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In adults, the use of the term discitis is generally discouraged as isolated infection of the spinal disc is a rare phenomenon. In general, the vertebral endplate is infected first with secondary involvement of the disc. For this reason terms such as spondylodiscitis or vertebral osteomyelitis are preferred due to their greater accuracy 18.
Spondylodiscitis has a bimodal age distribution, which many authors consider essentially as separate entities:
- older population ~50 years
- remote infection (present in ~25%)
- ascending infection, e.g. from urogenital tract instrumentation
- spinal instrumentation or trauma
- intravenous drug use (IVDU)
- long-term systemic administration of steroids
- advanced age
- diabetes mellitus
- organ transplantation
The typical presentation is back pain (over 90% of patients) and less common fever (under 20% of patients). Patients are often bacteremic from sources such as endocarditis and intravenous drug use (IVDU).
In the pediatric age group infection often starts in the intervertebral disc itself (direct blood supply still present) whereas in adults, infection is thought to begin at the vertebral body endplate, extending into the intervertebral disc space and then into the adjacent vertebral body endplate.
- Staphylococcus aureus (most common; 60%)
- Streptococcus viridans (IVDU, immunocompromised)
- gram-negative organisms, e.g. Enterobacter spp., E. coli
- Mycobacterium tuberculosis (Pott disease)
- less common organisms
- Cryptococcus neoformans
- Candida spp.
- Histoplasma capsulatum
- Coccidioides immitis
- Burkholderia pseudomallei (i.e. melioidosis): diabetic patients from northern Australia and parts of Southeast Asia
- Brucella spp.
- in patients with sickle cell disease consider Salmonella spp.
- although other organisms may also cause
- can occur anywhere in the vertebral column but more commonly involves lumbar spine
- single level involvement (65%)
- multiple contiguous levels (20%)
- multiple non-contiguous levels (10%)
Plain radiography is insensitive to the early changes of spondylodiscitis, with normal appearances being maintained for up to 2-4 weeks. Thereafter disc space narrowing and irregularity or ill definition of the vertebral endplates can be seen. In untreated cases, bony sclerosis may begin to appear in 10-12 weeks.
CT findings are similar to plain film but are more sensitive to earlier changes. Additionally, surrounding soft tissue swelling, intervertebral disc enhancement with contrast, collections (e.g. paraspinal and psoas muscle abscesses), and even epidural abscesses may be evident.
MRI is the imaging modality of choice due to its very high sensitivity and specificity. It is also useful in differentiating between pyogenic, tuberculous, and fungal infections, and a neoplastic process.
Signal characteristics include:
- low signal in disc space (fluid)
- low signal in adjacent endplates (bone marrow edema)
T2: (fat saturated or STIR especially useful)
- high signal in disc space (fluid)
- high signal in adjacent endplates (bone marrow edema)
- loss of low signal cortex at endplates
- high signal in paravertebral soft tissues
- hyperintensity within the psoas muscle (imaging psoas sign): this finding is ~92% sensitive and ~92% specific for spondylodiscitis
T1 C+ (Gd)
- peripheral enhancement around fluid collection(s)
- enhancement of vertebral endplates
- enhancement of paravertebral soft tissues
- enhancement around low-density center indicates abscess formation (hard to distinguish inflammatory phlegmon from abscess without contrast)
- hyperintense in the acute stage
- hypointense in the chronic stage
The DWI sequence can help to distinguish between the acute and chronic stages of the disease 7.
A bone scan and white cell (WBC) scan may be used to demonstrate increased uptake at the site of infection, and are more sensitive than plain film and CT, but lack specificity. Not infrequently, a WBC scan demonstrates cold spots, a non-specific finding. The classic appearance on multiphase bone scans is increased blood flow and pool activity and associated increased uptake on the standard delayed static images 15. Gallium-67 citrate has been used with some success but is hampered by higher dosimetry and inferior imaging characteristics (high effective dose, long half-life time, poor spatial resolution) 14,15.
PET and PET-CT
F-18-FDG PET has been demonstrated to possess high sensitivity in detecting spondylodiscitis. As such, infectious spondylodiscitis can virtually be excluded by a negative scan. Dual imaging with PET-CT may thus become the imaging modality of choice, especially in patients with prior surgery and/or implants, where MRI is contraindicated or hampered by artifact 8-11. Specificity is not as high 10,16, but monitoring of treatment results is possible 9.
Non-FDG PET-CT with Ga-68 citrate (an emerging, generator-based tracer) has shown promising results in pilot studies/small series 12,13.
Possible imaging differential considerations include:
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