Key Words
Tuberculosis, Pott Disease, gibbus deformity, cold abscess, Brucellosis
Introduction
Tuberculosis (TB) is classically considered an infectious disease of developing countries; however, it has made a resurgence in other areas of the world with the increased use of immunosuppressive drugs, increased immigration, and the relatively recent appearance of HIV. The musculoskeletal system is the most common extrapulmonary site of TB infection, with spinal involvement seen in 50% of skeletal cases. Tuberculous spondylitis refers to vertebral body involvement with TB. When compared with pyogenic infections of the spine, tuberculous spondylitis has a distinct pattern of spinal involvement on imaging, as well as a unique pattern of progression that warrants its own description.
Within endemic countries, tuberculous spondylitis typically affects children and young adults during primary lung infection; however, in Western countries adults are more commonly affected after reactivation of latent disease. As a result, the clinical diagnosis of early spinal TB can be difficult in patients without a known history of pulmonary TB. Less than half of these patients will have simultaneous pulmonary infection. Furthermore, the classic constitutional symptoms of TB (e.g., fever, night sweats, weight loss) are present in less than 40% of spinal cases and may not become clinically apparent for months after initial spinal involvement. The usual duration of illness ranges from 4 to 11 months. In some instances the diagnosis may be delayed by more than a year.
Most patients will seek medical care only after developing severe pain or neurologic complications. Radiologic examinations are one of the first and most important steps in establishing the diagnosis of tuberculous spondylitis. The goal of early diagnosis is avoiding significant morbidity associated with spinal instability, which may occur with delayed treatment. After suspicious imaging abnormalities are identified in the spine, percutaneous image-guided bone or soft tissue biopsies can be performed. Acid-fast staining or polymerase chain reaction can be used to quickly identify the organism when specifically suspected. Notably, TB is notoriously difficult to isolate on cultures, averaging 4 to 6 weeks to obtain results, with a sensitivity of 80%. As a result, microbiology data are often negative. Histopathology of the bone biopsy may show nonspecific granulomatous changes suggesting TB. The cryptic clinical and microbiology features of this disease accentuate the need for an accurate imaging diagnosis.
Lastly, spinal TB is far from a new disease. DNA analysis has identified TB strains from bone biopsies of ancient Egyptian mummies, making it one of the oldest known communicable diseases. However, newer multidrug resistant TB strains have been discovered, necessitating prolonged and aggressive treatment regimens.
Imaging Evolution: Overview
Tuberculous spinal infections are most often encountered in the lower thoracic or upper lumbar spine, although any spinal segment may be involved. TB infection is spread hematogenously either arterially or by Batson venous plexus, with the primary exposure site occurring in the lungs or genitourinary system. Similar to pyogenic infections, the anterior vertebral end plate is typically the first site of involvement in the spine, followed by involvement of the central vertebral body. Involvement of the posterior elements is rare ( Fig. 27.1 ).
After the vertebral body is involved, the infection will spread along the undersurface of the longitudinal ligaments to involve multiple adjacent vertebrae and into the epidural/paraspinal soft tissues to form abscesses. Disc destruction is typically not present in tuberculous spondylitis, which is thought to be due TB’s lack of proteolytic enzymes, which are present in most pyogenic bacterial infections.
Vertebral body collapse is much more common with TB than with pyogenic infections, creating the classic “gibbus deformity” of short segment kyphosis ( Fig. 27.2 ). This classical late stage appearance of severe focal kyphosis was first described by English surgeon Sir Percivall Pott in 1779 (hence the use of the term Pott disease). Neurologic complaints are much more common at this stage of the disease because the kyphosis can result in compression of the anterior spinal cord.
Imaging Evolution: in Greater Depth
Although magnetic resonance imaging (MRI) is the “gold standard” for infectious spondylitis imaging, radiographs and computed tomography (CT) may be the only modalities available in endemic regions and early disease may go undiagnosed in these instances. Many of the findings on radiographs and CT are similar, with the main advantage of CT being earlier recognition of paraspinal abnormalities and superior evaluation for calcifications that may be seen within tuberculous abscesses. The earliest finding on radiographs and CT is rarefaction and irregularity of the vertebral end plates. Late-stage findings include vertebral body collapse, osseous fragmentation, and disc height loss.
With MRI, the earliest finding of anterior end plate edema is often indistinguishable from other infectious and inflammatory spondyloarthropathies. However, as tuberculous vertebral osteomyelitis progresses, focal vertebral bone marrow edema and contrast enhancement will often develop into peripherally enhancing intraosseous abscesses with sparing of the intervertebral disc, in comparison to the more diffuse homogeneous enhancement of pyogenic osteomyelitis with early involvement of the adjacent intervertebral disc. Bone destruction will continue to be the predominant finding in tuberculous spondylitis, with eventual vertebral body collapse. Often more than one vertebral level will be involved at presentation.
Although discitis may occur in advanced disease (reported in up to 50%–70% of cases), one major study found disc height to be preserved in more than half of tuberculous spondylitis cases. Although infection may spread through the disc to involve the contiguous vertebral body (as typically seen in pyogenic infections), the unique pattern of multilevel vertebral osteomyelitis seen with tuberculous spondylitis is more likely to occur by subligamentous spread, best seen on postcontrast imaging ( Fig. 27.3 ). MRI of the entire spine should be performed in these cases to exclude distant skip lesions.
