Discitis And Osteomyelitis

MRI plays a pivotal role in the evaluation of infectious spondylitis particularly because the diagnosis is often a difficult one to make clinically. Patients with spinal infections present with nonspecific symptoms such as malaise, focal tenderness, radiculopathy, and back pain (38-40). These symptoms may be attributable to other etiologies such as degenerative disorders, spinal stenosis, and neoplasms. In fact, back pain (the most common symptom in patients with infectious spondylitis [40,41]) is also the second leading cause of physician visits affecting 5% of the population annually (42,43). Physicians therefore often rely on imaging modalities and on laboratory tests such as elevated erythrocyte sedimentation rate (ESR), white blood cell (WBC) count, or C-reactive protein (40). In a group of patients with pyogenic vertebral osteomyelitis evaluated by Caragee, 30% of immunocom-promised and 44% of immunocompetant patients had abnormal WBC counts while 89% of immunocompromised and 100% of immunocompetant patients had elevated ESR (44). It is important to note that WBC count and ESR can be normal in patients with chronic or partially treated infections (40). C-reactive protein is also accurate in identifying underlying infection (40).

MRI has been shown to be a highly accurate imaging modality in the diagnosis of infectious spondylitis with a reported sensitivity of 96%, specificity of 92%, and accuracy of 94% (45). This accuracy, along with the superior spatial resolution of MRI in assessing the extent of soft tissue and bone involvement and its ability in detecting early changes of infection have made it the imaging modality of choice in the evaluation of infectious spondylitis.

Spinal infections most commonly result from hematog-enous spread although they may also occur by traumatic or iatrogenic inoculation (during an invasive procedure) or uncommonly by extension from an adjacent infection (39,40,46). The hematogenous route typically originates from a genitourinary, gastrointestinal, skin, or respiratory source. Diabetics, intravenous drug abusers, and patients with chronic diseases such as sickle-cell anemia and acquired immunodeficiency syndrome (AIDS) particularly are prone to developing infections (38,40). Bacteria are the most common causes of spinal infections, with Staphylococcus aureus identified in 55-80% of cases (40,45,46). Parasitic and fungal infections are uncommon while viral infections are seen in greater frequency in AIDS patients (40).

The typical appearance and distribution of osteomyelitis and discitis is directly related to the vertebral arterial supply. Although retrograde venous infection of the spine through the inferior vena cava and Batson's plexus has been documented, arterial spread is the typical route of hematogenous infection (39,40,46-48). The vertebral bodies have a complex arterial supply with lumbar arteries and intercostal arteries terminating in multiple meta-physeal end arterioles adjacent to the subchondral endplates (39,48). Septic emboli lodge in end-arterioles producing infarctions susceptible to infection. This

Vertebral Osteomyelitis Phlegmon

Fig. 9. Discitis and osteomyelitis. Sagittal postcontrast Tl-weighted fat suppressed image shows a peripherally enhancing L4-5 disc abscess (arrowhead) with adjacent enhancement of the infected L4 and L5 vertebral bodies and erosion of the adjacent endplates. Enhancing ventral epidural tissue (arrow) represents epidural phlegmon.

Fig. 9. Discitis and osteomyelitis. Sagittal postcontrast Tl-weighted fat suppressed image shows a peripherally enhancing L4-5 disc abscess (arrowhead) with adjacent enhancement of the infected L4 and L5 vertebral bodies and erosion of the adjacent endplates. Enhancing ventral epidural tissue (arrow) represents epidural phlegmon.

accounts for adult spinal infections often originating in the endplates. The infection then spreads to the adjacent disc or vertebral bodies through arteries that traverse the discs (39).

Children have a rich network of collateral vessels which decreases the risk of embolic induced infarctions. These arteriole anastamoses atrophy by age 15 (47,49). In addition, unlike adults, children have a direct arterial supply to the intervertebral disc which is the most common site of pediatric spinal infection (39).

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