15 Postoperative Spondylodiskitis
Introduction
Postoperative spondylodiskitis (POSD) is an uncommon, yet significant postoperative complication. Spondylodiskitis is a bacterial infection affecting the intervertebral disk space, surrounding bony structures, or soft tissues. 1 Patients with POSD commonly present postoperatively with intense back pain and fever. Most cases occur following surgical procedures in the lumbar region, but also to a lesser extent in the cervical and thoracic regions. 1 – 4 A recent estimate of the incidence of POSD is 0.21 to 3.6% across all vertebral surgical procedures. 5 Although surgical site infections can occur after spine surgery, spondylodiskitis is a rare complication. Importantly, most spine surgical site infections do not lead to the development of spondylodiskitis. Of course, each spine procedure entails specific and often unique risks. 6 This chapter discusses postoperative spine infections, and specifically postoperative spondylodiskitis. The significance of postoperative infections, the risk factors associated with them, and the current diagnostic, treatment, and prevention strategies are discussed.
General Risk Factors for Spine Infection
There are several well-defined risk factors for surgical site infection (SSI) following spine sur gery: are older age, obesity (as measured by the body mass index, BMI), diabetes, cardiovascular disease, and smoking. 6 , 7 Malnourishment, immunosuppression, hypertension, low serum albumin, and low white blood cell count may also contribute to an increased risk of infection. 6 , 8 The risk factors relating to the surgery include longer operative times and increased levels of surgical invasiveness. 6 , 9 Long periods of immobilization postoperatively may increase the risk of infection. 8 Specific risk factors for spine SSI include the surgical approach (posterior approaches have a greater risk for infection than anterior approaches), and the length of the incision (longer incisions are associated with a greater incidence of infection). 8 Spine surgery in the setting of trauma is associated with a higher risk of SSI. 8
However, despite these well-defined risks for SSI, there are no patient or procedural risk factors identified for POSD. We have found that POSD is often a late occurrence following SSI in cases that have been treated in a subacute or delayed manner, although this may not be true in all cases. POSD may also occur in the early postoperative period in patients who are immunosuppressed.
Pathology and Bacteriology
Postoperative spondylodiskitis most often develops in the disk space following intradiskal procedures. Although rare, hematogenous spread can occur from other regions of the body. The specific bacteriology of POSD is largely unknown. We suspect that the most common bacterial causes of SSI are also the common causes of POSD. In one study, biopsy-confirmed infection in the disk space was observed to be caused by a single organism in 51% of patients, by two organisms in 16%, and by more than two organisms in 8%. 10
Diagnosis
Overview and Complicating Factors in Diagnosis
Postoperative spondylodiskitis is difficult to diagnose due to the indolent nature in which the infection presents and the limitations in diagnostic accuracy of existing investigative strategies. 11 In most cases, postoperative wound infections present with prolonged wound drainage, axial pain, and a variety of red flag symptoms including fevers or chills. Given the common nature of these symptoms in the acute postoperative period, the diagnosis of POSD can be confounded by other conditions. 12 Furthermore, low back pain in postoperative spine patients is generally an expected symptom. 13 Patients with POSD may exhibit atypical, nonmechanical back or neck pain that is worse when the patient is in the supine position and worse at night. This atypical pain is suggestive of an infectious process, as opposed to normal postoperative muscle healing. However, the axial pain from POSD may be initially overlooked because it is confounded by the patient’s baseline pain or new axial pain from the procedure. 13
Imaging
Plain Radiography
The first imaging modality of choice is plain radiographs. 12 , 14 Unfortunately, most studies agree that the effectiveness of plain radiographs is limited in the early diagnosis of spondylodiskitis due to the lack of end-plate destruction. Disk space narrowing is a late radiographic sign of diskitis. As the infection progresses, destruction of the bony end plates or fractures become more apparent. 12 , 14
Magnetic Resonance Imaging
The most frequently used imaging modality for diagnosing POSD is magnetic resonance imaging (MRI). 12 – 14 MRI illustrates the bone, nerve, disk, ligament, and muscle structures in anatomic detail. 12 MRI is very sensitive for early spondylodiskitis due to the presence of fluid and contrast uptake in the vertebral bodies and end plates. 14 Specific MRI findings in cases of spondylodiskitis include a high disk space and vertebral body T2 signal on short tau inversion recovery (STIR) images, as well as a low disk space and vertebral body T1 signal, and contrast uptake ( Fig. 15.1 ). MRI may show the presence of irregular edema and inflammation of the epidural space or an epidural abscess. 13 Nonetheless, MRI is not without its limitations. Bone edema on MRI can be caused by other, noninfectious sources, such as a fracture, spinal osteotomy, or spinal instrumentation. MRI is also susceptible to metallic artifact, so the resolution around instrumentation is often suboptimal. 15 Finally, MRI is highly sensitive but not specific. Thus, normal postoperative epidural fluid collection observed in MRI can be confused with POSD. Pathology identified on MRI of the spinal column may be indicative of other preexisting pathology (i.e., aging, degeneration from overuse, past traumatic injury) and not specifically spondylodiskitis. 13 , 14 Unfortunately, a recent study found that routine use of MRI may not be effective in monitoring the natural history of effectively treated osteodiskitis due to the lack of pertinent clinical correlation. 14
Computed Tomography
Computed tomography (CT) has demonstrated less specificity and sensitivity compared with MRI in the diagnosis of POSD. 12 Despite the increased preference for MRI, CT scans still hold some value in the diagnosis and treatment of spondylodiskitis. Bone involvement is a hallmark feature of osteodiskitis. CT scans (especially three-dimensional CT) can give a more complete picture of the degree of bony destruction. 12 , 14
Nuclear Scintigraphy
Nuclear scintigraphy imaging (NSI) involves the injection of radioactive isotopes. Nuclear medicine studies are generally not sensitive in the axial skeleton. 11 However, new advances in NSI may lead to decreased infection detection times and higher sensitivity. 14 One radioactive agent that has seen success is gallium 67, which is more preferentially taken up by areas of infection. 14
Emerging Imaging Modalities: 18F-FDG-PET
18F-fluorodeoxyglucose (FDG)–positron emission tomography (PET) is a variant of nuclear scintigraphy in which gamma rays emitted by a positron tracer are captured. It may offer significant potential as an imaging modality for detecting spondylodiskitis, perhaps with even greater sensitivity and specificity than MRI. 12 , 14 Results of 18F-FDG-PET testing are often available within 2 hours of tracer injection and the study is not affected by the presence of metallic artifact (i.e., from medical implants). 14 18F-FDG normally demonstrates minimal uptake in bone marrow, and therefore changes consistent with infection are likely to increase tracer uptake and highlight a specific region on the scan. 12 18F-FDG-PET is also capable of assisting in the differentiation between degenerative disease and infection when used in combination with MRI. 14 One currently observed drawback is that it may fail to differentiate between malignant tumors and infection due to similar uptake of the tracer. 12 , 14
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