Low back pain (LBP) is an acute or chronic pain in the lumbar or sacral regions, which may be associated with musculo-ligamentous sprains and strains, intervertebral disc displacement and other conditions. It is known that discography is the gold standard for the discogenic pain in the LBP patient, and a Dallas Discogram Scale for grading system1 is developed for the annular disruption. However, the discography is a traumatic examination, which can induce infection, cerebral spinal fluid leakage, retroperitoneal bleeding etc. In 1992, Aprill and Bogduk2 established the high intensity zone (HIZ) criteria, which is a high intensity zone (bright white) located in the substance of the posterior annulus fibrosus on T2 -weighted sagittal images of lumbar spine, clearly dissociated from the signal of the nucleus pulposus and is appreciably brighter than that of the nucleus pulposus. The results indicate that HIZ correlated significantly with the presence of Grade 4 annular disruption and with the reproduction of the patient's pain, which suggests that this sign is diagnostic of painful internal disc disruption. In contrast, Carragee et al3 recognized that the presence of a HIZ does not reliably indicate the presence of symptomatic internal disc disruption. In this context, an “evidenced-based radiology” methods was used to evaluated the comparative roles of HIZ in diagnosing the low back pain.
In this article, we used evidence-based radiology4-6 to evaluate the imaging diagnosis of LBP by HIZ. The process was mainly comprised of five steps: ask, search, appraise, apply, and evaluate.6 The associated literature were retrieved from 1992 to 2007. This project began in September 2007.
A focused clinical question design
According to the evidenced-based approach, the question was designed as follows: In patient with LBP, how does HIZ on MR T2WI compare with a reference standard in the diagnosis of discogenic LBP.
Evidence-base practice strategies and others were used. The databases included: Pubmed, Highwire, TRIP database, the National Institute of Clinical Excellence, the Scottish Intercollegiate Guidelines Network, the American College of Physicians Journal Club, Evidence-based Medicine Online, Clinical Evidence and UpToDate. For the lowest level of evidence, the Patient-Intervention-Comparator-Outcome (PICO) format was chosen and the patient components included low back pain, lower back pain, back pain, anular tear(s), anular fissure(s), and disc degeneration. The investigation components included high intensity zone(s), lumbar spine, lumbar intervertebral disc, and magnetic resonance imaging. The comparison parts consisted of discography, post-contrast computed tomography, computed tomography, granulation tissue and edge neovascularity. The outcome parts included discogenic pain, discogenic low back pain and discogenic back pain. This search strategy was performed on PubMed and the operator AND was used between the different concepts and the operator OR used within the similar concepts. In addition, retrospective search and google scholar were also applied. The British Journal of Radiology and Spine were searched manually from January 1992 to December 2007.
Studies were included in this project if they met the following crieteria: The subjects suffered from low back pain, and the patients had an MRI sagittal T2WI scan and dicography. The values of sensitivity and specificity, positive predictive value, and negative predictive value were available or could be calculated from the available data.
Studies were excluded from this project if they were in line with exclusion criteria: The subject had not received discography or MRI sagittal T2WI scan, and the data were not completely available so that the diagnostic accuracy parameters such as sensitivity and specificity could not be calculated.
Critical appraisal of retrieved literatures
Appraising the levels of the retrieved articles
The Oxford Centre for Evidence-based Medicine Levels of Evidence (May 2001)7 was used. The results of assigning levels of retrieved articles are shown in Table 1.
Appraising the validity from the materials and methods section
The retrieved articles were assessed for validity from the materials and methods section. The appraising criteria8 was applied as follows: Independent blinded comparison with reference standard, test evaluated in appropriate spectrum of patients, reference standard applied to all patients regardless of test result and test validated in second independent patient group. The results from appraising the validity in the retrieved articles could be seen in Table 2.
Appraising the strength from the results section
The important statistical parameters included the prevalence of HIZ, sensitivity, specificity, predictive values and likelihood ratios (LRs). A spreadsheet9 for the calculation was applied. The assessment results from the strength of the retrieved articles result section could be seen in Table 3.
Schellhas et al10 showed a high sensitivity of 0.95 and a narrow confidence interval (CI), 95% CI: 0.95-1.00, while the lowest sensitivity was 0.11 and its 95% CI was 0.01-0.21.11 A high specificity was demonstrated in the researches of Aprill et al2 and Peng et al,12 with a specificity of 0.97 and 1.00, respectively. The lowest specificity was 0.70.13 The positive predictive value was very high in the researches of Aprill et al2 and Peng et al12 (0.95 and 1.00 respectively). The highest negative predictive value was 0.97.10 For the research of Aprill et al,2 the positive LR was 18.37 and the negative LR 0.38, these values indicated a strong likelihood of discogenic low back pain if HIZ was positive and a small likelihood of the condition being present if HIZ was negative. The positive and negative LR was infinity and 0.65, respectively. In the research of Peng et al,12 these values suggested that the discogenic low back pain must be present if HIZ was positive and that discogenic low back pain could not be excluded if HIZ was negative.
Combining LRs and pre-test probability: (Graphs of conditional probability)
Aprill's study2 and Peng's study12 had high positive LRs but Aprill's study2 had a higher sensitivity and was assigned to 1b. So LRs of Aprill's study2 was the best evidence. Through the computer based literature search, the 1-month period pre-test clinical probability of LBP in general population was 39%,14 which was derived from a general population including 7 669 adults.
For the patients with a high pre-test clinical probability of LBP, the prevalence was applied as the pre-test probability to the graphs of conditional probability (GCP) for HIZ testing. If the result was positive, the post-test probability was 93.1%, which suggested further provocative discography; and if the result was negative, the post-test probability was 1.96%, which could exclude the discogenic low back pain (Figure).
In this context, the strength of the best evidence is assessed by the evidence-based approach. Aprill's study2 has a large positive LR and a small negative LR, and also has a moderate sensitivity and the highest specificity. For an LBP patient who failed conservative therapy, if the HIZ is positive, the discogenic low back pain is not able to be confirmed, and discography is suggested. However, if the HIZ is negative, the negative LRs are comparatively low (1.96%), the discogenic low back pain can be excluded. Therefore, a magnetic resonance imaging for HIZ is recommended as a routine implementation for the LBP patients.
Since Aprill and Bogduk2 reported that HIZ is diagnostic of painful internal disc disruption, many evaluations about the diagnostic roles of HIZ for discogenic low back pain have emerged. In 1996, Schellhas et al10 found that the HIZ is a reliable marker of painful outer anular disruption in patients with symptomatic low back pain. Saifuddin et al15 recognized that the HIZ is a marker of a painful posterior anular tear in 1998. Peng et al12 researched the pathogenesis and clinical significance of an HIZ of lumbar intervertebral disc on MRI in patients with discogenic LBP, and found that HIZ lesions are a disorganized, vascularized granulation tissue, and further confirmed that HIZ of the lumbar disc on MRI in the patient with low back pain could be considered as a reliable marker of painful outer anular disruption. However, Carragee et al3 investigated the asymptomatic and symptomatic group patients, and suggested that HIZ does not reliably indicate the presence of symptomatic internal disc disruption.
However, this evidence-based practice of the comparative role of HIZ in diagnosing the discogenic low back pain suggested that HIZ is limited in diagnosing the discogenic LBP. If HIZ is positive, the diagnosis of discogenic LBP is not able to be confirmed, however, if the test is negative, the diagnosis can be excluded.
A limitation of this evidence-based practice is in the study differences. Some of the studies were retrospective studies and the other were prospective studies, and the subject's inclusion criteria and the operators experience were different, although the reference standard is basically similar. All of these impacted the diagnostic accuracy. Another limitation of this study is that the prevalence of the low back pain is different in different populations and different areas, which directly affects the pre-test probability and post-test probability. Thus, we must be cautious of the clinical practice of these results when we face a patient with LBP.
Finally, literature bias can not be controlled. All the included literature are positive studies and negative studies are not available, and the related literature are not completely retrieved. So that literature bias may have affected our results. Thus, clinical practice is necessary for the improvement of this technique for the diagnostic accuracy of LBP.
The magnetic resonance signal intensity can reflect the hydration of intervertebral lumbar discs. Pearce et al16 classified disc degeneration into five grades by midsagittal T2-weighted images. According to the retrieved articles, HIZ is optimally recognized on the sagittal T2WI.
MRI technology generation
In all retrieved studies, GE, Siemens and Philips MR systems were used. The field strength is 0.5-1.5 Tesla.
Scan range: in all studies, the MR scan range was the lumbosacral area.
Series description: fast spin echo (FSE), spin echo (SE) and gradient echo pulse sequences were employed in these studies.
T2WI parameters: T2WI was obtained with TR 1800 ms-4500 ms and TE 21 ms-120 ms. The slice thickness was 4 mm or 5 mm and the slice gap was between 0.4 mm-1.5mm.
Scan plane: Coronal, axial and sagittal planes were included, and the sagittal plane must be chosen.
Additional parameters: In Aprill's study,2 two or four excitations and acquisition with 196×256 or 256×256 matrix were employed. Occasionally cardiac gating was used for motion suppression.
With the development of MR technology, a clear and valuable sagittal T2WI of the lumbar intervertebral disc is not difficult to obtain.
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