Utility of Follow-Up Imaging Studies in Spine Infections
Utility of Follow-Up Imaging Studies in Spine Infections
Abstract & Commentary
By Dean L. Winslow, MD, FACP, Chief, Division of AIDS Medicine, Santa Clara Valley Medical Center, Clinical Professor of Medicine, Stanford University School of Medicine, Section Editor, HIV, is Associate Editor for Infectious Disease Alert.
Dr. Winslow is a consultant for Bayer Diagnostics, and on the speaker's bureau for GlaxoSmithKline and Pfizer.
Synopsis: A retrospective cohort analysis of 79 patients with spine infection who underwent baseline and 4-8 week follow-up imaging was conducted. Twenty-seven (34%), 38 (48%), and 14 (18%) patients were judged to have images graded as improved, equivocal, or worse respectively. Absence of microbiological treatment failure was seen in 100%, 89%, and 56% of patients respectively.
Source: Kowalski TJ, et al. Do Follow-Up Imaging Examinations Provide Useful Prognostic Information in Patients with Spine Infection? Clin Infect Dis. 2006;43:172-179.
This paper presents the results of a retrospective cohort analysis from the Mayo Clinic of 79 patients with bacterial spine infection who underwent baseline and follow-up imaging studies with MRI. Thirty-five infections (44%) were due to Staphylococcus aureus, 9 (11%) due to coagulase- negative staphylococci, and 16 (20%) were culture-negative. Twenty-seven (34%), 38 (48%), and 14 (18%) of follow-up imaging studies were judged as improved, equivocal, or worse, respectively. One year survival free of microbiologically-confirmed treatment failure was 100%, 89%, and 56% across these same groups. In another analysis, looking across radiographic strata, only 3 (6%) of 52 patients who were judged to have had clinical improvement at time of follow-up imaging experienced treatment failure. Elevated levels of inflammatory biomarkers were present in 2 of these 3 patients.
Commentary
I recently finished a 2-month stint as the attending physician on ID consult service at our county hospital, which included the first 2 weeks of July with a bright, young, brand new first-year ID fellow, and 2 excellent medical residents. As always, one of the toughest questions the ID attending is usually asked is, "How long should you treat XYZ infection?" With the exception of a handful of diseases where the pathophysiology is relatively homogenous (such as bacterial endocarditis due to a particular organism), most of us fall back on the wisecrack answer most commonly attributed to the legendary Max Finland at Boston City Hospital, "Long enough." After using that answer almost daily followed by the equally unhelpful answer ("clinical experience") to the follow-up question of how do you know how long is "long enough," I was glad to see this article in CID which helps answer this question for bacterial spinal infections.
The study does have some limitations, including its retrospective nature and the fact that only staphylococcal and culture-negative infections were included in the series. However, Kowalski and colleagues used a reasonable and simple grading scale for assessing improvement, equivocal, and worsening MRI findings in which signal abnormality and abscess size in the paraspinal/psoas musculature and/or epidural space were considered. A somewhat arbitrary set of criteria for interpretation of biomarker improvement (25% reduction in ESR or CRP from baseline or final value of ESR < 40 mm/h and CRP < 1 mg/dL), but this seemed helpful and congruent with the prognostic information gleaned from follow-up MRI imaging. Using similar inflammatory biomarker criteria provides sensitivity for predicting treatment failure, but has low specificity since many patients who are cured do not experience normalization of ESR and/or CRP in the first 4-8 weeks of treatment.1 Despite this limitation, this study shows that only one patient in the cohort's relapse/treatment failure would have been missed by relying on serial biomarkers of inflammation in the absence of follow-up imaging. Kowalski et al postulate that a cost-effective approach derived from this study would be to order follow-up imaging only on those patients who are at high risk for treatment failure based on clinical status and inflammatory biomarker response.
Magnetic resonance imaging is a relatively new imaging technology which is exquisitely sensitive for use in assessing the brain, spinal cord, and axial skeleton, and has appropriately become the imaging technique of choice for infections involving these structures.2 Ironically, one limitation of MRI is its exquisite sensitivity and the fact that the scan does not rapidly return to normal following an adequate course of antimicrobial therapy. This retrospective study sheds some light on how to best use MRI (in conjunction with ESR, CRP, and, yes, "clinical judgment" is still in the equation) in managing spinal infections due to staphylococci.
References
- Carragee EJ, et al. The Clinical Use of Erythrocyte Sedimentation Rate in Pyogenic Vertebral Osteomyelitis. Spine. 1997;22:2089-2093.
- Carragee EJ. The Clinical Use of Magnetic Resonance Imaging in Pyogenic Vertebral Osteomyelitis. Spine. 1997;22:780-785.
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