Vertebral Osteomyelitis
What is Vertebral Osteomyelitis?
Vertebral osteomyelitis is the most common infectious process affecting the spine outside of the meninges. However, the diagnosis is frequently missed or delayed until major bony destruction (or neural compromise as a consequence) has occurred.
The insidious nature of the disease and nonspecific presenting symptoms contribute to difficulty in diagnosis. The infective process may involve the vertebral body (vertebral osteomyelitis), the intervertebral disk (spondylodiscitis), or the spinal epidural space (spinal epidural abscess).
Pyogenic (pus-forming) vertebral osteomyelitis is reported to occur in about 1 per 100,000 population, but the true incidence is probably higher and should always be remembered in the differential diagnosis of low back pain.
How do you get Vertebral Osteomyelitis?
The cause of the infection is via two pathways. The first is from direct injury such as with trauma, or introduction of the infectious agent during surgery. The second and more common cause is via the bloodstream (hematogenous seeding) from another primary source (ie from an infected intravenous cannula site, or in drug abusers).
Although the disease is more common among patients with diabetes, intravenous drug abusers, those on chronic steroids or other immunosuppressive therapy, and patients with AIDS or other immunodeficiencies, it is also not uncommon among the otherwise healthy elderly and is occasionally seen in healthy young adults.
Staphylococcus aureus remains the most common causative organism, but an increasing proportion of cases are due to gram-negative and anaerobic organisms such as Proteus, Escherichia coli and Pseudomonas. Pseudomonas is frequently isolated in osteomyelitis occurring in intravenous drug abusers. Group B Streptococcus infections are frequently isolated in diabetic patients.
How is Vertebral Osteomyelitis diagnosed?
The most common presenting sign is back pain and malaise, often of 3 months duration or longer. Pain is often well localized to the affected level and the nature is not unlike most degenerative spinal conditions. A high index of suspicion is essential to make a timely diagnosis. Back pain that awakens a patient at night is a hallmark of infection or tumour. Pain associated with infection tends to be relentless and not related to activity level. Most patients have percussion tenderness over the involved segments. Fevers are noted in less than half of patients.
Laboratory studies may be helpful. The white blood cell (WBC) count is helpful when elevated, but it is often normal. The erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are elevated in more than 90% of patients. Although nonspecific, they are often useful in monitoring response to treatment by serial measurements. Blood cultures are positive in less than one fourth of adults with vertebral osteomyelitis.
Imaging is an essential tool in the diagnosis of vertebral osteomyelitis. Plain x-rays are often the first line of evaluation, but radiographic findings lag 2 to 3 weeks behind the onset of symptoms. Films may show evidence of bone loss (osteolysis) in the vertebra, loss of disc space height, end plate blurring, and subchondral reactive bone formation. Long-standing disease causes vertebral destruction, leading to collapse, kyphosis, and abscess formation.
A hallmark feature of osteomyelitis used to differentiate it from a neoplastic process is the crossing of the infectious process along the disc space to involve adjacent vertebrae.
Radionuclide studies can detect vertebral osteomyelitis long before plain radiographs. Technetium bone scans and gallium scans have a relative accuracy of 86% in diagnosing pyogenic vertebral osteomyelitis with a higher false negative rate for tuberculosis infections. Antibiotic administration prior to scanning decreases the accuracy of gallium scans.
Computed tomography (CT) scans provide excellent detail of local bone anatomy with relatively good definition of soft tissue structures. Multiple lytic areas surrounded by reactive bone adjacent to a narrowed disk space and paravertebral soft tissue thickening distinguish infection from malignant destruction. When combined with myelography, enhanced CT scans provide a useful view of the spinal canal and in addition, cerebrospinal fluid samples may be obtained for study. However the requirement for a CT-myelogram is becoming very much less common since the advent of MRI scanning. It may now be used in those patients who are too obese for the MRI scan, are claustrophobic or may have a metallic implant inside their body that precludes them going in the magnet of the scanner.
Magnetic resonance imaging (MRI) is the modality of choice for spinal infections. The major advantages of an MRI study are that it provides excellent visualization of the neural elements and determines whether the inflammatory process extends beyond the margins of disk and bone. MRI also provides excellent regional anatomic information. Scans performed with and without intravenous gadolinium are diagnostic in 90% to 95% of cases.
Biopsy and culture should be done to obtain tissue for bacteriologic processing, as well as to exclude the possibility of neoplasm. Computed tomography or fluoroscopic guided needle biopsies are sometimes necessary, with positive cultures in 68% to 86% of cases. When technically difficult, or if initial needle biopsy is negative, open biopsy may be required if index of suspicion is high for vertebral osteomyelitis. All samples should be sent for pathologic examination to exclude malignancy and stains for acid-fast bacilli, fungal organisms, and pyogens should be preformed.
How is Vertebral Osteomyelitis treated?
Medical treatment of vertebral osteomyelitis is successful in many patients. The choice of antibiotic is guided by the culture and sensitivities. Current recommendations are for 6 weeks of intravenous antibiotic, possibly supplemented by oral antibiotics thereafter. Shorter lengths of parenteral (intravenous) treatment have been associated with increasing rates of recurrence. Serial imaging and measurements of ESR levels are performed to ensure treatment effectiveness. In addition to antimicrobial therapy, successful treatment includes appropriate immobilization of the spine. Suitable bracing with or without bed rest is usually sufficient. Patients should be observed for change in neurologic status and radiographic progression of the disease. Failure to show adequate clinical response with improvement of temperature, symptoms, and ESR within 2 to 4 weeks of treatment means that medical therapy can be considered to have failed.
Indications for surgical treatment include the need for open biopsy, significant and/or progressive neurologic deficit, vertebral collapse with or without spinal deformity, significant intraosseous or paraspinal abscesses, or failure of clinical response to medical treatment.
The surgical goal is to remove all infected or necrotic tissue. The approach is dictated by anatomy and by the location of the infection. Most often the pathology is located within the vertebral body and intervertebral disks; therefore, the approach is generally anterior.
Anterior decompression should include removal of all accessible infected and/or necrotic tissue, resecting back to healthy bleeding bone. Paraspinal abscesses should be debrided aggressively. Supplemental segmental posterior instrumentation and fusion should be considered with anterior resections of two or more vertebral bodies. The use of anterior instrumentation in the face of active infection has been questioned, but the possibility of eliminating prolonged bed rest and avoiding a second surgical procedure suggest that this may not be an unreasonable approach.