Differentiation between Tuberculous and Pyogenic Spondylodiscitis: The Role of the Anterior Meningovertebral Ligament in Patients with Anterior Epidural Abscess

DISCUSSION

Mycobacterium tuberculosis is a Gram-positive acid-fast bacillus, which can primarily infect bone or secondarily spread to the axial skeleton via the Batson venous plexus from remote sites of disease. Several patterns of vertebral body infection are described in M tuberculosis spondylodiscitis, including central, anterior, posterior, and paradiscal. Tuberculosis occurs more often in the thoracic spine, followed by the lumbar spine.¹⁰ Late in the disease, osteolysis results from bone necrosis, causing spinal instability and the typical gibbus deformity to develop.¹⁰ Epidural abscess may be present in TB spondylodiscitis, particularly in association with paradiscal infection and can be difficult to differentiate from pyogenic osteomyelitis in the absence of other stereotypical features.¹⁰

Differentiation between pyogenic and TB spondylodiscitis is often a diagnostic challenge because few imaging features are shown to reliably distinguish the 2 entities. Moreover, approximately 43%–70% of percutaneous biopsies are culture-negative in the setting of pyogenic and TB spondylodiscitis, even with high clinical and radiologic suspicion, with only marginally improved percentages reported for open biopsies.^11,12 In this study, we found that preservation of the anterior meningovertebral ligament had high sensitivity (83.3%) and specificity (100%) for the diagnosis of tuberculous infection in the setting of anterior epidural abscess, though these results must be interpreted with some degree of caution given the small sample size. The single case of culture-positive TB infection with a destroyed anterior meningovertebral ligament also demonstrated extensive vertebral body height loss (75% loss of vertebral body height). We postulate that the vertebral body damage itself was likely responsible for injury to the anterior meningovertebral ligament, rather than primary destruction due to the infection.

Several prior studies have examined imaging features that differentiate pyogenic from TB spondylodiscitis. Features favoring TB infection over typical pyogenic infection include intervertebral disc space sparing, posterior element involvement, extensive paraspinal soft-tissue involvement, multiple vertebral body involvement, a heterogeneous vertebral body enhancement pattern, and subligamentous spread.^1,5,13,14 The status of the anterior meningovertebral ligament represents a novel addition to the existing body of literature and may serve as collateral information in cases in which both diagnoses are being entertained.

The anterior meningovertebral ligament has been shown to have an impact on the spread of disease in the context of noninfectious pathology. In the setting of degenerative disease, the anterior meningovertebral ligament limits transverse migration of extruded disc material.⁸ Schellinger et al⁶ initially described progressive destruction of this midline septum in the setting of infection and neoplasm, and Kim et al⁹ showed that a unilobed or bilobed appearance of a ventral epidural collection had a 76%–82% sensitivity and 73%–80% specificity in identifying neoplastic compared with nonneoplastic anterior epidural space lesions, including hematoma and infection. Hong et al¹⁵ noted that TB associated anterior paraspinal abscess of the thoracic spine will not encase the intercostal arteries due to an inability to breach the anterior longitudinal ligament. In the current study, the observed destruction of the anterior midline meningovertebral ligament in the setting of pyogenic spondylodiscitis is consistent with that previously reported; the observed preservation of the ligament in TB suggests that mycobacterial infection behaves more similar to neoplasm than to trauma or bacterial infection.

The pathophysiologic basis of the observed effects on the anterior meningovertebral ligament is likely related to the unique features of the microorganism. An explanation commonly given for the observed sparing of the intervertebral disc during early stages of infection is that TB characteristically lacks proteolytic enzymes.^16⇓-18 Additionally, as an aerobic organism, M tuberculosis requires greater arterial supply to survive and is therefore more slow-growing than other infectious organisms,¹⁹ taking up to 11.2 months before clinical signs present, compared with 6.4 months in patients with pyogenic spondylodiscitis.¹ Lack of proteolytic enzymes is an important factor that may explain why M tuberculosis infection typically spares the anterior meningovertebral ligament. Although the meningovertebral ligament varies in thickness between different spinal segments, increasing in thickness at the level of the lumbar spine, we found no significant difference between the pyogenic and TB cases in terms of segmental involvement.

There are several limitations to this study. First, the sample size is limited, with only 15% of total cases positive for tuberculosis. However, this limitation reflects the relatively low prevalence of tuberculosis in the studied population. The relatively high number of pyogenic cases improves the specificity and negative predictive value of this finding, while reducing the sensitivity and positive predictive value. Of note, the search criteria used to develop the cohort included only patients who underwent acid-fast bacillus testing. Therefore, while we did not capture all epidural abscesses in our institution during the study period; the study specifically addresses those cases in which both pyogenic and TB infections were differential considerations and in which a radiologic sign might be of greatest utility in discriminating the 2 organisms. Finally, although all studies were performed at the same institution, not all studies were performed on the same scanner or with identical parameters, and the variability in image quality may potentially impact interpretation.