Spinal infections

1

How are spinal infections classified?

• Host immune response: Pyogenic versus granulomatous

• Anatomic location: Vertebral body, disc, epidural space, subdural space, facet joint, paraspinous soft tissue

• Infectious route: Hematogenous, local extension, direct inoculation

• Host age: Pediatric versus adult

• Duration: Acute versus chronic spinal infection

2

What are the three most frequent routes by which bacterial infection spreads to the spinal column?

The most common method for bacteria to spread to the spine is by the hematogenous route. Common sources of infection include infected catheters, urinary tract infection (UTI), dental caries, intravenous drug use, and skin infections.

The second most common route is direct inoculation via trauma, puncture, or following spine injections or surgery.

The third most common route is local extension from an adjacent soft tissue infection or paravertebral abscess.

The disc is nearly always involved in pyogenic vertebral infections. The nucleus pulposus is relatively avascular in adults, providing little or no immune response, and thus is rapidly destroyed by bacterial enzymes. Bacterial infection may begin in the vertebral body and extend into the disc space or initially develop in the disc and spread to adjacent vertebrae. In contrast, granulomatous infections typically do not involve the disc space.

3

Define risk factors for developing pyogenic vertebral osteomyelitis.

Risk factors for development of pyogenic vertebral osteomyelitis (PVO) include advanced age, diabetes, individuals on hemodialysis, intravenous drug use, immune compromise, individuals with HIV, recent or concurrent extraspinal infection (UTI, pneumonia, endocarditis, soft tissue infection), long-term steroid use, recent surgical procedures, and history of malignancy.

4

Describe the clinical presentation of pyogenic vertebral infection.

The most consistent symptom at presentation is spinal pain, which is noted in 90% of patients. Pain generally has an insidious onset. Pain is often exacerbated by activity, although it may be variably affected by recumbency or worse at night. Paravertebral muscle tenderness, spasm, and limited spinal motion are common physical findings. Fever is documented in approximately 50% of patients. Neurologic deficits are noted in up to one-third of patients at initial presentation. Weight loss is often present and occurs over a period of weeks to months. Spinal deformity is a late presenting finding. A delay in diagnosis is common, with 50% of patients reporting symptoms for more than 3 months before diagnosis. The lumbar region is the most common site of PVO (50%), followed by the thoracic region (35%), and cervical region (15%).

5

What is the most common pyogenic organism responsible for spinal infection?

Staphylococcus aureus is the most common organism and has been identified in over 50% of cases. However, infections due to a diverse group of gram-positive, gram-negative, and mixed pathogens may occur. Gram-negative organisms ( Escherichia coli, Pseudomonas spp., Proteus spp.) are associated with spinal infections following genitourinary infections or procedures. Intravenous drug abusers have an increased incidence of Pseudomonas infections. Anaerobic infections are common in people with diabetes and following penetrating trauma. Antibiotic-resistant organisms are noted with increasing frequency, including methicillin-resistant S. aureus (MRSA), vancomycin-resistant S. aureus (VRSA), and vancomycin-resistant Enterococcus (VRE).

6

When pyogenic vertebral infection is suspected, what diagnostic tests are indicated?

An algorithm for evaluation of a suspected spinal infection includes:

• Laboratory tests: Complete blood count (CBC), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), aerobic and anaerobic blood cultures

• Imaging studies: Magnetic resonance imaging (MRI) is the imaging study of choice. If MRI is unavailable or contraindicated, next best options include technetium bone scan with or without a gallium scan, positron emission tomography (PET), or computed tomography (CT)

• Biopsy: CT-guided or open biopsy

• Echocardiogram: Indicated for patients with cardiac risk factors, history of intravenous drug use, or failure to improve with initial treatment

7

Discuss the relative value of different laboratory tests in the diagnosis of pyogenic vertebral infection.

The white blood cell count on a CBC is not a sensitive test for diagnosis of PVO as it is elevated in fewer than half of patients. The ESR is elevated in more than 90% of patients with infection but is nonspecific and may be normal in the presence of low-virulence organisms. CRP is typically elevated in pyogenic infections and is considered more specific than ESR, and is helpful for monitoring treatment as it normalizes more quickly than ESR following adequate infection treatment. Blood cultures, although helpful if positive, yield the causative organism in only one-quarter to one-half of cases.

8

Describe the roles of the various imaging studies in the diagnosis of pyogenic vertebral infection.

Radiographs: Positive radiographic findings are not evident for 2–4 weeks after the onset of symptoms. The earliest detectable radiographic finding is disc space narrowing, followed by localized osteopenia, and finally destruction of the vertebral endplates. Radiographs remain valuable to rule out other noninfectious etiologies responsible for back pain symptoms and to follow-up nonoperative and operative treatment. Upright radiographs are used to evaluate spinal alignment and stability.

MRI: Considered the imaging modality of choice for diagnosis of vertebral infection. It provides detailed assessment of the vertebral body, disc space, spinal canal, and surrounding soft tissue not provided with any other single test. The typical findings associated with pyogenic vertebral infection are decreased signal in the vertebral body and adjacent discs on T1-weighted sequences and increased signal intensity noted in these structures on T2-weighted images. Paravertebral abscess, if present, also demonstrates increased uptake on T2-weighted images. Gadolinium contrast is a useful adjunct in diagnosing infection because the disc and involved regions of adjacent vertebral bodies enhance in the presence of contrast ( Fig. 66.1 A,B).

Radionuclide studies: Technetium-99m bone scanning is informative in the early diagnosis of PVO as it demonstrates positive findings before the development of radiographically detectable changes. It has utility as a screening study if MRI is unavailable or contraindicated and may be performed in conjunction with a gallium scan. PET with 18F-fluoro-2-deoxy-2D-glucose (FDG) is another alternative diagnostic study when MRI is not possible. However, radionuclide studies do not provide sufficiently detailed information for surgical planning.

CT: Plays a role in defining the extent of bony destruction and localization of lesions for biopsies.

9

What is the role of biopsy in the diagnosis of pyogenic vertebral infections?

In the absence of positive blood cultures, biopsy of the site of presumed vertebral osteomyelitis or discitis is essential to provide a definitive diagnosis, identify the causative organism, and guide treatment. The biopsy ideally should be performed before initiation of antibiotics. If antibiotics have been given, they should be discontinued for 3 days before the biopsy. Exceptions to this practice include patients with sepsis or impending spinal cord compression. CT-guided needle biopsy needle biopsy is safe and effective and yields the etiologic organism in 70% of cases. If a closed biopsy is negative after two attempts, an open biopsy is an option.

10

What tests should be done on tissue samples from an open biopsy?

Tissue samples should be sent for Gram stain, aerobic and anaerobic cultures, and acid-fast stain. Mycobacterial, fungal, and brucellar cultures are recommended in patients with suspected spinal infection based on epidemiologic factors, host risk factors, or characteristic radiographic findings. Bacterial cultures should be observed for at least 10 days to detect low-virulence organisms. Tuberculosis (TB) cultures may take weeks to grow. Histology studies should also be performed to detect neoplastic processes and to differentiate acute versus chronic infection.

11

What are the goals of treatment of pyogenic vertebral infection?

The goals in treating PVO include early definitive diagnosis, eradication of infection, relief of axial pain, prevention or reversal of neurologic deficits, preservation of spinal stability, and correction of spinal deformity (if present).

12

Describe the nonoperative treatment of pyogenic vertebral infection.

Nonoperative treatment includes antibiotic administration, treatment of underlying disease processes, nutritional support, and spinal immobilization with an orthosis. Antibiotic selection is based on identification and sensitivity testing. If cultures are negative, treatment with broad spectrum antibiotics is recommended. Consultation with an infectious disease specialist is recommended. Intravenous antibiotics generally should be continued for 6 weeks, provided that satisfactory clinical results and reduction in ESR and CRP occur. In the setting of a broadly sensitive organism and rapid clinical resolution, intravenous antibiotics may be replaced with a highly bioavailable oral antimicrobial therapy regimen after several weeks.