Intradural-Extramedullary Tumors
- "Intradural-extramedullary" just means the tumor lives inside the dural sac but outside the cord — it pushes the cord rather than growing from it.
- The tell on MRI: the tumor splays the cerebrospinal fluid (CSF) into a sharp cap above and below it, and shoves the cord to the far wall.
- In adults, the two heavyweights are meningioma and nerve sheath tumors (schwannoma and neurofibroma). Most are benign and enhance avidly.
- The scary diagnosis to never miss is drop metastases — tumor seeding the CSF, which paints the cord and roots with lumps and crumbs of enhancement.
The spinal cord lives inside a long, fluid-filled sleeve called the thecal sac — picture a wet sock with the cord running down the middle of it and CSF sloshing all around. "Intradural-extramedullary" is just three Latin words doing the work of one idea: the tumor is inside the sock but not inside the cord. It's a passenger riding in the CSF, not a growth sprouting from the cord itself. That single location fact does most of your diagnostic homework before you've even named the tumor.
Why location is the whole ballgame
Spinal tumors get sorted into three apartments, and which apartment a mass lives in narrows your differential dramatically. Inside the cord is intramedullary. Outside the dural sac entirely (in bone or epidural fat) is extradural. Our zone is the middle floor.
The way you prove a mass is in this middle floor is by reading the CSF. When a tumor sits inside the sac but outside the cord, it widens the CSF space on its own side and caps it — the fluid forms a clean little crescent of high signal above and below the mass on T2. Meanwhile the cord gets bullied across to the opposite wall.
Quick spatial cheat sheet. An intramedullary tumor expands the cord and pinches the CSF thin. An intradural-extramedullary tumor displaces the cord and widens the CSF on its side with sharp caps. An extradural mass pushes the whole dural sac inward, tapering the CSF from outside. Read the CSF, and the lesion tells you where it lives.
The usual suspects
In adults, two tumor families own this neighborhood, and they're cousins more than strangers.
Meningiomas grow off the meninges, love the thoracic spine, and skew toward older women. On MRI they're typically broad-based against the dura, isointense to cord, and enhance briskly and uniformly. A tail of enhancement creeping along the adjacent dura — the "dural tail" — is a classic supporting clue, though it isn't unique to meningioma.
Nerve sheath tumors — schwannomas and neurofibromas — grow off the nerve roots instead. Their signature trick is squeezing through the neural foramen and ballooning out either side, giving the famous dumbbell shape (a blob inside the canal, a blob outside, pinched in the middle like a balloon dog). Schwannomas, especially the bigger ones, can go cystic or hemorrhagic and enhance unevenly. A bright T2 rim with a darker center — the "target sign" — leans toward a nerve sheath tumor.
| Tumor | Loves | Classic look |
|---|---|---|
| Meningioma | Thoracic spine, older women | Broad dural base, uniform enhancement, possible dural tail |
| Schwannoma | Any level, off a single root | Dumbbell shape, can be cystic, target sign on T2 |
| Neurofibroma | Nerve roots, often multiple | Dumbbell shape, target sign, multiplicity hints at a syndrome |
When you see multiple nerve sheath tumors, or a nerve sheath tumor in a young patient, your brain should whisper "neurofibromatosis." Multiplicity is a syndrome flag, not a coincidence.
The one you cannot miss: drop metastases
Here's where the friendly tour gets serious. The CSF is a highway, and some tumors use it to travel and seed. When a malignancy — often a brain tumor like medulloblastoma or ependymoma, or sometimes a systemic cancer — sheds cells into the CSF, those cells settle along the cord and nerve roots and grow there. We call these drop metastases or leptomeningeal carcinomatosis.
On post-contrast MRI they look like the cord and roots got dusted with enhancing crumbs: nodular bumps, a sugar-coating sheen along the cord surface, or thickened, clumped nerve roots in the lower sac. Contrast is non-negotiable here — without gadolinium, a thin coating of seeding can hide in plain sight.
Tiny CSF drop metastases can be nearly invisible on non-contrast sequences. If there's any clinical worry about CSF seeding, the study isn't complete without post-gadolinium imaging — and ideally imaging the whole neuraxis, since seeds can land anywhere along the highway.
How to look, and why it matters
The workhorse is contrast-enhanced spine MRI — T2 to find the mass and read the CSF caps, T1 with gadolinium to characterize it and to hunt for sneaky seeding. These tumors share a zip code with their neighbors, so it's worth knowing how they differ from intramedullary tumors that grow from the cord, and from extradural tumors and metastases that press in from the bone.
The reason any of this earns urgency: a mass in a bony tube has nowhere to grow but into the cord. A slowly enlarging meningioma or schwannoma can quietly squeeze until the cord can't take it, which is its own emergency — see cord compression. So the takeaway is two-part: use the CSF caps to place the tumor in the middle floor, then use contrast to tell a benign passenger apart from CSF seeding you absolutely cannot let walk out the door.