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Volume 4 Issue 11 - November/December 2006
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Vertebral Osteomyelitis
 
  • Vertebral osteomyelitis should be considered in patients with unremitting focal back pain,
    especially if accompanied by fever

  • Plain film radiography is recommended for initial diagnostic imaging

  • MRI is indicated if x-rays are normal and clinical suspicion is high; if MRI is contraindicated,
    CT is recommended

  • Combined gallium and bone scan is recommended when MRI and/or CT are equivocal

  • CT-guided biopsy is recommended to confirm the diagnosis and identify the infectious agent



Plain Film Radiography
MRI
CT
Nuclear Scintigraphy
CT-Guided Biopsy
Follow-up Imaging
Scheduling
Further Information
References

I
n the vast majority of patients, back pain uncomplicated by neurological symptoms will resolve within 4-6 weeks without any lasting consequences. Unfortunately, for the few patients whose back pain is due to a more serious condition, such as vertebral osteomyelitis, diagnosis is often delayed. Vertebral osteomyelitis accounts for 2-4% of all cases of bone infection and is most often found in men over 50 years. Although rare, incidence is thought to be rising due to an aging population, a higher number of immunocompromised patients, and the increased use of intravascular devices and injected drugs.

Vertebral osteomyelitis should be considered in all patients experiencing unremitting and/or focal vertebral pain that is not relieved by lying down, particularly if accompanied by fever or paravertebral symptoms indicating a psoas or other paraspinal extension. Patients with vertebral osteomyelitis typically experience mild pain to start with, which progressively worsens over a period of weeks or even months. The key clinical finding is localized tenderness of the infected bone, with palpation being more sensitive than percussion. Infection in the cervical region is more likely to be associated with neurological deficits. In most cases, the infection is blood-born. Spread from Batson’s venous plexus (draining the genitourinary tract) has been postulated. Infection may also spread from a contiguous infection or, occasionally, be a result of direct inoculation.

Blood cultures as well as tests for leukocytosis and elevated erythrocyte sedimentation rate are recommended. However, none of these tests are sensitive and, if negative, do not rule out osteomylelitis.

 
Figure 1. Plain film radiograph of spinal discitis / osteomyelitis. Lateral view of the lumbar spine demonstrates L 3-4 disc space narrowing (arrow) and end plate irregularity.
 
Figure 1. Plain film radiograph of spinal discitis / osteomyelitis. Lateral view of the lumbar spine demonstrates L 3-4 disc space narrowing (arrow) and end-plate irregularity.

Plain Film Radiography
Radiography should be ordered for all patients with suspected vertebral osteomyelitis to evaluate the spine for this and other conditions. Generally, the earliest radiographic signs of vertebral osteomyelitis are loss of definition of a vertebral end-plate and narrowing of the associated disc space. Although these signs do not appear until 10-21 days after the onset of infection, most patient are not seen early in the course of infection because back pain from vertebral osteomyelitis is generally mild at first and the symptoms disregarded. The signs may be less conspicuous in the thoracic spine and if pre-existing degenerative conditions of the spine are present. If radiography is not diagnostic and the patient is febrile or has pain that is unremitting and not explained by the plain film findings, further imaging should be considered.

Comparison of Imaging Modalities for the Diagnosis of Vertebral Osteomyelitis
Modality
Strengths
Weaknesses
Plain film x-ray
Sensitive when infection well established
Readily accessible
Signs do not develop until 10-21 days after start of infection
MRI
Most sensitive for early detection (edema)
No radiation exposure
Moderate specificity
Contraindications to MR, e.g. claustrophobia, pacer, etc.
CT More sensitive than plain film for detecting bone and disc erosions
Less sensitive than MR to soft tissue lesions and abscesses
Iodinated contrast administered
Gallium-Bone Scan May be useful if CT and/or MRI equivocal
Low spatial resolution
Requires 2 days


Figure 2. MRI of lumbar spine discitis/osteomyelitis.  A. Sagittal T1-weighted images of the lumbar spine in the same patient as figure 1 demonstrate T1-hypointense signal (solid arrows) centered around the L3-4 interspace.  B. Post gadolinium sagittal fat-suppressed T1-weighted images shows marrow (dashed arrows) and disc enhancement with endplate erosions.
 
Figure 2. MRI of lumbar spine discitis/osteomyelitis.  A. Sagittal T1-weighted images of the lumbar spine in the same patient as figure 1 demonstrate T1-hypointense signal (solid arrows) centered around the L3-4 interspace.  B. Post gadolinium sagittal fat-suppressed T1-weighted images shows marrow (dashed arrows) and disc enhancement with endplate erosions.

MRI
MRI is the most sensitive imaging modality and can shows signs of bone marrow edema and intervertebral disc signal changes on non-contrast images early in the course of infection. Contrast MRI scans may show diffuse enhancement in infected bone and discs and help to demonstrate the presence of paraspinal soft tissue and epidural extent of disease. However, MRI is not completely specific and the diagnosis can be confused with, for example, tumor, spondylosis, compression fracture, or post-radiation changes. In one study, the primary diagnosis of discitis or vertebral osteomyelitis was made in 67% of the cases where it was initially suspected and was an alternate diagnosis in 26%. MRI is also less specific if performed <2 weeks after the onset of infection.

CT
If MRI is contraindicated, CT is useful as a sensitive modality for detecting erosions of bone and disc, and is more sensitive than radiography in this respect. In addition, it can be helpful for characterizing compression fractures, which may be confused with osteomyelitis. Contrast should be used if possible.  However, CT is much less sensitive than MRI for the detection of epidural abscesses or soft tissue lesions.

Nuclear Scintigraphy
Scintigraphy may be useful in equivocal cases or when different sites of infection must be identified in multifocal disease. A combination of 67Ga citrate and 99mTc-MDP bone scanning is very sensitive and more specific than gallium alone.
 
Figure 3. Percutaneous CT –guided biopsy of discitis / osteomyelitis.spine in the same patient as figure 1 demonstrate T1-hypointense signal (solid arrows) centered around the L3-4 interspace.  B. Post gadolinium sagittal fat-suppressed T1-weighted images shows marrow (dashed arrows) and disc enhancement with endplate erosions.
 
Figure 3. Percutaneous CT –guided biopsy of discitis / osteomyelitis.


CT-Guided Biopsy
If there are radiological indications of discitis/osteomyelitis and blood cultures are negative, CT-guided biopsy may be indicated to confirm the diagnosis and determine the cause of the infection. If the diagnosis was made with plain film radiography, CT is recommended for planning a biopsy. Core biopsy samples can be taken from an abnormal disc, soft tissue around the bone, and the vertebra itself when technically possible. If there is an abscess, it can be aspirated. Biopsy procedures are generally performed under local anesthesia with conscious sedation. Patients must have normal coagulation studies and be taken off anti-coagulant medication.

Follow-up Imaging
Follow-up imaging is not indicated in most patients because MRI findings of discitis/osteomyelitis may persist and can even appear worse despite a clinical response to therapy. However, follow-up MRI may be considered if epidural or paraspinal infection is present, if surgery is being considered for any reason, if the microbiological agent has not been identified (particularly if there has not been a clinical response to empiric therapy), and in any patient with no clinical response or clinical worsening.

Routine scans at the end of treatment are not indicated, because imaging abnormalities are expected to persist when the recommended duration of therapy is completed. Additional imaging at this time may be indicated if there is a concern that the extent of paraspinal disease or its response to therapy may warrant further treatment (medical or surgical).



Scheduling
Radiographic imaging for suspected osteomyelitis can be performed at all MGH facilities. MRI can be performed at Mass General Imaging in Waltham, Chelsea, or Charlestown, or the main MGH campus. CT can be performed at all these facilities except Mass General Imaging in Charlestown. Nuclear medicine studies can be performed at Mass General West Imaging in Waltham or the main MGH campus. All studies can be ordered online via the Radiology Order Entry (http://mghroe ) or by calling 4-XRAY (617-724-9729). CT-guided biopsies are performed only on the main campus and can be scheduled by calling 4-PAIN (617-724-7246).


 
Further Information
For further questions on imaging studies for osteomyelitis, please contact Tara M. Lawrimore, M.D. , Musculoskeletal Radiology (617-727-7717), or James A. Scott, M.D. , Nuclear Medicine (617-726-8758).

We would like to thank Drs. Lawrimore and Scott as well as Nesli Basgoz, M.D., of the Infectious Disease Unit for their advice and assistance in the preparation of this article.









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References
   

Baleriaux, DL and Neugroschl, C. (2004) Spinal and spinal cord infection. Eur Radiol 14: E72-83

Carragee, EJ. (1997) The clinical use of magnetic resonance imaging in pyogenic vertebral osteomyelitis. Spine 22: 780-5

Lew, DP and Waldvogel, FA. (2004) Osteomyelitis. Lancet 364: 369-79Tyrrell, PN, Cassar-Pullicino, VN and McCall, IW. (1999) Spinal infection. Eur Radiol 9: 1066-77

Winters, ME, Kluetz, P and Zilberstein, J. (2006) Back pain emergencies. Med Clin North Am 90: 505-23