Keywords
spondylodiskitis - spinal infection - radiologic investigation
Introduction
Nonspecific pyogenic spondylodiskitis is a rare infectious disease, representing 2
to 7% of skeletal infections.[1] However, in the last few decades an increase in the incidence of this disease has
been reported. This is due to demographics, improved diagnostic possibilities, and
advances in medicine in general.[2]
[3]
[4]
[5]
[6] In the past, the incidence of pyogenic spondylodiskitis was reported as a ratio
of 1:250,000, whereas newer studies reveal an increase up to 5:100,000.[7]
[8] Men are affected more frequently than women, and the peak age is 60 years of age
and older.[9] The lumbar spine is affected most frequently, followed by the thoracic and the cervical
spine.[10]
According to the etiopathogenesis as well as clinical findings and imaging results,
the condition should be distinguished between diskitis, the isolated infection of
the disk; spondylitis, the isolated infection of the vertebral body; and spondylodiskitis,
the mixed picture of both entities ([Fig. 1]). Isolated infections of the disks are mainly found in children related to the still
existing vascular supply of the disk and possibly in cases after interventional intradiskal
procedures.[11] Isolated spondylitis is commonly seen in specific infections such as tuberculosis.
Spondylodiskitis represents the most common form of nonspecific pyogenic spondylodiskitis.
Fig. 1 Schematic drawing of the possible locations of infection of the spine: infection
of the intervertebral disk, “diskitis”; infection of the vertebral body, “spondylitis”;
and infection of both the vertebral body and the intervertebral disk, “spondylodiskitis.”
Abscess formation is possible and plays an important role in determining the therapeutic
approach. Paraspinal abscesses, mainly located in the psoas muscle (whereby localization
is possible everywhere in the surrounding tissue), can often be observed. Intraspinal
abscess formation requires attention. It can be a result of spondylodiskitis, of facet
joint infection or can be observed as primary abscess without any other infectious
focus.[12] The extent and localization can be highly variable, and neurologic impairment must
be ruled out in such a situation.
Isolated facet joint infection as a source of a spondylodiskitis is rarely described.
However, in the context of an increase of invasive therapeutic procedures, there are
an increasing number of reports in the recent literature.[12]
[13]
One major problem in this context remains the point of definitive proof of the diagnosis.
It is well known that early diagnosis of this disease is associated with both better
outcomes and more successful nonoperative therapeutic options . In contrast, delayed
diagnosis is associated with bony defects, instability, and more concomitant deformities.[1]
[10]
[11] In addition, undiagnosed spondylodiskitis can cause secondary infections such as
endocarditis. Accordingly, related inflammation should always be proven and treated.[14]
[15] In this context, a standardized diagnostic protocol including current imaging techniques
is mandatory to confirm the diagnosis as early as possible.[14]
Detailed knowledge about the pathogenesis is helpful in the correct interpretation
of specific imaging findings. Because the spread of germs is predominantly hematogenic,
initial germ colonization takes place in the terminal intravertebral arteries near
the subchondral layer. The resulting inflammatory reaction leads to an edema. Infection-caused
microinfarctions and local necrosis lead to destruction of the subchondral bone with
infection of the adjacent disk. Proteolytic enzymes enhance the destruction of the
disk. Additional involvement of the adjacent vertebra and/or the surrounding tissue
is possible.[3]
[9]
In this article we discuss the imaging techniques for the diagnosis of spondylodiskitis
and the differential diagnosis of nonspecific pyogenic spondylodiskitis. The focus
should be on a feasible and pragmatic approach for the treating clinician.
Radiography
The results of radiographic investigations are often unsatisfactory, based on nonspecific
findings and low sensitivity. In the early phase of infection, no specific radiographic
findings are commonly visible, and the differentiation to degenerative pathologies
(Modic type 1) remains a challenge.[1]
[10]
These are the earliest typical radiographic signs described in the literature[1]
[10]
[16]:
-
Loss of definition (irregularity) of the end plates
-
Unspecific porotic changes (demineralization) in the subchondral layer
-
Suspect changes typically begin anterosuperiorly
In the subacute and chronic phase, various changes such as loss of disk space height,
end-plate erosion, bony destruction, reactive sclerosis ([Fig. 2]), paravertebral soft tissue mass, defects, and resulting deformity can be observed.
The degree of these changes depends on the progression of the disease and the treatment
course.[15]
[16]
[17]
[18]
Fig. 2 Lateral and anteroposterior radiograph of the thoracolumbar spine with advanced destruction
of end plates and disk in level L1–L2 in a 78-year-old male patient. A ureter splint
is shown at the side.
Plain radiographs, particularly in the standing position, are often used as a first
overview in patients with persistent back pain. In case of spondylodiskitis, typical
findings as mentioned appear later during the course of the disease. Thus this investigation
is helpful for a differential diagnosis. In addition, plain radiographs are recommended
to evaluate both the alignment in the sagittal and the coronal plane, and the degree
of destruction.[19] Thus mechanical stability can be estimated.[20] However, the main problem of radiographs remains the delay of imaging findings in
relation to the clinical course of 2 to 8 weeks after onset of infection.[1]
[10]
[16]
Computed Tomography
Computed tomography (CT) is very useful to evaluate the extent of bony changes such
as end-plate erosion, sequestra, bone defects, reactive sclerosis, and reactive ossification
as shown in [Fig. 3].[1]
[10]
Fig. 3 Computed tomography scan with (left) sagittal and (right) coronal reconstruction.
An 83-year-old male patient with pyogenic spondylodiskitis in level L1–L2 with marked
erosion of the end plates.
Contrast-enhanced CT enables improved assessment of affected soft tissue paravertebrally.
Surrounding swellings and thickenings of the paravertebral fat tissue, increased enhancement
and abscess formation, typically in the psoas muscle, should be sought. Gas inclusions
are suggestive of inflammatory soft tissue infection. Even if intraspinal changes
in CT are difficult to detect, epidural abscesses should be screened for specifically.
CT can detect bone changes earlier than radiographs. Preexisting osteochondritic changes
can complicate the identification of spondylodiskitis. Intradiskal gas as a vacuum
phenomenon and well-defined sclerosis of the end plates without bone resorption can
primarily be observed in degenerative intervertebral disks.
Multiplanar reconstruction techniques and three-dimensional reconstruction techniques
allow for a spatial visualization that could be helpful, especially for operative
planning.[15]
[17]
Although CT is more sensitive regarding the previously mentioned changes compared
with radiographs, the accompanying radiation exposure needs to be consided.[10] Disadvantages are the limited possibility to evaluate the disk and neural structures.[1]
CT is currently used in those patients where bony destruction is presumed or with
a contraindication for magnetic resonance imaging (MRI).[21] Furthermore, if an abscess is present, a CT-assisted puncture can be performed to
obtain tissue samples for microbiological diagnostics. In addition, CT is often recommended
as the first method for a puncture of suspicious tissue (intervertebral disk, vertebral
body) to identify causative germs.[16]
Magnetic Resonance Imaging
Magnetic Resonance Imaging
MRI has the best sensitivity (up to 96%) and specificity (up to 94%) compared with
other imaging techniques.[8]
[15]
[16] It provides precise anatomical information: The disk, neural structures, epidural
space, and surrounding soft tissue are clearly depicted.[16] High resolution, the ability of multiplanar reconstruction, and the absence of radiation
exposure are further advantages of the MRI. Thus MRI is seen as modality of choice
to prove spondylodiskitis and accordingly recommended by the Infectious Diseases Society
of America.[14]
[15]
[18]
[19]
The classic MRI findings of spondylodiskitis according to the literature are on T2-weighted
images hyperintense signal alterations of the disk and the adjacent vertebral bodies
with a worse delimitation of the normal intradiskal cleft.[16]
[18]
Fat-suppressed fluid-sensitive MRI sequences, most commonly short tau inversion recovery
(STIR) or turbo inversion recovery magnitude (TIRM), help differentiate bony edema
and circumscribed fluid collections from surrounding signal hyperintensity soft tissue
([Fig. 4a]).
Fig. 4 Sagittal magnetic resonance image of the lumbar spine in a 76-year-old female patient
with spondylodiskitis in L2–L3. Typically present are (left) short tau inversion recovery
hyperintense, (middle) T1-weighted natively hypointense alterations in the end plates,
and the disk with a marked enhancement (right) after contrast (gadolinium) administration.
In native T1-weighted sequences, correlated reduced signal intensity can be observed
in the adjacent vertebral bodies. Hypointense erosions and the loss of end-plate definition
can also be detected ([Fig. 4b]). In the further course of the disease, a lowering of the disk space height can
be observed in all sequences.
Gadolinium enhancement improves the information value in general. The enhancement
of the vertebral body, disk, and soft tissue improves the diagnostic accuracy ([Figs. 4c] and [5]).[16] In MRI, a precise examination of the intraspinal structures is possible. An increased
enhancement of the dura and possible epidural fluid collections should be explored
([Fig. 6]).
Fig. 5 Axial magnetic resonance image in a 67-year-old male patient with spondylodiskitis
in L4–L5. Surrounding thickening of the soft tissue with increased enhancement.
Fig. 6 T1-weighted sagittal postgadolinium magnetic resonance image. Pyogenic spondylodiskitis
in L3–L4 with epidural expansion in a 52-year-old man.
In addition to excellent spatial resolution, the advantage of MRI is the differentiation
between infectious and degenerative pathologies.[15] However, although MRI usually shows abnormal findings in the initial phase of infection,
they do not always suggest spinal infection.[8] Very early findings are nonspecific and prone to misinterpretation. In particular,
the differentiation of activated osteochondrosis Modic type 1, which can mimic infections,
remains a major challenge. In Modic type 1, bone marrow edema is typically localized
in the subchondral region and presents as a sharp margin with a still well-defined
vertebral end plate. The degenerated intervertebral disk also has a lower signal in
the T2-weighted image in contrast to spondylodiskitis. However, both contrast-enhanced
MRI with sophisticated investigation protocols and, if the diagnosis remains uncertain,
early follow-up MRI are useful for confirmation of the diagnosis.[11]
According to Leone et al, both the initial edema as well as hyperemia are the earliest
signs of inflammation.[15] Thus the diagnosis can be made early in a relatively safe way.[7] Among imaging techniques, MRI is the method of choice to detect spondylodiskitis.[10]
[22]
Multifocal infection is reported to appear up to a double-digit percentage range.
To detect possible multilevel infectious foci, whole-spine MRI protocols are suggested.[23] To gain an overview, for example, STIR-weighted MR images are recommended to detect
suspicious areas ([Fig. 7]). Afterward, these regions can be evaluated following the usual standardized protocol
for detailed information.[23]
Fig. 7 Whole-spine magnetic resonance (MR) imaging revealed multilevel spondylodiskitis
in the cervical, thoracic, and lumbar spine in a 78-year-old male patient. With sagittal
short tau inversion recovery sequence as a search tool, a good overview of the entire
spine can be obtained.
Disadvantages of the MRI are the reduced visualization of cortical bone involvement
and the rising number of contraindications related to the increasing number of patients
with implants who are not suitable for MRI.[15]
Nuclear Medicine
If the radiologic findings are not clear and a persistent suspicion of a spinal infection
continues, radionuclide imaging procedures may facilitate the diagnosis. Different
techniques and tracers (e.g., technetium-99m-diphosphonates, gallium-67) are available
to support the confirmation of the suspected diagnosis.[16] They differ regarding distribution pattern and uptake rate. Thus the sensitivities
and specificities vary between the techniques. Combined investigations can improve
the diagnostic accuracy in some situations. The addition of three-phase technetium-99m
scintigraphy with CT (single-photon emission computed tomography) improves the diagnostic
performance by superior localization of the infection. This additionally helps exclude
differential diagnoses such as tumor, metastasis, and, in particular, degenerative
disorders.[24] The combination of three-phase technetium-99m scintigraphy with gallium-67 scintigraphy
or the combination of three-phase technetium-99m scintigraphy with a scan with indium-111
labeled white blood cells are also reported in the literature. Disadvantages are the
reduced spatial visualization and the inferiority in detection of an epidural abscess
formation.[1]
[16] Therefore some authors recommend these techniques only if MRI is contraindicated.[1] Others point out the high sensitivity in excluding infection if no uptake takes
place.[24]
It was found that 18F-fluorodeoxyglucose-positron emission tomography (F18-FDG-PET)
is sensitive and allows an examination of the whole body in one session. It was also
described as a useful tool to differentiate degenerative from infectious end-plate
abnormalities, although the drawback is also the lack of anatomical information.[1]
[24] To overcome the low spatial resolution, a simultaneous acquisition with CT can be
performed with a reported sensitivity up to 95%.[1]
[24]
[25]
[26] With the recent development of integrated PET/MRI scanners, new possibilities for
multimodal molecular imaging have emerged. PET/MRI ([Fig. 8]) and the application of the so-called one-stop shop principle enable analysis of
simultaneously acquired metabolic and morphological parameters with excellent soft
tissue definition along with a significantly increased diagnostic certainty of spondylodiskitis.
Particularly in cases with inconclusive MRI findings, the combination of F18-FDG-PET
with MRI supports the diagnostic certainty markedly regarding both false-positive
and false-negative results with a sensitivity of 100%.[27]
Fig. 8 Simultaneous F18-FDG-positrone emission tomography/magnetic resonance imaging in
a 79-year-old male patient with suspected edema in the (left) short tau inversion
recovery sequence in L1–L2 and L2–L3. Spondylodiskitis was detected by an increased
tracer uptake in L1–L2. In L2–L3, spondylodiskitis was ruled out (right).
A drawback of isolated 18F-FDG-PET is the reduced image quality of detailed anatomical
information. In combination with CT or MRI, spatial resolution can be improved markedly.
This leads to both a safe assessment of the exact localization and the extent of infection
and a good assessment of the condition of the soft tissue.
Some of the biggest advantages of this technique are the ability of very early diagnostic
proof and its high diagnostic sensitivity, particularly in distinguishing between
early stages of infection and degenerative changes. As mentioned earlier, the prognosis
of pyogenic spondylodiskitis can be improved with an early diagnosis and thus immediate
therapy. The sensitivity is reported as 95% and a specificity > 87%.[28] In addition, if there is a suspicion of spondylodiskitis of more than one segment,
F18-FDG-PET/CT or MRI is helpful to clarify which of the levels is indeed affected
from inflammation. Moreover, the PET component can also provide significant benefits,
especially in patients with prior surgery and internal fixation, where MRI assessment
may be significantly hampered. In addition, 18F-FDG showed potential for monitoring
response to treatment.[29]
Unfortunately, these techniques are costly and not available everywhere. At this time
PET/CT, and in particular PET/MRI, remain reserved for selected cases.
An important point applicable for all these techniques is the scanning of the entire
body including the whole spine in one session. This allows the detection of additional
clinically uneventful infection foci.[24]
Discussion
Today, different radiologic modalities are available for the diagnosis of nonspecific
pyogenic spondylodiskitis. These modalities have various advantages and disadvantages,
and knowledge of the pros and cons of each technique or rather of their combination
facilitates an accurate diagnosis ([Table 1]). In the early phase after onset of infection at the spine, the diagnosis can be
challenging, particularly in the case of present degenerative disk changes (Modic
type 1). However, sophisticated MRI examination protocols available today and, in
uncertain cases, 18-FDG-PET with CT or MRI allow for a definitive diagnosis.
-
In cases of suspected spondylodiskitis, an MRI of the entire spine with fat-suppressed
fluid-sensitive MRI sequences is recommended as a search tool. If areas suspect for
inflammation are detected, the standardized in-house protocol should be used for further
evaluation. The use of gadolinium is recommended.
-
A conventional radiograph of the affected region, whenever possible in a standing
position, is useful to assess the alignment of the spine, to exclude advanced degrees
of destruction and to obtain an overview regarding degeneration, reduced bone quality,
and so on.
-
CT is proposed if relevant bony defects are to be estimated, particularly for preoperative
planning with the possibility of multiplanar reconstruction or in case of contraindications
for MRI. Contrast enhancement improves the information value, particularly with regard
to indicative inflammatory soft tissue reactions.
-
18-FDG-PET enables a whole-body investigation in one session. In case of uncertain
cases, it is helpful to differentiate between the early stage of infection and degenerative
Modic type 1 changes. In addition, this investigation is an option if MRI is contraindicated
and CT remains inconclusive. If multilevel infection is suspected, this technique
allows a safe identification of affected levels.
-
The combination of 18-FDG-PET with CT or MRI improves the spatial resolution significantly,
although it is reserved for selected cases.
Table 1
Advantages and drawbacks of the different radiologic investigation modalities
|
Modality
|
Advantages
|
Drawbacks
|
|
Radiograph: standing position whenever possible
|
General advantages
• Quick investigation
• Broad availability
• Inexpensive
Medical advantages
• Alignment evaluation
• Hints for destructive processes in later stages of disease
• Progress assessment
|
General disadvantages
• Radiation exposure
Medical disadvantages
• Low spatial resolution
• Poor assessment of the soft tissue
• Low sensitivity
• Detection of late-stage changes
|
|
CT: contrast enhanced if possible
|
General advantages
• Quick investigation
• Broad availability
• Inexpensive
Medical advantages
• Safe and early evaluation of bony changes
• Allows for multiplanar reconstruction
|
General disadvantages
• Radiation exposure
Medical disadvantages
• Reduced ability to assess neural structures and intradiskal changes
• Reduced sensitivity in early stages
|
|
MRI with gadolinium
|
General advantages
• Broad availability
• Radiation free
• Inexpensive
Medical advantages
• Evaluation of the whole spine
• Excellent soft tissue contrast
• Detection of early-stage infection
|
General disadvantages
• Long investigation time
Medical disadvantages
• Contraindications may exist (e.g., cardiac pacemaker, claustrophobia)
• Artifact prone
• Reduced assessment of bone quality
|
|
F18-FDG-PET
|
General advantages
• Scanning of the whole body in one session
Medical advantages
• High sensitivity and specificity
• Very early diagnostic proof
• Potential for monitoring response to treatment
• In combination with CT or MRI: high spatial anatomical resolution
|
General disadvantages
• Radiation exposure
• Not available everywhere
• Expensive
Medical disadvantages
• Reduced detailed anatomical information without CT or MRI
|
Abbreviations: CT, computed tomography; MRI, magnetic resonance imaging; F18-FDG-PET,
18F-fluorodeoxyglucose-positron emission tomography.
Conclusion
MRI with gadolinium is defined as the gold standard in the diagnosis of pyogenic spondylodiskitis.
An evaluation of the whole spine should be standard. Conventional radiographs, CT,
and radionuclide imaging procedures are helpful to complete the diagnosis and to answer
specific questions.
