Tethered Cord & Dysraphism
- "Dysraphism" just means the back of the neural tube didn't zip all the way shut during development. The spine seam stayed partly open.
- The split is open (an exposed, fluid-leaking defect like a myelomeningocele — diagnosed in the delivery room, not the reading room) versus closed (skin-covered, often hiding under a normal-looking back).
- A tethered cord is the punchline of many closed forms: the cord is anchored caudally and can't ride up as the child grows, so it gets stretched.
- On MRI, two findings do most of the work: a conus medullaris that ends too low (its tip below the L2–L3 disc space) and a thick filum terminale.
- Skin clues on the lower back — a dimple, a tuft of hair, a fatty lump, a birthmark — are the body politely waving a flag that says "look underneath me with imaging."
Early on, the spinal cord builds itself by rolling a flat sheet of tissue into a tube and zipping it shut from the middle outward, like a sleeping bag closing in both directions. Most of the time the zipper finishes its job. Dysraphism is the family of conditions where it doesn't — the back seam of the neural tube stays partly open. Everything downstream of that is just a question of how open, and what got stuck where.
Open vs. closed: which side of the skin is the problem on?
The single most useful sorting question is whether neural tissue is exposed to the outside world.
In open spinal dysraphism, it is. The classic example is a myelomeningocele: the cord and its coverings balloon out through a bony defect, uncovered by skin, often weeping cerebrospinal fluid (CSF). This is diagnosed clinically at birth — nobody is waiting on your MRI for the headline. The radiology often comes later, to map the anatomy and to check the brain, because open defects travel with a Chiari II malformation (a small, crowded posterior fossa with hindbrain pushed down) almost as a package deal.
In closed spinal dysraphism, the skin is intact. This is the sneaky branch — the back can look completely normal, and the diagnosis hinges on either a subtle skin marker or a kid who starts having problems and gets imaged. These are the ones that land on a radiologist's screen as a real puzzle.
A handy mnemonic of physiology, not magic: open defects leak, so they declare themselves loudly and early. Closed defects are covered, so they whisper — and the whisper is usually a skin finding on the lower back.
The tethered cord: a cord that can't grow up
Here's the bit of developmental trivia that makes everything click. In a fetus, the spinal cord runs the whole length of the canal. As the spine grows faster than the cord, the bottom of the cord — the conus medullaris — appears to "ascend," settling by birth at roughly the L1–L2 level. Picture an elevator that should rise to the first floor and park.
In a tethered cord, the bottom of the cord is anchored down — glued, snagged, or weighed down — so it can't make that climb. The elevator cable is stapled to the basement. As the child grows, the cord gets stretched like taffy, and that chronic tension slowly damages it: leg weakness, foot deformities, bladder trouble, scoliosis, back pain.
What does the anchoring? Often a thickened, fatty filum terminale (the filum is normally a wispy thread tethering the cord tip to the sacrum; here it's a fat rope), or a lipoma fused to the cord, or scar from a prior open defect that was repaired.
"Tethered cord" is a consequence, not a single disease. Your job is to spot the low cord and then name the thing tethering it.
What the MRI is actually showing you
MRI is the workhorse here, because it shows the cord, the filum, fat, and fluid all at once. Two measurements carry most of the diagnosis:
| Finding | Normal | Tethered cord |
|---|---|---|
| Conus tip level | At or above the L2–L3 disc (usually ~L1–L2) | Sits abnormally low (tip below L2–L3) |
| Filum terminale thickness | Thin, wispy thread | Thickened (often fatty) |
The bright signal of fat on a standard T1 sequence is your friend — a fatty filum or a lipoma lights up white, and fat-suppressed sequences let you prove that bright thing really is fat by watching it go dark. Always glance at the skin surface too: a dermal sinus tract (a little channel from the skin to the canal) is both a tethering culprit and an infection highway.
Look at the back. Seriously, look at the back.
The cheapest screening tool predates MRI by a few million years: the skin over the lower spine. Closed dysraphisms love to advertise with a midline lumbosacral stigma — a deep dimple above the gluteal crease, a tuft of hair, a soft fatty bulge, a birthmark, or a little skin tag. None of these guarantees anything underneath, but they're the reason a baby gets the ultrasound or MRI that finds the tethered cord.
Not every low dimple is a crisis, and not every "low conus" is pathologic in isolation — there's a normal range and some borderline cases. But a simple sacral dimple low in the crease is usually benign, whereas a dimple higher up, off midline, or paired with another skin marker deserves imaging. Don't let a normal-looking back talk you out of looking when the clinical story (or a second skin finding) says otherwise.
Why it matters, and when to image
In infants up to a few months old, the back is still cartilaginous enough that ultrasound can peek straight through and see the cord pulsing (or not) — a great first-line, no-sedation screen. After the bones ossify, MRI takes over and stays the reference standard. The whole point of catching a tethered cord early is that the damage from chronic stretch is the kind you want to prevent, not treat after the fact; surgical untethering aims to release the anchor before the deficits set in.
If you want the broader scaffolding for reading these studies, start with the approach to spine MRI and the spine anatomy and levels page, since "where exactly does the conus end" is a question you can only answer if you can count vertebrae confidently. And because a tethered cord and a fluid cavity in the cord can travel together, it's worth knowing syringomyelia too.
The takeaway is small enough to carry in one hand: dysraphism is an incompletely closed neural tube, the tethered cord is the cord that got anchored and stretched because of it, and the two MRI findings that crack the case are a conus that ends too low and a filum that's too thick.