Radiologia Brasileira - Publicação Científica Oficial do Colégio Brasileiro de Radiologia

INTRODUCTION

The imaging evaluation of the musculoskeletal system has been object of a range of studies recently published in the Brazilian radiological literature^(1–7). Among such studies, some related to the vertebral spine are highlighted^(8–10).

Spondylodiscitis represent 2%–4% of all cases of skeletal infection⁽¹¹⁾ and its radiological diagnosis is based on the demonstration of the involvement of the vertebral body and adjacent intervertebral disc, although several non-infectious diseases may mimic this condition⁽¹¹⁾.

Three forms of dissemination are described, as follows: hematogenous spread from a distant septic focus; direct inoculation (either by surgery or trauma); contiguity with an adjacent septic focus. Generally, the infection starts in the anterior portion of the vertebral body because of its rich arterial supply, and spreads through the medullary spaces, affecting the intervertebral disc by contiguity, most frequently involving the lumbar and dorsal segments of the spine (50% and 35% of cases, respectively)⁽¹²⁾. Generally, late diagnosis is the rule (on average, two to six months after the symptoms onset)⁽¹³⁾. A fast diagnosis allows the institution of appropriate treatment and may avoid complications such as vertebral collapse and medullary compression syndrome.

In this context, imaging investigation is critical for the diagnosis and follow-up of the lesions so as to reduce the necessity of invasive procedures. Magnetic resonance imaging (MRI) is the method of choice because of its high sensitivity and specificity⁽¹⁴⁾, as well as good tissue resolution and multiplanar capacity⁽¹⁵⁾. Also, MRI can be useful to suggest the origin of the infection – although this is not always possible –, aiding in the differentiation between tuberculous and pyogenic infections, thus allowing efficacy in the treatment of the patient⁽¹⁶⁾.

Main MRI findings include hyposignal on T1-weighted and hypersignal on T2-weighted images from the vertebral bodies and adjacent intervertebral discs, as well as enhancement after intravenous paramagnetic contrast medium injection, besides paravertebral masses/abscesses, all of them nonspecific⁽¹⁷⁾. However, as some details are correctly individualized and each finding is appropriately interpreted, they may become useful tools to guide the radiologist in the differential diagnosis.

The present pictorial essay is aimed at gathering and demonstrating some imaging findings in order to appropriately highlight evidences of higher suspicion degree of etiological agents, according to the available literature.


PYOGENIC SPONDYLODISCITIS

Pyogenic spondylodiscitis affects most frequently the lumbar spine, involving only one vertebral segment (one intervertebral disk and adjacent vertebral bodies). Most frequently, Staphylococcus aureus is the etiological agent implied in the infection, responsible for 55%–90% of the cases⁽¹⁸⁾. Other relevant agents include Streptococcus, Pneumococcus, Enterococcus, Escherichia coli, Salmonella, Pseudomonas aeruginosa and Klebsiella⁽¹⁸⁾.

Because of the high concentration of proteolytic enzymes intrinsic to the virulence of these biological agents, the disc involvement occurs early in the course of the disease and can be demonstrated concomitantly with the corresponding vertebral body lesion at very initial stages of infection. The vertebral body involvement tends to be more homogeneous in relation to the alterations in signal intensity on T1- and T2-weighted sequences and to the enhancement of its medullary spaces⁽¹⁹⁾. Considering the remarkable worsening in the general condition as well as important features of the symptoms affecting the patient, imaging studies are requested at the initial phase of the disease where, although the appearance of the lesion is more aggressive, it is less disseminated in multiple vertebral bodies, so the finding of large abscesses or paravertebral masses is not expected. In cases where such findings are present, however, the lesion is poorly defined, depending on the aggressiveness of the implied etiological agent⁽¹¹⁾.

Thus, MRI findings which lead to the suspicion of pyogenic spondylodiscitis include segmental involvement (Figures 1 and 2), presence of poorly defined paravertebral mass (Figure 3), early intervertebral disc involvement (Figure 2), and homogeneous enhancement/alteration of signal of affected vertebral bodies (Figures 1 and 2).




TUBERCULOUS SPONDYLODISCITIS

The spine is the main site of bone involvement by tuberculosis, responsible for 50% of the cases⁽²⁰⁾. The differential diagnosis with pyogenic spondylodiscitis is clinically and radiologically difficult to be achieved, particularly in cases where the etiological agent is less aggressive, like in cases of brucelosis.

The scarcity of proteolytic enzymes, which is an intrinsic characteristic of Mycobacterium tuberculosis, results in a remarkably late, indolent infection of the intervertebral disc⁽²¹⁾. Thus, tuberculous spondylodiscitis may originate large paravertebral abscesses or granulomatous masses with well defined borders⁽¹⁷⁾, extending into the subligamentous space through several (typically more than three) vertebral bodies at the time of the imaging diagnosis. An irregular pattern of involvement may also be observed¹⁵, besides a more heterogeneous involvement of vertebral bodies, provided there is time for the bone tissue to react against the infection. Thus, focal areas of altered signal intensity are observed on T1- and T2-weighted images, besides focal enhancement of part of medullary spaces of the vertebral body. Such findings allow the demonstration of discrepancy in the involvement between discs and adjacent structures such as vertebral bodies and paravertebral region. Thus, disc involvement is noticeable in later phases of the disease. At early phases, the disc is usually intact, although it is already possible to identify the presence of extensive bone involvement or large paravertebral abscesses/masses which, typically, tend to present well defined contours. At early phases of the disease, the differential diagnosis with, for example, neoplastic (Figure 6), degenerative (Figure 7) or inflammatory etiologies may be hardly made. In such cases, morphological criteria, presence of signal from affected regions on T2-weighted images, and the characteristics of the gadolinium enhancement may be useful to shorten the list of diagnostic hypotheses⁽²²⁾.




Inflammatory spondylites such as ankylosing spondylitis, psoriatic spondylitis and reactive spondylitis, over its natural history, may present alterations similar to the ones of degenerative spondylitis. However, the vertebral involvement is typically associated with, and sometimes preceded by either symmetrical or asymmetrical, bilateral sacroiliitis, depending on each etiology⁽²³⁾.

Usually, neoplastic lytic lesions with metastasis to the axial skeleton are not associated with reactive sclerosis or periosteal reaction; and, typically, the first sites of involvement are the vertebral pedicles⁽²³⁾.

Degenerative spondylites, like infectious cases, may present enhancement of the intervertebral disc on the post-gadolinium sequences. However, the affected vertebral plateaus tend to present hyposignal on T2-weighted images and no contrast-enhancement after intravenous gadolinium injections⁽²³⁾.

Considering the complex dissemination of the etiological agent of tuberculosis through the body, with different dissemination patterns depending on the phase of disease onset, some atypical patterns (classified according the mode and initial site of infection) are described in addition to the classical paradiscal infection where the disc involvement occurs by contiguity with adjacent vertebral plateaus, classified according their initial involvement⁽²³⁾. Thus the central, anterior subperiosteal and appendicular patterns of dissemination are observed, all of them corresponding to the findings focused by the present essay, but their description is not included in the scope of this study.

The MRI findings suspicious for tuberculous spondylodiscitis include multisegmental subligamentous involvement (Figure 4); presence of well defined paravertebral mass/abscess (Figures 5 and 8); relatively spared disc at early phases of the disease (Figure 4); and heterogeneous enhancement/change in signal of vertebral bodies (Figures 5 and 8).




CONCLUSION

The characteristics of MRI findings play a relevant role in the diagnosis of complications inherent to infectious processes established in the vertebral spine, as well as in the determination of the involvement extent. However, it is not rare that radiologists investigate evidences suggesting the probable implied etiological agent. The etiological diagnosis of spondylodiscitis is hardly established on the grounds of isolated imaging findings criteria since many times they are nonspecific. However, the disproportion of the degree of involvement of the intervertebral disc in relation to the corresponding vertebral plateaus, the homogeneity of the vertebral bodies signal and enhancement, as well as the paravertebral masses volume and contours⁽¹⁹⁾ should be appropriately taken into consideration as useful – although not definitive – tool in the investigation of a possible etiologic agent (Table 1). Thus, the MRI specificity for the diagnosis of spondylodiscitis is directly dependent on the characteristics of the signal, on the anatomic distribution, on the proportionality between vertebral bodies and intervertebral discs involvement, on the homogeneity of enhancement of the medullary spaces of vertebral bodies and, invariably, on the clinical history of the patient, although it is not always available at the moment of the imaging reporting⁽¹⁷⁾.




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1. MD, Resident of Radiology and Imaging Diagnosis, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.
2. Associate Professor, Department of Radiology, School of Medicine – Universidade Federal do Rio de Janeiro (UFRJ), MD, Neuroradiologist, Grupo DASA, Rio de Janeiro, RJ, Brazil.
3. Full Professor of Radiology, Universidade Federal Fluminense (UFF), Niterói, RJ, Adjunct Coordinator, Course of Postgraduation in Radiology, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.
4. Associate Professor, Department of Radiology, School of Medicine – Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.

* Study developed in the Service of Radiology at Hospital Universitário Clementino Fraga Filho – Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.

Mailing Address:
Dr. Cristiano Gonzaga de Souza
Rua Nossa Senhora do Carmo, 155, Vila Tereza
Cataguases, MG, Brazil, 36772-016
E-mail: souzacataguases@gmail.com

Received November 24, 2012.
Accepted after revision January 18, 2013.