part of a wider study of the diagnostic accuracy of clinical examination variables in relation to available reference standard diagnoses [24], [25] in chronic low back pain, a subset of patients received a screening local anesthetic injection into the ZJ joint or medial branch targets. The results of clinical tests were compared with reduction in pain after the screening ZJ blocks. Pain intensity was measured on 100-mm pain visual analogue scales (VAS). A positive response was based on
Results
Physical examinations and screening ZJ blocks were carried out on 151 chronic low back pain patients. Thirty-one were excluded from the main analysis as they received another intervention in the same procedure session and did not return for differentiating and confirmatory blocks or received blocks at another facility before examination in the current study. Table 1 contains demographic and other descriptive characteristics with comparisons between included and excluded patients. Included
Discussion
Our results for the ER test replicate those in an earlier study [14], but are in contrast to the results of Revel et al. [9]. The low specificity means that the test has no diagnostic value for ZJ-mediated back pain. However, the high sensitivity (100%) allows the clinician to rule out (SnNout) [36] a 95% pain reduction after a screening block if the right and left ER tests are negative. The ER test is typically considered positive when pain is provoked by extension combined with rotation
Study limitations
The pragmatic nature of this study has resulted in several significant weaknesses:
The high exclusion rate was caused by the confounding influences of other procedures such as sacroiliac joint blocks conducted in the same session as the screening ZJ block. In most cases, the combined blocks were not expected to result in a substantial reduction in pain, as the source of nociception was considered to be elsewhere, eg, the intervertebral disc. We were wrong in this expectation. Surprisingly, of
Conclusions
Diagnosis of symptomatic ZJ joint by noninvasive means remains elusive. Clinical findings have predictive power only for ZJ blocks using a stringent pain reduction or ablation standard. This study has mainly negative value in that clinicians may rule out a positive response to a screening ZJ block when the ER test or one of four clinical prediction rules are negative, or in the presence of pain centralization. One clinical prediction rule (CPR5) shows a fivefold advantage in selecting patients
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Supported by the International Spinal Injection Society, a New Zealand Society of Physiotherapists scholarship, and the New Zealand Manipulative Physiotherapists Education Trust Fund. Nothing of value received from a commercial entity related to this manuscript.