Case 35 - Discitis, Osteomyelitis
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Classic history. Fever, back pain, IV drug abuser.
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Here we see on the T1-weighted scan,
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T2-weighted scan and STIR imaging,
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the imaging features of discitis and osteomyelitis.
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We see on the T1-weighted scan dark signal intensity
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in the endplates, as well as the disc.
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On the T2-weighted scan, high signal intensity in the
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disc, as well as the endplates.
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This is not as well seen on the T2-weighted scan as the STIR
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image because with the STIR image, we suppress
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the fat of the normal vertebral bodies.
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But the edematous vertebral bodies is not able
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to be suppressed because of the fluid content.
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So you notice that the L4 vertebra and the
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L5 vertebra are both high in signal intensity
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on the STIR image. The disc is also bright in signal intensity.
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You also see irregularity of the inferior endplate of
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the L4 vertebra and the superior endplate.
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So this irregularity and erosion is yet another finding that will
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help us to distinguish between degenerative changes,
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which may cause bone edema.
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Remember,
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we have the modic type changes which are bright on
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T2 and dark on T1 in modic type 1 changes.
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But this irregularity of the endplate is one
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of the helpful findings. In addition,
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we want to look at the post-gadolinium scans.
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Let's look at the gadolinium-enhanced
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image of the same patient.
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You notice that on this gadolinium-enhanced image, you
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see marked enhancement of the intervertebral disc.
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This would not be occurring in the
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typical degenerative category.
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Once we've looked at the diagnosis and the endplates
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in the disc, we have discitis/osteomyelitis complex,
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or what some people will refer to as the
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DOI, Disguised Osteomyelitis Infection.
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We want to look at the epidural space and this
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is best seen on the sagittal and the axial scans.
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We're going to look at the axial post contrast scans.
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Here we have the axial post contrast scans.
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And as we scroll down to the level in which
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there is the abnormal infection,
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you will see that there is soft tissue anterior to the thecal sac,
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which is demonstrated along the back of the vertebral
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body of L4 representing epidural infection.
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When we look at the scan along the paraspinal soft tissue,
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you see that there is enhancement in the
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medial aspect of the psoas muscle,
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both on the left side as well as the right side,
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as well as this inflammatory epidural collection that's
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compressing the thecal sac, just at the L4-L5
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level. So this is discitis/osteomyelitis with epidural phlegmon,
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epidural abscess,
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as well as paraspinal involvement
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extending into the psoas muscles.
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In order to confirm that this is enhancement
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and not fat in the psoas muscle,
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you might want to look at the T2-weighted scans.
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On the T2-weighted scans,
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one would expect the edema of the infection to be bright
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in signal intensity as opposed to the muscle,
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which is usually very dark in signal intensity.
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So let's confirm that.
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We're going to scroll through the axial T2
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weighted scans, and as we get down to the L4-L5 level,
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we can see some of the bright signal intensity in the medial
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aspect of the psoas muscle representing the inflammation.
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Now, this is not a psoas abscess.
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We're not seeing necrosis within the psoas musculature,
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but we are seeing the inflammation
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in the paraspinal soft tissues.
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So these are the findings that we're going
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to rely on in a patient who has
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suspected fever, back pain for discitis/osteomyelitis.
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Sometimes, if I'm a little bit unsure about whether it
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represents degenerative change and modic type endplate
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changes versus an infection, I will specifically say
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in the impression of the report, "Recommend correlation
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with the patient sed rate, C-reactive protein,
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and white blood count. Because discitis/osteomyelitis usually elicits a
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pretty active elevation of the erythrocyte sedimentation
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rate and the C-reactive protein levels,
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as well as elicits an elevated white blood count.
David M Yousem, MD, MBA
Professor of Radiology, Vice Chairman and Associate Dean
Johns Hopkins University
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