Discitis-Osteomyelitis

Introduction

Infections of the spine are not uncommon in clinical practice. There are many terms used to describe essentially the same pathologic entity at different stages of evolution. In adults, pyogenic infections typically spread hematogenously to the vertebral endplate (vertebral osteomyelitis), continue through the intervertebral disc to involve the adjacent vertebral body (discitis-osteomyelitis), and may develop abscesses in paraspinal and epidural spaces (psoas or epidural abscesses) ( Fig. 26.1 ).

Less common routes of transmission include direct inoculation from spinal procedures, direct extension from nearby infection, and hematogenous spread to the facet joints (facet septic arthritis). In the pediatric population, the disc may be the initial site of infection due to its residual vascularization.

A distinction should be drawn between pyogenic and non-pyogenic infections. Tuberculous spondylitis, a form of non-pyogenic infection, has similar consequences, although different imaging evolution and distinct clinical management, which will be discussed separately.

The clinical diagnosis of a spinal infection can be challenging. The typical presentation is insidious and nonspecific focal back pain, with fever present in only half of patients. Laboratory findings of leukocytosis and elevated erythrocyte sedimentation rate (ESR)/C-reactive protein (CRP) are also nonspecific, but may help exclude infectious etiologies if normal. Blood cultures are useful when positive and can also be used to identify the most effective antibiotic therapy. The causative bacteria are often Staphylococcus aureus ; however, other organisms may be encountered, particularly with immunocompromised patients or intravenous (IV) drug users. As a result of these clinical factors, there is typically a delay in presentation, diagnosis, and definitive therapy of spinal infections. If left untreated, infections of the spine can have devastating consequences related to spinal instability, which threatens the integrity of the spinal canal and its contents.

Imaging Evolution: Overview

Magnetic resonance imaging (MRI) has become critical to the early diagnosis of spinal infections. Early pathologic findings of bone marrow edema, T2 hyperintense disc signal, and abnormal disc enhancement are only identifiable by MRI. As such, MRI with and without contrast is the imaging study of choice for the early diagnosis of suspected spinal infections. Ultimately, the utility of computed tomography (CT) and radiographs for the initial diagnosis of infectious spondylitis is limited by the fact that a normal examination does not exclude the diagnosis of an early evolving spinal infection. Also, MRI is most valuable to differentiate an infectious process from the more commonly encountered degenerative processes of the spine that may present in a similar fashion.

Late imaging manifestations of endplate erosion and disc height loss may be identified by radiographs or CT, although complications of advanced disease, such as paraspinal abscess and spinal canal stenosis, are best seen by MRI. As such, CT should not be relied upon to assess the complete extent of infection unless MRI is contraindicated. Of note, CT and radiographs can be useful in preoperative evaluation prior to treatment of spinal instability in the setting of known spinal infection. Gallium-67 SPECT and F-18 FDG PET studies may also provide useful information when the diagnosis is in doubt or when MRI is contraindicated.