Acute Nontraumatic Back Pain: Infections and Mimics

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Acute nontraumatic back pain has a broad differential diagnosis, but the primary concern in the emergent setting is the accurate and timely identification of spinal infection. Prompt diagnosis facilitates optimal medical and/or surgical management and can reduce the risk of long-term musculoskeletal and neurologic complications. MRI is the mainstay imaging modality for assessing a suspected infection. Diagnosing infection can be challenging owing to subtle imaging findings and because several noninfectious entities can mimic infection.

This online presentation reviews the anatomy, pathophysiology, and characteristic appearances of spinal infections and noninfectious mimics involving the vertebral column, facet joints, and epidural space (Fig 1). Helpful imaging clues and the use of diffusion-weighted imaging (DWI) are described to help establish the correct diagnosis. Several cases are presented in a quiz-based format to allow the reader to determine if the presented case demonstrates an infection or a mimic.

Figure 1.

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Imaging findings of infection involving the vertebral column include endplate destruction, iliopsoas edema, and epidural phlegmon. In particular, edema or fluid in the psoas musculature (MRI psoas sign) is a finding consistent with early spondylodiscitis, which may precede osseous destructive changes and if present serves as a feasible biopsy target to confirm infection and guide antibiotic treatment (Fig 2).

Figure 2a.

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Figure 2b.

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DWI is helpful for assessing suspected infection. The DWI claw sign (paired clawlike hyperintense regions in adjacent vertebral bodies) has a high negative predictive value for excluding infection. In contrast, restricted diffusion centered in the endplate edema is highly suspicious for infection. DWI can help differentiate postoperative soft-tissue paraspinal abscess from noninfectious seroma. Noninfectious mimics of infectious discitis-osteomyelitis include Modic type 1 endplate edema, acute Schmorl node, Andersson lesion pseudarthrosis in ankylosing spondylitis, radiation osteitis, longus colli calcific tendinitis, Paget disease, and SAPHO (synovitis, acne, pustulosis, hyperostosis, osteitis) syndrome.

Facet septic arthritis is uncommon, characterized by facet effusion, bone destruction, thickened peripheral enhancement of the joint, edema, and phlegmon and/or abscess development within the adjacent paraspinal muscles. In contrast, noninfectious acute osteoarthritis of the facet joint represents an extremely common noninfectious cause, characterized by edematous changes in the facet joint and surrounding soft tissue, without bone destruction. In addition, metastases to the facet joint or articular processes are rare but can mimic acute infection. Infection may spread to the epidural space hematogenously or by contiguous spread from adjacent spondylodiscitis or from recent instrumentation. Noninfectious causes with overlapping imaging appearances include epidural hematoma and neoplastic and nonneoplastic masses with epidural extension.

This online presentation reviews the imaging appearance of spinal infection and noninfectious mimics involving the disk-endplate complex, facet joints, and epidural space.

Disclosures of Conflicts of Interest.—J.C.M.Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: royalties from Cambridge University Press. Other activities: disclosed no relevant relationships. D.S.T.Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: fees for independent medical-legal consulting. Other activities: disclosed no relevant relationships.