Approach Considerations
Lab studies used in the diagnosis of Pott disease include the following:
- Tuberculin skin test (PPD) - Results are positive in 84-95% of patients with Pott disease who are not infected with HIV
- Erythrocyte sedimentation rate (ESR) - May be markedly elevated (>100 mm/h)
- Microbiologic studies - Used to confirm the diagnosis
With regard to the above-mentioned microbiologic studies, bone tissue or abscess samples are obtained to stain for acid-fast bacilli (AFB), and organisms are isolated for culture and susceptibility. Procedures guided by computed tomography (CT) scanning can be used to guide percutaneous sampling of affected bone or soft-tissue structures. These study findings are positive in only about 50% of the cases.
Biopsy
Percutaneous, CT scan ̶ guided needle biopsy of bone lesions is a safe procedure that also allows therapeutic drainage of large paraspinal abscesses. Obtain a tissue sample for microbiologic and pathologic studies to confirm diagnosis and to isolate organisms for culture and susceptibility.
Histologic findings
Because microbiologic studies may be nondiagnostic of Pott disease, anatomic pathology can be significant. Gross pathologic findings include exudative granulation tissue with interspersed abscesses. Coalescence of abscesses results in areas of caseating necrosis.
Drainage
Some cases of Pott disease are diagnosed following an open drainage procedure (eg, following presentation with acute neurologic deterioration).
Scintigraphy
Radionuclide scanning findings are not specific for Pott disease. Gallium and technetium bone scans yield high false-negative rates (70% and up to 35%, respectively).[20]
Radiography
Radiographic changes associated with Pott disease present relatively late. The following are radiographic changes characteristic of spinal tuberculosis on plain radiography[15] :
- Lytic destruction of anterior portion of vertebral body
- Increased anterior wedging
- Collapse of vertebral body
- Reactive sclerosis on a progressive lytic process
- Enlarged psoas shadow with or without calcification
Additional radiographic findings may include the following:
- Vertebral end plates are osteoporotic.
- Intervertebral disks may be shrunk or destroyed.
- Vertebral bodies show variable degrees of destruction.
- Fusiform paravertebral shadows suggest abscess formation.
- Bone lesions may occur at more than 1 level.
CT Scanning
CT scanning provides much better bony detail of irregular lytic lesions, sclerosis, disk collapse, and disruption of bone circumference.[16]
Low-contrast resolution provides a better assessment of soft tissue, particularly in epidural and paraspinal areas.
CT scanning reveals early lesions and is more effective for defining the shape and calcification of soft-tissue abscesses. In contrast to pyogenic disease, calcification is common in tuberculous lesions.
MRI
Magnetic resonance imaging (MRI) is the criterion standard for evaluating disk-space infection and osteomyelitis of the spine and is most effective for demonstrating the extension of disease into soft tissues and the spread of tuberculous debris under the anterior and posterior longitudinal ligaments. MRI is also the most effective imaging study for demonstrating neural compression.[17, 18]
Contrast-enhanced MRI findings are useful in differentiating tuberculous spondylitis from pyogenic spondylitis. MRI findings in Pott disease include thin and smooth enhancement of the abscess wall and a well-defined paraspinal abnormal signal. Thick and irregular enhancement of the abscess wall and an ill-defined paraspinal abnormal signal suggest pyogenic spondylitis.[19] The images below are studies of a man aged 31 years with spinal tuberculosis.
MRI of a 31-year-old man with tuberculosis of the spine. Images show the thoracic spine before and after an infusion of intravenous gadolinium contrast. The abscess and subsequent destruction of the T11-T12 disc interspace is marked with arrowheads. Vertebral body alignment is normal. Courtesy of Mark C. Diamond, MD, and J. Antonio Bouffard, MD, Detroit, Mich.
MRI of the T11 in a 31-year-old man with tuberculosis of the spine. Extensive bone destruction consistent with tuberculous osteomyelitis is evident. The spinal cord has normal caliber and signal. No evidence of spinal cord compression or significant spinal stenosis is distinguishable. Courtesy of Mark C. Diamond, MD, and J. Antonio Bouffard, MD, Detroit, Mich. Taylor GM, Murphy E, Hopkins R, et al. First report of Mycobacterium bovis DNA in human remains from the Iron Age. Microbiology. Apr 2007;153:1243-9. [Medline].
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