Diskitis Workup

• Hematology

  • Elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are the most consistent laboratory abnormalities seen in cases of diskitis.
  • The mean ESR for patients with diskitis is 85-95 mm per hour. ESR utility can be extended by serial measurements during treatment.
  • A 50% decline in ESR can usually be expected with successful treatment, and ESR often continues to decline after treatment.
  • Frequently, ESR may not return to normal levels despite adequate therapy.
• Leukocytosis is often present in systemic disease but is frequently absent in diskitis cases. Diskitis is generally accompanied by a normal peripheral white blood cell (WBC) count if the primary site of infection has been treated.
• Microbiology

  • Blood cultures must be obtained on a frequent basis for any patient suspected of harboring an infected disk.
  • Appropriate therapy may be instituted for positive blood cultures without the need for invasive tests.
  • Unfortunately, blood cultures are positive in only one third to one half of diskitis cases.
• Sputum and urine cultures are necessary to locate any other sources of infection, including respiratory and genitourinary sites.

• Plain radiography

  • Although radiographic films of the spine can be very useful in diagnosing diskitis, abnormalities are visible only after several weeks following the onset of disease.
  • The most common early finding on plain films is disk-space narrowing, followed by irregularities and erosion of the adjacent endplates and calcification of the anulus around the affected disk.
  • As osteomyelitis progresses, bone density decreases, with loss of the normal trabeculation of the vertebra. If bone destruction continues, subluxation (with possible instability of the spine) becomes evident.
• Nuclear medicine

  • Gallium-67 and technetium-99m have been utilized in the detection of diskitis with similar results. Radionuclide scanning has demonstrated a high degree of sensitivity shortly after the onset of symptoms. Diffuse initial uptake is followed by more focal uptake on delayed scans. Technetium-99m has been recommended more often because of its lower cost and smaller radiation dose.
  • Because of the availability and sensitivity of other tests, radionuclide scans may be most useful in the workup of patients with fever of unknown origin.
  • Indium-111 WBC scintigraphy has been shown to have a low sensitivity for diskitis and is not the test of choice.
• CT scan

  • CT scanning has the ability to detect diskitis earlier than plain radiographs.
  • Findings include hypodensity of the intervertebral disk and destruction of the adjacent endplate and bone, as seen in the image below, with edematous surrounding tissues. Axial CT scan in a patient with diskitis demonstrates extensive destruction of the vertebral endplate. Note the preservation of the posterior elements, including facet joints, lamina, and spinous process. This is characteristic for pyogenic diskitis and less common in tuberculosis (Pott disease).
  • Organisms at the affected site can also produce a gas that is easily detected on CT scans.
  • The advantage of CT scans over radiographs is that associated paraspinal disease can also be detected, especially when combined with intravenous contrast or myelography.
  • Use of CT scanning can supplement magnetic resonance imaging (MRI), as it is better at distinguishing between bone and soft tissue than MRI.
  • CT can help monitor successful treatment, which is accompanied by increased bone density and sclerosis.
• MRI

  • The most sensitive and specific test for diskitis is MRI. T1-weighted images, as seen in the image below, show narrowing of the disk space and low signals consistent with edema in the marrow of adjacent vertebral bodies. T2-weighted images show increased signals in both the disk space and the surrounding vertebral bodies. Sagittal T1-weighted MRI of the lumbar spine in a 74-year-old man, revealing diskitis of the L4-L5 disk space. Note extensive destruction of the endplates of the adjacent vertebral bodies. No compression of the thecal sac is present, which is an important consideration when contemplating surgical intervention.
  • MRI is very useful in helping distinguish between infectious diskitis, neoplasia, and tuberculosis. Diffusion-weighted imaging is useful in distinguishing between degenerative and infectious endplate abnormalities. Compared with positron emission tomography, diffusion-weighted MRI costs less, has faster imaging times, and lacks ionizing radiation.^[9]
  • Disk space involvement directs attention to infection, as it only is involved late in tuberculosis and very rarely in neoplasia.
  • With the use of intravenous contrast, as seen in the image below, MRI, like CT, can detect paraspinal disease (eg, paraspinal phlegmon, epidural abscess). Contrast-enhanced sagittal T1-weighted MRI image in a 55-year-old woman shows thoracic diskitis with an associated epidural abscess and spinal cord compression. Because of the significant cord compression, this patient underwent surgical decompression.
  • A large amount of paraspinal soft-tissue swelling and a psoas abscess are often associated with spinal tuberculosis.
• Bone scans are not specific for infection over inflammation; therefore, they are ineffective in postoperative patients.

• Echocardiography can detect bacterial endocarditis, which is a common source of diskitis and embolic infection throughout the body.

• Needle biopsy

  • Needle or trocar placement into the infected area is a minimally invasive test used to obtain histologic confirmation of the disease and tissue samples for culture.
  • Yield and safety of the procedure are maximized by the use of CT scanning for guidance (see image below). Trajectory of a needle in a biopsy of the infected disk space guided by CT scan. Care is taken to avoid the thecal sac and nerve roots.
  • As in blood cultures, positive tissue cultures occur in only half of biopsies, especially if antibiotic therapy has already been initiated. In such cases, needle biopsy can be repeated or the patient can be referred for open surgical biopsy.
• Surgical biopsy

  • Open biopsy has been found in some studies to have the highest yield in terms of positive cultures and diagnosis confirmation.
  • Open biopsy is the most invasive test.
  • While some surgeons prefer to combine open biopsy with surgical debridement, no difference has been found between antibiotics and debridement when compared with antibiotics alone in cases of early diskitis.

The histologic findings of diskitis are similar to those of any bacterial pyogenic infection. Local destruction of the disk and endplates occurs with infiltration of neutrophils in the early stages. Later, a lymphocytic infiltrate predominates.