Spondylodiskitis (spondylodiscitis, infectious spondylitis) is an infection that involves 1 or more of the extradural components of the spine. Although it affects a small proportion (2-7%) of all patients with osteomyelitis, it is important because of its potential morbidity and mortality. Infectious spondylitis most commonly appears as spinal osteomyelitis and/or diskitis. Its complications include paraspinal and epidural abscess formation. [1, 2, 3, 4, 5, 6]
Pyogenic spinal infections most commonly are caused by Staphylococcus aureus (in 60% of patients) and Enterobacter species (in 30% of patients). Osteomyelitis caused by Salmonella is most often seen in patients with sickle cell disease. Pseudomonas aeruginosa, Serratia species, and Candida species most often affect patients with a history of intravenous drug abuse. Mycobacterium tuberculosis causes most nonpyogenic spinal infections; however, fungi (eg, Cryptococcus species, Aspergillus species, coccidioidomycosis) also may cause infections. [1, 7, 8, 9, 2] The characteristics of spondylodiskitis as they appear on radiographs, computed tomography (CT) scans, and magnetic resonance imaging (MRI) scans are demonstrated in the images below.
Although plain images, radiographs, CT scans, and nuclear medicine studies can help to establish the diagnosis of spondylodiskitis, MRI is considered the modality of choice for evaluating the presence and severity of spinal infection. MRI is especially effective for evaluating the neural structures of the spine (ie, spinal cord, nerve roots) and extradural soft tissue.
Along with appropriate history taking, physical examination, and positive blood cultures, findings on MRI or radionuclide studies may confirm the diagnosis. 
CT scanning is most useful for characterizing vertebral osteomyelitis in patients with subacute or chronic illness. CT scanning provides radiologic guidance for interventional procedures (ie, biopsy, drainage).
Conventional radiographs are insensitive to the acute changes seen in cases of spondylodiskitis, but they may be used on a limited basis in the follow-up of chronically ill patients. [11, 12, 13, 14, 15, 16, 17]
Limitations of techniques
MRI is relatively expensive. In addition, it is contraindicated in patients with certain implanted medical devices (eg, pacemakers), and it is not tolerated by all patients because of claustrophobia or morbid obesity. Good MRIs require the cooperation of the patient; the quality of MRI is degraded by motions of the patient. Use of a gadolinium-based intravenous contrast agent is strongly recommended.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis.
NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see FDA Information on Gadolinium-Based Contrast Agents or Medscape.
CT scanning is quick and inexpensive, although it exposes the patient to ionizing radiation. In addition, it is associated with a low risk of an allergic reaction to the intravenous contrast agent, though detection of bone destruction or a paraspinal mass does not require the use of contrast material.
Radionuclide studies may be time consuming, and spatial resolution may be low.
With plain radiographs, results are often normal or nonspecific in patients with acute spinal infection.
Invasive methods may be required to establish a diagnosis of spondylodiskitis. Spinal biopsy may be performed as an open surgical procedure or as an image-guided percutaneous procedure. The morbidity rate associated with percutaneous image-guided biopsy (eg, with CT scanning or fluoroscopy) is lower than the morbidity rate associated with open biopsy. Percutaneous biopsy may be highly sensitive and specific. Multiple samples should be obtained and submitted in appropriate containers for Gram staining, acid-fast bacillus smears, fungal staining, and culturing. [18, 4]
Conventional radiography is insensitive to the early changes of spondylodiskitis. In the acute setting, radiographs are normal. In the subacute period (1-3 wk), reduced disk-space height and/or endplate erosion may be evident. In the chronic period (>10 wk), vertebral body sclerosis or collapse may be observed on either side of a narrowed disk space. Paraspinal soft tissue opacity may develop. Gradual disk obliteration may occur with fusion (ie, osseous, fibrous). Partial restoration of disk height may be observed with successful treatment. (See the image below.)
Plain radiographic findings suggestive of subacute or chronic spondylodiskitis, such as disk-space loss, endplate erosion, and vertebral sclerosis, should be correlated with the patient's clinical history. In cases of spondylodiskitis, radiographic findings lag behind clinical response to treatment by approximately 1 month.
In spondylodiskitis, CT scans may appear normal early in the course of disease. Disk-space narrowing or decreased attenuation in the disk is often the earliest manifestation of disease. After the administration of iodinated contrast material, the abnormal disk space, vertebral marrow, or paravertebral soft tissues may enhance.
As the disease progresses, destruction of the vertebral body and fragmentation of vertebral endplates are observed. Destruction may be either diffuse or permeative. The presence of paraspinal soft tissue lesions and/or collections aids in the diagnosis of spondylodiskitis. In children, extrusion of the vertebral body may be observed. 
Overall, CT scanning is more sensitive (68%), more specific (97%), and more accurate (80%) than plain radiography in identifying areas of vertebral osteomyelitis in animal models.
Because of its ease and speed of use, lower cost, and availability, CT scanning is an excellent method for evaluating the bony changes of spondylodiskitis and for directing radiologic intervention (ie, aspiration or biopsy; see the image below) in spinal infections. In the diagnosis of active bacterial disk-space infections, the sensitivity and specificity of CT-guided percutaneous aspiration may be as high as 91% and 100%, respectively; however, percutaneous aspiration is less reliable in the diagnosis of nonbacterial infections.
Gas in the disk space (ie, vacuum phenomenon) may occasionally be observed on CT scans. Although this finding is most often associated with degenerative disk disease, it may also occur with infection.
CT scanning is less sensitive than MRI in depicting soft tissue structures, but its sensitivity may be improved with the use of intravenous contrast material.
Magnetic Resonance Imaging
MRI is the modality of choice for evaluating spinal infections. In particular, MRI is useful in detecting abnormalities during the acute stage of spinal infection. MRI is 96% sensitive, 92% specific, and 94% accurate in the evaluation of spondylodiskitis. During the acute stage, MRI may demonstrate increased signal intensity on T2-weighted images in the vertebral body or disk space caused by infarction, abscess, or edema (see the image below). On T1-weighted images, decreased signal intensity may be observed in the disk and adjacent endplates as a result of edema, and the margin between the disk and the endplate may be diminished. The multiplanar capability of MRI allows excellent anatomic localization of the extent of infection, as well as full evaluation of the spinal cord and nerve roots. MRI is also useful for the evaluation of postoperative spinal infections and for the follow-up evaluation of treatment of spondylodiskitis. Fat-suppression sequences with contrast-enhanced T1-weighted imaging should be routinely performed. [20, 21, 22, 23, 24, 5]
Paraspinal and epidural abscesses generally appear isointense to muscle on T1-weighted images and hyperintense on fat-saturated T2-weighted or short-tau inversion recovery (STIR) images.
Abscesses of the vertebral body may appear more conspicuous on diffusion-weighted images (DWIs) than on conventional T1- or T2-weighted images.
On contrast-enhanced MRIs, an epidural abscess may appear as an area of peripheral enhancement (as seen in the image below) surrounding an area of fluid signal intensity; it may also appear as an area of abnormal enhancement of disk, vertebral marrow, or paraspinal soft tissue.
Signal and enhancement changes may persist after clinical resolution of the infection; such findings gradually decrease over weeks to months. This illustrates the fact that contrast enhancement is not always indicative of active infection. These changes do not occur as rapidly as with gallium scanning.
False-negative findings may occur in patients with spondylodiskitis caused by organisms of low virulence (eg, diphtheroids, coagulase-negative staphylococci).
The usefulness of ultrasound is limited in spondylodiskitis. Occasionally, ultrasonography may be useful for localizing large paraspinal fluid collections for purposes of aspiration and/or drainage. Color Doppler sonography may show areas of hyperemia surrounding abscesses.
Radionuclide studies are useful for early detection (3-15 days) of spinal infection. However, establishing this diagnosis may be more difficult in the spine than in other areas of the skeleton because of the presence of active bone marrow. On 3-phase technetium-99m (99mTc) scintigraphy, osteomyelitis is characterized by nonspecific arterial hyperemia and the progressive focal skeletal uptake of tracer. Bone scan specificity may be increased by combining 99mTc with an indium-111–labeled WBC or gallium-67 (67Ga) scans. 67Ga functions as an acute-phase reactant, accumulating in areas of inflammation. 111In-labeled WBCs also accumulate in areas of infection and inflammation. Areas of infection should appear hotter on the 67Ga scan than on the accompanying 99mTc scan. Infective lesions may appear hot or cold on 111In WBC scans. 67Ga scans may normalize after satisfactory treatment, and they may be used to monitor treatment response. The use of single-photon emission CT enhances the 3-dimensional localization of infection. [25, 26, 27, 28, 29, 30]
The degrees of confidence of the aforementioned scans are as follows:
Table. Degrees of confidence (Open Table in a new window)
|Scan||Sensitivity (%)||Specificity (%)||Accuracy (%)|
|111 In WBC||17||100||31|
|Combined 111In WBC and 99mTc||90||91||Not available|
|Combined 67Ga and 99mTc||90||100||94|
Nuclear scintigraphy is sensitive for vertebral osteomyelitis; however, increased bone turnover from surgery, fracture, or degenerative changes decreases specificity.
False-negative results may occur when atherosclerosis or primary bone destruction compromises blood flow to a lesion.
Use of 111In WBCs may increase the specificity of bone scanning; however, the images may be falsely negative in patients with chronic osteomyelitis or falsely positive in patients with inflammatory or reactive conditions such as rheumatoid arthritis and fractures.