Imaging of Spinal Infections in Children

Abstract

Infections of the spine in children consist of osteomyelitis and the so-called spondyloarthritis or diskitis. The hallmark of radiographic diagnosis is disk space narrowing with destruction of the two adjacent vertebral body surfaces. 3 , 5 , 26 , 29 Localization of infection to the osseous and articular structures of the vertebral column is not as common in children as adults. The most common route of contamination of the spine is by a hematogenous pathway, either by an arterial route or Batson's paravertebral venous system. The arterial pathway can be implicated by the localization of early infection in the subchondral region of the vertebral body, where numerous arterial loops are present. Infection may also be distributed along the course of the ascending and descending nutrient branches of the posterior spinal artery. The direction and extent of flow through the valveless venous plexus are significantly influenced by changes in intra-abdominal pressure, which may account for the contamination of the spine that may be observed in a child with urinary tract and other pelvic infections. 17 Infection of spine may also occur secondary to spread from a contiguous contaminated source or from direct implantation. Subligamentous spread of infection with subsequent bony invasion is more characteristic of tuberculosis than pyogenic infection. Direct implantation of organisms can occur during punctures of the spinal canal. Postoperative infection may develop following laminectomy or other instrumentation such as scoliosis repair. The cervical spine may be infected by direct extension from prevertebral abscesses, especially those associated with sharp foreign bodies in the pharynx. Vertebral and disk infections account for approximately 2% to 4% of all cases of osteomyelitis with equal incidence in boys and girls. The symptoms include fever; malaise; weight loss; back pain, which may be intermittent or constant; or hip contracture, which occurs secondary to psoas muscle irritation. The erythrocyte sedimentation rate is elevated, but serum leukocyte count may be normal and organisms may not be recovered from blood cultures. The most common early focus of hematogenous osteomyelitis of the spine is the anterior subchondral region of the vertebral body adjacent to the intervertebral disk. The diagnosis of infection is suggested by rapid loss of disk height and adjacent lysis of bone (Figs. 1 and 2) . Involvement of two contiguous vertebral bodies almost always represents transdiskal infection rather than multicentric foci. With vertebral osteomyelitis, bone scintigraphy usually shows diffuse uptake of the radiopharmaceutical within the involved vertebra in blood pool and delayed images. A butterfly pattern of uptake on gallium scintigraphy has been described in vertebral osteomyelitis. White blood cell imaging of osteomyelitis of the spine has been disappointing. Most foci of infection demonstrate increased activity, but photopenia has also been reported in osteomyelitis of spine. MR imaging is the modality of choice for early diagnosis of vertebral osteomyelitis. MR imaging is more sensitive in the detection of vertebral osteomyelitis than either conventional radiography or CT scan and nuclear scintigraphic studies. 21 The appearance of vertebral osteomyelitis on MR images has been characterized as (1) confluently decreased signal intensity of the vertebral bodies and associated interspace with poor distinction between these on short TR, short TE images; and (2) abnormal increased signal of the disk on long TR, long TE images with an abnormal configuration (i.e., absent intranuclear cleft); and (3) increased signal of the vertebral end plates at the abnormal disk level on long TR, long TE images. 2 , 22 , 29 In advanced stages of infection, such as tuberculous spondylitis, a soft tissue mass of low signal intensity on T1-weighted images and of high signal intensity on T2-weighted images is frequently observed. Unlike neoplastic disease, infection is associated with loss of disk space height and the low signal intensity in transnuclear cleft normally seen on T2-weighted images. Increased T2 signal within the vertebral body marrow often precedes abnormal uptake on technetium 99m scintigraphy. Gadolinium chelate enhancement further increases sensitivity (Figs. 2 B, 3 B, and 4 A). Soft tissue extension from spinal osteomyelitis in the form of epidural abscess or paravertebral abscess is well demonstrated by MR imaging (see Fig. 4 A) and enhanced CT scan. Following treatment, a radiodense “ivory” vertebra may be seen. The affected disk space may remain relatively intact, or complete bony ankylosis may occur.