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Pathways of Spread
Anatomy-Based Imaging Issues
Spread of infection may occur along one of many different tracts, including direct extension , lymphatic spread , hematogenous spread , and along the cerebrospinal fluid pathways . Direct extension , as its name implies, occurs when bone or soft tissue comes into contact with a directly adjacent infection leading to a soft tissue abscess or osteomyelitis. For the spine, this route is typically seen adjacent to a decubitus ulcer where there is adjacent osteomyelitis. An infection of the disc space can extend into the adjacent paravertebral soft tissues and produce psoas muscle abscesses. Direct extension is also the mechanism for epidural abscess involvement cranial or caudal to the site of disc space infection. This route can also be seen for an intramedullary spinal cord abscess where the infection occurs through congenital dysraphism or a dermal sinus tract. Lymphatic spread is of limited importance in the spine relative to the much more commonly seen direct extension and hematogenous spread. Lymphatic spread may be seen in cases of retroperitoneal node enlargement from pelvic or abdominal primary neoplasms.
Hematogenous Spread
Hematogenous spread is the major pathway of infection spread to the axial skeleton; which route is more important (arterial or venous) is controversial. The arterial route is classically more important for spread of spinal infection. Vertebral bodies have areas that function physiologically in a similar manner to long bone metaphyses. The metaphyseal equivalent bone occurs near the anterior longitudinal ligament and has an end-arteriole network making it susceptible to bacterial seeding. These areas have distal nonanastomosing vessels that have slow flow, and occlusion of these vessels will lead to avascular necrosis. In the vertebral bodies, segmental arteries usually supply 2 adjacent vertebral bodies and the intervening disc, giving the typical disc space infection patterns. The venous route is classically through the Batson plexus, which is a longitudinal network of valveless veins running parallel to the spinal column. These veins lie outside of the thoracoabdominal cavity. These veins communicate with multiple aspects of the venous system, including the vena cava, portal venous system, azygos system, intercostal veins, and pulmonary and renal veins. Flow direction within the plexus is variable due to the variable intrathoracic and intraabdominal pressures. The pharyngovertebral plexus serves the same physiologic purpose. The contiguity of the cerebrospinal fluid spaces between the intracranial vault and the thecal sac allows for direct communication of the neoplasm and infection. Intracranial neoplasms may seed throughout the cervical, thoracic, or lumbar thecal sac. Likewise, even distal neoplasms involving the caudal thecal sac may propagate cephalad and extend into the intracranial cerebrospinal fluid space.
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