Diskitis Workup

Elevations in the erythrocyte sedimentation rate (ESR) and the C-reactive protein (CRP) level are the most consistent laboratory abnormalities seen in cases of diskitis (discitis). The mean ESR for patients with diskitis is 85-95 mm/hr. The utility of the ESR can be extended by performing serial measurements during treatment. A 50% decline in the ESR can usually be expected with successful treatment, and the ESR often continues to decline after treatment. Frequently, the 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.

Procalcitonin (PCT) has been evaluated as a diagnostic tool and monitoring parameter for spondylodiskitis and for discrimination between bacterial infection and aseptic inflammation of the spine, but a study by Maus et al did not find it to be useful for these purposes. ^[7]  A study by Jeong et al found serum PCT to be less sensitive than serum CRP in patients with spinal infection. ^[8]

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.

Magnetic resonance imaging

The most sensitive and specific test for diskitis is magnetic resonance imaging (MRI). T1-weighted images (see 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.

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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 (PET), diffusion-weighted MRI costs less, has faster imaging times, and lacks ionizing radiation. ^[9]

Disk-space involvement directs attention toward infection; it only is involved late in tuberculosis and very rarely in neoplasia.

With the use of intravenous (IV) contrast (see the image below), MRI, like computed tomography (CT), can detect paraspinal disease (eg, paraspinal phlegmon or epidural abscess). A large amount of paraspinal soft-tissue swelling and a psoas abscess are often associated with spinal tuberculosis.

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.

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Computed tomography

CT has the ability to detect diskitis earlier than plain radiography does. Findings include hypodensity of the intervertebral disk and destruction of the adjacent endplate and bone (see 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).

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Certain bacterial organisms can also produce gas in the site that is easily detected on CT scans. However, this is not pathognomonic, because as it can be present in degenerative spine disease.

The advantage of CT over radiography is that it can also detect associated paraspinal disease, especially when combined with IV contrast or myelography. CT can serve as a supplement to MRI, in that it is better able to distinguish between bone and soft tissue than MRI. CT can help monitor successful treatment, which is accompanied by increased bone density and sclerosis.

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.

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. The yield and safety of the procedure are maximized by employing CT for guidance (see the image below). 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.

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.

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Surgical biopsy

Open biopsy is the most invasive test. In some studies, it has been found to have the highest yield in terms of positive cultures and diagnosis confirmation. ^[10] Whereas 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.