American Journal of Neuroradiology

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Case of the Week

Section Editors: Matylda Machnowska¹and Anvita Pauranik²
¹University of Toronto, Toronto, Ontario, Canada
²BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada

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January 6, 2022

Infectious Spondylodiscitis

Background:
- Spondylodiscitis, an infection of the disc and vertebra, occurs most commonly from hematogenous dissemination of a remote site of infection and less commonly from direct implantation related to intervention or trauma.
- Pyogenic infections are the most frequent cause.
- Less commonly caused by nonpyogenic etiologies such as tuberculous or fungal infections
- It accounts for only about 1% of skeletal infections; however, there is a recent trend towards higher incidence from advanced age, diabetes, malignancy, immunosuppression, IV drug abuse, and spinal interventions.
Clinical Presentation:
- Presentation can be insidious, with back pain being the most frequent, albeit a nonspecific, symptom.
- More specific symptoms and signs such as fever, elevated WBC count, CRP, and positive blood cultures are less sensitive and hence, in many instances, diagnosis can get delayed, with progression of disease and epidural extension of infection leading to higher morbidity.
Key Diagnostic Features:
- Radiographs and CT
  - Insensitive to early disease
  - Progressive disease may be detected by endplate destruction, disc space narrowing, vertebral compression, and paravertebral abnormal soft tissue.
  - Healed remote infections may manifest as sclerosis and vertebral fusion.
- MRI
  - High sensitivity and relatively high specificity in diagnosis
  - Early findings: Signal change in the disc and marrow—low on T1 and high on T2/STIR with enhancement
  - Late findings: Morphologic alterations of loss of disc height, endplate erosion/destruction, and vertebral collapse
  - MRI can accurately depict epidural and paraspinal extension of infection and diagnosis of abscess formation.
  - Atypical findings: Late or absence of disc involvement, scalloping of the vertebral margins, and large paraspinal/psoas abscesses should bring into consideration nonpyogenic etiologies.
  - Limitations: Artifacts from spinal hardware, small field of view limiting the ability to detect significant extraspinal infections in the same sitting
- PET/CT
  - PET/CT can be a "one-stop shop" for infection imaging, as it also aids in detection of additional synchronous foci of infection in the spine and elsewhere in the body. It is becoming the test of choice for imaging evaluation of fever of unknown origin.
  - Focal moderate-to-avid uptake in the disc space and vertebral endplates
  - Progressive disease can show uptake in the paraspinal soft tissues. Central photopenia in abscess. PET is poor in the detection of epidural infection.
  - PET/CT has also shown promise in evaluating treatment response better than MRI. PET is valuable when MRI is contraindicated due to patient safety factors or if spine hardware limits MRI quality. PET/MRI, which offers the advantages of both modalities, is becoming increasingly available.
  - Limitations of PET compared with MRI are its poor anatomic detail, precluding evaluation of small epidural phlegmon and abscesses.
Differential Diagnoses:
- Marrow edema is a nonspecific finding that can be seen in other differentials mentioned below. Presence of high T2 signal within the disc, endplate destruction, postcontrast enhancement in the disc or vertebra, paraspinal/epidural phlegmon, and abscess aid in distinguishing spondylodiscitis from these other differentials:
- Type I Modic degenerative changes: Typically present without endplate destruction, although Schmorl nodes can be present; lower T2 signal in disc and absence of abnormal signal in paraspinal soft tissues on MRI; typically less avid than discitis on PET
- Acute compression fractures and vertebral trauma: Acute onset; a fracture line may be seen; PET uptake can be present in acute osteoporotic compressions, but is less avid than in discitis.
- Inflammatory spondyloarthropathy: Presence of a "shiny corner" sign; syndesmophytes; lack of disc involvement
- Metastatic disease: More likely to be multifocal and masslike; no specific predilection for the disc or endplates
Treatment:
- CT-guided biopsy may be performed to confirm the diagnosis or identify the pathogen.
- Early stages of the disease can be treated with antibiotic therapy and imaging utilized for disease surveillance.
- Advanced cases with intraspinal extension or spinal instability would require surgical intervention.