Vertebral Osteomyelitis

Etiology

Vertebral osteomyelitis is typically caused by a single pathogen, with S aureus being the most common etiologic agent, especially in cases of hematogenous dissemination.[2][3][4] Thus, in a patient with S aureus bacteremia within the past 3 months and compatible spine MRI findings, vertebral osteomyelitis should be assumed without the need for disc space aspiration or other tissue sampling.[2] 

Conversely, coagulase-negative staphylococci and P acnes are the most common pathogens in exogenous osteomyelitis following spinal surgery, particularly in patients with spinal fixation devices.[5] Vertebral osteomyelitis caused by alternative pathogens, such as Mycobacterium tuberculosis or Brucella, may be seen in endemic regions and immunocompromised patients.[2] Mycobacterium avium complex is a common pathogen in patients with HIV.[2] Fungal vertebral osteomyelitis, although rare, can occur in patients from endemic regions (eg, histoplasmosis and blastomycosis), immunocompromised individuals (eg, Aspergillus fumigatus), IV drug users, and those with indwelling intravenous catheters (eg, Candida and Aspergillus).[6]

Risk Factors for Vertebral Osteomyelitis

The risk factors for vertebral osteomyelitis include:

• Advanced age
• Immunosuppression
• Diabetes
• Long-term corticosteroid use
• Malignancy
• Malnutrition
• IV drug use

Epidemiology

The incidence of vertebral osteomyelitis varies by region and population. In the United States, the estimated incidence of vertebral osteomyelitis is 4.8 per 100,000, with a noted increase over the past few decades.[1] Globally, the incidence of vertebral osteomyelitis varies between 1 and 7 per 100,000, depending on the country.[1] The incidence of the disease increases with age, and a slightly higher incidence is observed in males compared to females. Similarly, vertebral osteomyelitis mortality varies depending on the country, the causative organism, and whether the patient is in a high-risk or low-risk group. The 1-year mortality can reach up to 11%.[1] In the pre-antibiotic era, mortality estimates were over 25%.[2]

Pathophysiology

Hematogenous dissemination is the primary route of pathogen spread in vertebral osteomyelitis, in contrast to extremity infections, where direct inoculation through adjacent skin ulcers is a more common form of pathogen dissemination. Additionally, vertebral osteomyelitis can result from iatrogenic infections following spinal surgery or procedures such as facet or epidural injections for pain management.[7] Once infectious pathogens gain access to the vertebral column, the infection can spread to adjacent paraspinal tissues, nerve roots, the epidural space, and even the intradural space, leading to inflammation, abscess formation, and both soft tissue and bone destruction.[5]

Most infections affect the vertebral body, with only 5% involving the posterior structures of the spine. This disparity is primarily due to the rich blood supply to the vertebral bodies. In addition to arterial spread, retrograde seeding through the venous system via the Batson plexus can also transmit the infection to the spinal cord. In rare instances, the infection may spread continuously from the retropharyngeal space to the vertebral body.

History and Physical

The initial clinical assessment of a patient with vertebral osteomyelitis often reveals nonspecific and nonfocal findings, especially in the early stages of the disease. Back pain is the most common presenting symptom, while fever occurs in only 35% to 60% of cases.[5] Pain may initially be absent or diffuse, but as the disease progresses, it typically localizes to the affected vertebral level.

The most common site of infection is the lumbar spine (58%), followed by the thoracic spine (30%) and the cervical spine (11%).[5] Interestingly, spine tenderness with palpation has a reported sensitivity of as low as 20% in affected patients.[8] Neurological impairment, including sensory loss, weakness, or radiculopathy, is present in 33% of cases.[5]

Evaluation

The differential diagnosis for back pain in the outpatient setting is extensive and includes conditions such as spondylosis (degenerative disc disease and facet arthrosis), vertebral body or pars interarticularis fractures, and muscle spasms. An elevated temperature does not significantly narrow the diagnostic differential, as fever can occur in viral syndromes, retroperitoneal infections, vertebral osteomyelitis, and many other conditions. Nonspecific symptoms, a nonfocal physical examination, and a wide differential diagnosis all contribute to an increased likelihood of delayed diagnosis.

Laboratory Studies

A complete blood cell count (CBC) is commonly ordered to evaluate febrile back pain, but it has low sensitivity for detecting vertebral osteomyelitis. Notably, the neutrophil count is often not elevated,[5] with up to 40% of patients exhibiting a white blood cell count within the reference range.[2] In contrast, elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels are much more sensitive, with reported sensitivity rates ranging from 94% to 100%.[2] These inflammatory markers are particularly valuable for diagnosing vertebral osteomyelitis and are frequently used to monitor treatment progress and success. Blood cultures (aerobic and anaerobic) are also indicated in the routine evaluation of febrile back pain. For clinically stable patients—those who are not septic and exhibit no signs of neurological impairment—antibiotic therapy should be deferred until blood culture results are available to guide targeted treatment.[2]

In endemic regions, diagnostic testing should include Brucella serology and M tuberculosis assessments, such as a purified protein derivative test or an interferon-gamma release assay (IGRA).[2] Fungal assays are beneficial in immunocompromised patients or patients in endemic geographic areas, particularly if the initial blood cultures are negative.[2]

Imaging Studies

Imaging studies are a critical component in evaluating vertebral osteomyelitis, complementing laboratory findings. Although radiographs are typically the first imaging modality performed due to their accessibility and quick acquisition time, they are often insensitive to detect vertebral osteomyelitis. However, radiographs can be valuable for identifying alternative causes of back pain.[5] 

Magnetic resonance imaging: MRI is the preferred modality for evaluating spinal infections, with an accuracy exceeding 90%.[9] Multiple abnormal MRI findings are common in most patients with vertebral osteomyelitis. Typically, infections in the intervertebral disc space rapidly spread to the 2 adjacent vertebral body endplates.[10] These endplates demonstrate decreased signal intensity on T1-weighted images, increased signal intensity on T2-weighted images, and corresponding enhancement on post-contrast imaging.[10] 

The infected disc space typically shows decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images, with contrast enhancement that may appear patchy, diffuse, or linear. In chronic infections, the disc space height and vertebral endplates may collapse. In most cases, soft tissue edema or an abscess is present in the anterior paraspinal region or anterior epidural space, which may extend cranially or caudally across multiple spinal levels.[11]

Computed tomography: CT evaluation has relatively limited utility in assessing vertebral osteomyelitis, as findings may appear normal early in the disease and can underestimate the extent of soft tissue and bone involvement in advanced cases. However, CT is more sensitive than radiography and can detect subtle vertebral endplate erosion or the formation of soft tissue abscesses. CT findings of intervertebral disc space narrowing are nonspecific, as this feature is observed in both degenerative disc disease and osteomyelitis. In such cases, MRI can enhance diagnostic specificity by providing more detailed imaging of soft tissues and bone structures.[2]

While MRI is the preferred imaging modality for evaluating vertebral osteomyelitis, some patients may have contraindications to MRI and require alternative imaging options. In such cases, nuclear medicine offers an effective alternative. Gallium-67 single-photon emission computed tomography (SPECT) has demonstrated comparable diagnostic sensitivity to MRI and is an excellent substitute when MRI is unavailable or contraindicated.[9] Bone scintigraphy with technetium-99m (^99mTc) and indium-111 (¹¹¹In) are nuclear medicine studies that offer lower diagnostic sensitivity compared to other imaging methods. Fluorodeoxyglucose F 18 (¹⁸F-FDG) positron emission tomography (PET) is also an acceptable alternative for evaluating vertebral osteomyelitis.[12][13] ¹⁸F-FDG accumulates at sites of infection and inflammation, including areas affected by autoimmune and granulomatous diseases.[14] However, the lack of specificity is a limitation of PET, as radionucleotide uptake can occur in various inflammatory and neoplastic conditions. Therefore, clinicians should interpret PET results alongside clinical and imaging findings to ensure accurate diagnosis.

Image-Guided Biopsy

If imaging findings suggest vertebral osteomyelitis but blood cultures are negative, percutaneous needle aspiration or biopsy is recommended to identify the causative microorganism before initiating antibiotic therapy.[2] CT is the most effective radiological imaging modality for guiding a percutaneous image-guided biopsy.[15] MRI or nuclear medicine studies can help in selecting the appropriate site for the CT-guided aspiration or biopsy. CT guidance offers superior accuracy in sampling, as it allows continuous assessment of the needle’s position relative to the anatomical target.[2]

Treatment / Management

Utilizing recommended management approaches is crucial to optimizing clinical outcomes. Antibiotic therapy is the cornerstone of vertebral osteomyelitis treatment. Various regimens combining parenteral and oral antibiotics tailored to the specific pathogen identified. Most experts recommend 6 weeks of antimicrobial therapy,[16] as longer treatment durations have not shown increased efficacy for typical cases.[2] (A1)

Surgical management is rarely indicated, except in cases involving neurological compromise, significant vertebral destruction with spinal instability, large epidural abscess formation, intractable back pain, or failure of medical therapy.[2] Traditionally, surgery was associated with a higher risk of persistent infection or recurrence. However, recent studies suggest that surgical intervention does not increase the risk of adverse clinical outcomes in patients with known or suspected vertebral osteomyelitis.[17](A1)

Spine bracing is commonly used to provide additional stability during treatment. After treatment, serial CT and MRI scans are typically required to monitor healing. Rehabilitation to restore muscle strength is often recommended as part of the recovery process.

Differential Diagnosis

When evaluating vertebral osteomyelitis, the following differential diagnoses should also be considered:

• Chronic nonbacterial osteomyelitis
• Ewing sarcoma
• Gout
• Gaucher disease
• Malignancy
• Pseudogout
• Septic arthritis
• Sickle cell anemia
• Vitamin C deficiency
• Vertebral compression fracture
• Vaso-occlusive pain episode

Prognosis

Recovery from vertebral osteomyelitis is often prolonged, even with appropriate treatment, and some patients may require multiple surgical interventions. Surgical procedures carry significant risks, including the potential for paralysis. Approximately 15% of patients with vertebral osteomyelitis experience permanent neurological deficits. Many patients remain disabled, with a diminished quality of life.

Complications

Complications of vertebral osteomyelitis may include:

• Recurrence of infection
• Fracture
• Paralysis
• Permanent neurological deficits
• Chronic pain
• Disability