Discitis-Osteomyelitis & Epidural Abscess
- Spine infection usually starts in the disc and the two endplates hugging it, then spreads — so think of it as one disease, "discitis-osteomyelitis," not two.
- The classic pattern is the opposite of degeneration: the disc lights up, the endplates blur and erode, and the marrow on either side floods with edema.
- Contrast-enhanced MRI is the workhorse. It tells you whether you have a treatable phlegmon or a drainable epidural abscess.
- An epidural abscess squeezing the cord is a surgical emergency — pus has no patience, and neither should you.
Here is a tidy little fact of spine anatomy: the intervertebral disc in an adult has no real blood supply of its own. It's an unplugged appliance. So when bacteria go looking for a home in the spine, they don't land in the disc first — they land in the vascular bone right next to it, the endplate, and then leak into the defenseless disc. That's why the infection grabs the disc and both neighboring vertebrae at once. The disease is greedy and symmetric, and that greediness is exactly how we recognize it.
One disease wearing two name tags
You'll hear "discitis" and "vertebral osteomyelitis" thrown around as if they were separate problems. In adults they're almost always the same fire, just measured at different walls of the same room. The bug seeds the endplate (osteomyelitis), then crosses into the disc (discitis), then floods the marrow of the vertebra above and below. Radiologists smush it into one mouthful: discitis-osteomyelitis. Most cases come from bacteria riding in through the bloodstream — Staph aureus is the usual suspect — so it loves people with reasons for bugs in the blood: IV drug use, recent procedures, diabetes, dialysis.
What it looks like, modality by modality
The single most useful idea: spine infection is the photographic negative of degenerative disc disease. Wear-and-tear keeps the disc dark and dehydrated; infection makes the disc bright, swollen, and angry, while it erodes the endplates that degeneration merely thickens.
| Modality | What you'll see |
|---|---|
| Radiograph | Lags by 1-2 weeks, then shows blurred, eroded endplates and a narrowed disc space. Early on it's heartbreakingly normal. |
| CT | Endplate destruction and erosion in fine detail; paravertebral soft-tissue swelling. Great for bone, weaker for the cord. |
| MRI | The star. Bright disc and marrow edema on fluid-sensitive sequences, low T1 marrow signal, and avid enhancement after contrast. |
MRI is where the diagnosis is usually made, and it's worth knowing why each sequence earns its place — a quick detour through the approach to spine MRI pays off here. On T1 the normally bright fatty marrow goes dark, because edema and infection elbow the fat out. On fluid-sensitive (T2/STIR) images the disc and marrow turn bright. And after gadolinium, the disc, endplates, and any soft-tissue collection light up.
Diffusion-weighted imaging can help when things are murky. Frank pus restricts diffusion (it's thick and crowded with cells), which can separate a drainable abscess from a more solid inflammatory phlegmon. If diffusion as a concept feels hazy, the DWI/ADC physics page explains why pus glows.
The dangerous neighbor: epidural abscess
Infection rarely respects property lines. When it pushes backward out of the bone into the epidural space — the thin fatty gap between the dura and the bony canal — you get an epidural abscess, a collection of pus parked right next to the spinal cord. This is the part that turns a bad week into an emergency.
On MRI the giveaway is a collection that enhances around its rim but not in the middle — a bright ring around a darker core of pus. Picture a jelly donut, where the wall is inflamed and enhancing and the filling is the abscess itself. That rim-enhancing pattern, combined with mass effect that flattens the thecal sac or the cord, is the finding you cannot afford to miss.
A compressive epidural abscess with new or worsening neurological signs is a surgical emergency. Pressure on the cord can cause permanent paralysis within hours, and unlike many imaging findings, this one doesn't politely wait for the morning list. Flag it immediately.
The trap that catches everyone
The single most common mimic is the thing infection most resembles: ordinary degenerative endplate change. Reactive Modic endplate changes (covered on the endplate/Modic changes page) can also brighten the marrow and confuse a tired reader at 2 a.m.
Degenerative endplate change keeps the disc dark and desiccated and leaves the endplate cortex intact. Infection does the reverse: the disc turns bright and swollen, the endplate cortex erodes and blurs, and you get enhancement plus paravertebral soft-tissue inflammation. When the disc itself is the brightest, most enhancing thing in the picture, stop calling it degeneration.
The one thing to carry out the door
If you remember nothing else: spine infection is disc-centered and symmetric — it grabs the disc and the two vertebrae hugging it, brightens what degeneration darkens, and erodes what degeneration thickens. Then ask the follow-up question every time: is there pus in the epidural space pressing on the cord? Because that question is the difference between antibiotics and an operating room.