The SpineInflammatory disease
Most patients with inflammatory diseases of the spine present with pain. Depending on the site and extent of the pathological process, neurological deficits and other symptoms may be found. Spondylitis can be caused by various organisms like bacteria, tuberculous bacilli, fungi and parasites. Moreover, aseptic spondylitis may be found in ankylosing spondylitis and rheumatoid arthritis. Erosive osteochondritis is a special type of aseptic spondylitis mimicking septic spondylitis in MRI, but major bone destruction and signs of infection are missing. Aseptic, destructive spondylarthropathy is related to ß2-microglobulin-associated amyloid deposits in patients with chronic renal failure and has to be differentiated from septic spondylitis as well.
The correct diagnosis of septic spondylitis is mandatory to allow proper antibiotic and/or surgical treatment. Any delay bears the risk of a severe complication, especially para- and tetraplegia. Epidural extension of abscesses, gibbus formation and sequestration of bony elements, as well as granulation and scar tissue may cause encroachment of the spinal cord. In rare instances ascending meningitis and meningoencephalitis can originate from pyogenic vertebral osteomyelitis. Ankylosing spondylitis or erosive osteochondritis bear no risk of neurological complications whereas ß2--microglobulin amyloid deposits in dialysis-associated, destructive spondyl arthropathy can cause cord compression with neurologic deficits.
Plain films
For diagnosis, differential diagnosis and follow-up of inflammatory diseases of the spine, analysis of the endplates plays a key role. Erosions, destructions, subchondral or regional sclerosis and osteopenia as well as gibbus formation are clearly visualized on plain films. Healing and progression, respectively, can also be monitored by plain film radiography, which is indispensible for diagnosis and follow-up. In the early phase of septic spondylitis, plain films are frequently negative and at the time of onset of clinical symptoms only 25 % of the patients show unambiguous destructions of the endplates. The aseptic types of spondylitis, on the other hand, are characterized by a chronic course and changes at the discovertebral junction are usually present, when patients complain of clinical symptoms.
Frequently reduction of the disc height is an early sign of septic spondylitis, even if erosions of the end plates are missing. In the lumbar spine, for example, a gradual increase of disc height is found from L1 to L5 and the L5/Sl disc is smaller. Segmental thinning of the disc space is found in spondylitis and can be easily assessed on the plain film.
Bone scintigraphy
The sensitivity of 99m-technetium bone scintigraphy is high (more than 90%) for the detection of osteomyelitis, discitis and aseptic spine diseases. However, bone scans are non-specific and are also positive in neoplastic and degenerative spine disorders, postoperatively and after posttraumatic or osteoporotic fractures. With Indium-labeled leukocyte scintigraphy, specificity is improved which may be helpful in the few cases when organisms play a role in the pathogenesis. With scintigraphic methods, precise assessment of extent and localization of inflammatory spine processes is not possible and the involvement of bony elements by infection cannot be differentiated from soft tissue infection.
Computed tomography (CT)
For the specific diagnosis of spondylitis, CT is less useful than plain film radiography. Early destructive changes at the end plates, an important sign of spondylitis, may be missed by axial scanning due to partial volume averaging. Destruction of the vertebral bodies on CT are often ambiguous and tumors as well as infection may be responsible. Destruction within a vertebral body and geographic lesions with sclerotic margins are readily depicted by CT. CT is extremely useful for the assessment or exclusion of paravertebral and epidural abscess extension which cannot be detected on conventional films. With i.v. contrast enhancement, extraosseous abscesses are better detected by demonstrating their enhancing rims. Even with i.v. contrast, however, epidural involvement may be missed on CT due to beam hardening artefacts especially in the cervicothoracic region requiring intrathecal contrast enhancement for more complete evaluation.
Percutaneous biopsy can be performed under CT guidance allowing precise localization of the suspected foci and planning of the best line of approach. Drainage of paraspinal fluid collections and abscesses can be performed under CT -guidance as well.
Magnetic resonance imaging (MRI)
Multiplanar imaging capabilities and high sensitivity for soft tissue and bone marrow lesions make MRI the modality of choice for the diagnosis of inflammatory spine disorders. Pathological alterations of the disc space are demonstrated by MRI as well. Frequently, characteristic patterns of disc space, bone marrow and paravertebral lesions allow the specific diagnosis of spinal infections. Therefore, in many institutions MRI has replaced CT in the evaluation of inflammatory spine diseases.
In pyogenic spondylitis, ankylosing spondylitis and some types of degenerative disc disease, a uniform pattern of signal changes within the bone marrow is found in MRI. Low signal intensity on short TR, short TE sequences (T1-weighted) and high signal on long TR, long TE sequences (T2-weighted) is found. Bony sclerosis is detected on susceptibility sensitive GE-sequences with high sensitivity. In our institution, opposed phase GRE sequences have proved very useful. On opposed phase GRE sequences, normal red bone marrow exhibits low signal intensity by subtraction of the water and fat component. On opposed phase GRE sequences bone marrow edema is characterized by high signal intensity whereas in even minor bony sclerosis low signal is found. Inversion recovery sequences with short inversion time (STIR) and T2-weighted sequences with selective fat suppression pulses are the most sensitive sequences for the detection of bone marrow edema and inflammatory lesions within the bone marrow.
Contrast enhancement is demonstrated to good advantage on opposed sequences like GRE sequences, e.g. with TE = 17 ms at 1.0 T or opposed T1-weighted sequences with an asymmetric 180 refocusing pulse. On "in phase" T1-weighted SE-sequences the conspicuity of marrow lesions is decreased after contrast enhancement. Recent reports indicate, that contrast-enhanced MRI examinations and frequency-selective fat saturation techniques are superior to every other imaging protocol for the detection of inflammatory spine disorders.
Pathology
Pyogenic spondylitis
In pyogenic spondylitis, hematogenous spread of bacteria and other microorganisms within the bone marrow of the vertebral bodies is the most common portal of infection. Direct inoculation is less frequent than other causes, whereas postoperative infection is the most important. However, disc or vertebral puncture can also be responsible for inoculation of pathogenetic organisms. A dense paravertebral net of valveless venous collaterals exist which communicates with the inferior vena cava and the pelvic veins (Batson's venous plexus). This enables organisms and malignant cells to invade the vertebral bone marrow retrogradely. Nevertheless, it is now generally accepted that the arterial route is the most important for hematogenous osteomyelitis of the spine entering the vertebral body through the central dorsal nutrient foramen. From the vertebral bodies, the discs are affected by direct invasion of the organisms. In degenerative disc disease, secondary vascularization of the disc is possible. Granulation tissue may penetrate radial tears of the annulus fibrosus, which contributes to the stabilization of the disc. In rare instances, hematogenous spread of infection direct to the discs is therefore also possible in adults. In children and young adolescents up to the age of 5 years the disc is still vascularized providing a hematogenous route by which the organisms can be carried directly to the disc.
Pyogenic infections of the spine affect patients most frequently in the fifth and sixth decades of life. The offending organisms can be cultured from blood samples, needle or open biopsies in about half of the cases. The most common organism is Staphylococcus aureus (80-90%), but other organisms like Streptococcus, Salmonella, or Klebsiella are found as well.
On plain films, ill-defined destruction can be detected after 2 to 3 weeks (Fig. 29). In most of the cases, intramedullary foci are located in the anterior and subchondral portion of the vertebral bodies, in which high vascularity exists. From the intramedullary focus infection penetrates early to the disc. In this very early phase of pyogenic spondylitis, an increase of disc height may be found in rare instances. A typical sign of early spondylitis is a reduction of the height of the disc which may be the single radiological sign of spondylitis. From the disc spaces infection penetrates the adjacent end plates of the vertebral bodies and extends into the bone marrow. Irregular destructions of the end plates are visualized on radiographs and tomograms. Depending on the host's resistance (immunological competence) and the virulence of the organisms, the destructions can progress to vertebral collapse and/or gibbus formation. Weeks or 2-3 months after the onset of symptoms, bony sclerosis indicating attempted healing of spondylitis is radiographically depicted.
Sclerosis, eburnation, severe narrowing of the disc space or even bony ankylosis, as well as osteophytosis are found in healed spondylitis. However, radiographic findings may be ambiguous and complete healing can only be diagnosed, when long term follow-up examinations indicate no further changes.
In pyogenic spondylitis on T1-weighted images MRI shows low signal intensity within the marrow of the vertebrae adjacent to the involved disc (Fig. 29). Disc space narrowing is a common finding and the signal intensity of the disc is low; thus contrast between the disc and adjacent vertebral bodies is reduced or lost. In the early phase of infection within a normal, non-degenerated disc, swelling and increase of disc space, may be found in rare instances. On follow-up examinations and frequently on initial examination, a considerable loss of the height of the disc space is found which progresses rapidly. Granulation tissue and abscesses may extend into the subligamentous and epidural space. Moreover, erosions of the cortical endplates are visualized. Infrequently, only one vertebra or more than two are affected.
On T2-weighted and STIR-images, abnormally high signal intensity is detected in the bone marrow of the affected vertebrae and definite signal increase is found following Gadopentate injection (Fig. 29). In the disc space, high intensity fluid collections are frequently found. The "intranuclear cleft", which is present in about 30% of normal discs, can no longer be identified. The shape of the disc is irregular and extension of the high signal intensity disc within the bone marrow of the adjacent vertebral body may be found. Opposed T1-weighted sequences and frequency selective fat saturation techniques support the use of paramagnetic contrast media and are definitely useful in assessing the extension of epidural involvement or abscesses and show involvement of the spinal canal and soft tissues (Fig. 29).
Healing of spondylitis following surgical or conservative treatment is associated with long standing signal alterations in MRI, even if there is no clinical evidence of persistent infection. After several months, high signal intensity marrow is found in the vertebral bodies on T1-weighted images which is higher in signal intensity than the uninvolved vertebrae, presumably due to conversion of haematopoietic marrow to fatty marrow. High signal intensity on T2-weighted and contrast enhanced examination decreases and the soft tissue and epidural involvement resolves.
Tuberculous spondylitis
Tuberculous spondylitis is characterized by a chronic course of clinical symptoms and radiological changes. Spondylitis has become the most frequent manifestation of osteoarticular tuberculosis and it is often associated with tuberculosis of other organs, especially of the lung. Low grade fever, nocturnal sweating and moderate elevation of the erythrocyte sedimentation rate are found.
Widespread osteopenia of the bones involved by osteoarticular tuberculosis and of the adjacent skeletal elements has been described as typical for tuberculosis in many textbooks. However, this is not a constant finding and differentiation between pyogenic and tuberculous spondylitis is frequently not possible based upon radiological findings.
After i.v. administration of Gadopentate, intraosseous abscesses with ring-like, peripheral enhancement are frequently found. The disc space can be preserved for prolonged periods. The cortical endplates are focally destroyed. In the thoracic vertebral column, destruction of the anterior aspects of two adjacent vertebrae results in gibbus formation. In the lumbar spine axial compression may be found due to axial forces of weight bearing. Large abscesses are frequently found in the pre- and paravertebral region, which may extend into the psoas muscle, under the longitudinal ligaments and along the pleura-lined spaces of the thorax. Tuberculous abscesses may spread over long distances, especially when originating from the lumbar vertebrae and fistulae may appear within the groin, above the iliac crest, at the medial side of the thigh and within the popliteal fossa. Extension and localization of tuberculous abscesses are readily assessed by coronal and sagittal MRI. Involvement of the posterior elements of the vertebrae and of the suboccipital area are very unlikely in pyogenic spondylitis, but can be found in tuberculous spondylitis.
Fungal spondylitis
Candida and Aspergillus are saprophytic fungi which may cause spinal infection in immunocompromised patients. Candida and aspergillus spondylitis cause morphological findings similar to pyogenic spondylitis.
If more than one area in the spine is involved, however, aspergillus infection should be taken into consideration. There is only little contrast enhancement within the disc, but anterior and posterior bulging of the disc with protrusion of the annulus fibrosus and enhancement of the anterior and posterior longitudinal ligament and of the subligamentous space are frequently detected.
Blastomycosis, Cryptococcosis and Coccidioidomycosis are endemic in South Africa, South America and the United States, but rarely encountered in Europe. Definite diagnosis of granulomatous fungal infections requires culture of the organism or microscopic proof.
"Inflammatory" Disc Degeneration
Pyogenic spondylitis has to be differentiated from acute, erosive degenerative disc disease with edema of the vertebral bone marrow adjacent to the endplate. This disorder simulates the signal patterns of infectious spondylitis, named by Modic as Type I reaction (Fig. 30). The underlying mechanism is rapid disc degeneration without the creation of adequate spondylophytes. The rapid loss of segmental stability, which can occur spontaneously, after surgery with removal of the disc material or after chemonucleolysis, causes transient disc vascularization and bone marrow edema (see chapter on spinal anatomy). The vascularized granulation tissue within the disc, as well as bone marrow edema exhibit Gadopentate enhancement (Figs. 30, 31). Bone marrow edema and disc vascularization may be found together or only one of these features may be present. No epidural or paravertebral abscess is found. Gas within the discspace on CT is a frequent finding and it is strongly suggestive of erosive degenerative disc disease (Fig. 31). No major destruction of the vertebrae is found in this entity, but with time, dense sclerosis will develop known as spondylosclerosis hemispherica.
Dialysis-associated, destructive spondylarthropathy
Bone destruction due to ß2-microglobulin (ß2-M)-related amyloid deposition increase in frequency with long-term haemodialysis. ß2-microglobulin-associated amyloidosis plays an etiological role in carpal tunnel syndrome, subchondral bone cysts, pathological fractures and destructive, erosive, non-infectious spondyloarthropathy. Destructive, erosive, non-infectious spondyloarthropathy in patients on chronic haemodialysis was first described by Kuntz (1984). The cervical spine,
especially the atlanto-axial region and the lower
cervical spine are most frequently involved, however, the lumbar and
thoracic spine are also commonly affected. In patients with end stage
renal disease, ß2-M accumulates and serum concentrations are elevated. ß2-M is usually excreted by the kidneys and dialysis membranes eliminate it inadequately. Eventually, triggered by crystals, ß2-M related amyloid is deposited in the disc space and in the ligaments. After narrowing of the disc space, the endplates are eroded and pseudocystic hernations into the trabecular bone of the
vertebrae with sclerotic margins are found. Amyloid
 | Figure 33.
Ankylosing spondylitis.
Complete ossification of the ventral annulus fibrosus and the apophyseal joints. During a skiing accident a fracture passing through the C6/C7 disk space, the lamina C7 and the spinous process of C6 had occurred. |
infiltrates in the dens axis increase the risk of dens fracture (Fig. 32). Amyloid deposits in the transverse or posterior longitudinal ligament or in the ligamenta flava can encroach upon the epidural space and myelopathy may result.
Ankylosing spondylitis
Ankylosing spondylitis is a chronic inflammatory disorder of unknown origin affecting predominantly the spine and the sacroiliac joints. The frequency of ankylosing spondylitis (about l %) in men is much higher than in women and there is a definite association with the histocompatibility antigen HLA-B27. The clinical onset of the disease is insidious and usually between 15-35 years of age. The patient complains of stiffness and low back pain.
Sacroiliac joint abnormalities and spinal ligament calcification and ossification are characteristic of ankylosing spondylitis. Marginal syndesmophytes are orientated vertically and may extend up the entire length
 | Figure 34.
Ankylosing spondylitis. Ventral subligamental osteitis creates new bone formation at the anterior vertebral border resulting in squaring of the vertebral body. Osseous bridging at the ventral annulus fibrosus is also seen. |
 a |  b |
Figure 35.
T1-weighted MR image (570/15 ms) pre- and postcontrast in ankylosing spondylitis with an Anderson lesion at the T11/TI2 level. Parallel to the endplates all discs exhibit Gadopentate enhancement (b). Erosive, destructive changes with widening of the disk space and bone marrow edema in the region of the Anderson les ion is present. |
of the
spine. Eventually, osseous ankylosis of the apophyseal joints and of the intervertebral disc spaces (bamboo
spine) is found (Fig. 33).
Erosions of the vertebral margins heal by
proliferation of sclerotic bone, which stands out in marked contrast to the rest of the vertebral body (shiny corner). Healed erosions and deposition of new bone beneath the inflamed longitudinal ligaments (osteitis) contribute to squaring of the vertebral bodies (Fig. 34).
On MRI, widespread bone marrow edema and inflammatory changes of the disc space is found (Fig. 35). The disc spaces are narrowed and marked reactive sclerosis is present. Occasionally, localized discovertebral erosions and destructions are seen, in some cases ending in a disc pseudarthrosis (Andersson lesion), resembling infective discitis (Fig. 35). Following minor trauma, fractures through the porotic bones just beneath the endplates or through the discs may occur, and may result in spinal pseudarthrosis.
Stig Holtås, Maximilian F. Reiser and Axel Stäbler