Syringomyelia
- Syringomyelia is a fluid-filled cavity inside the spinal cord — think of a sausage with a long tunnel hollowed out down its middle.
- On MRI it follows cerebrospinal fluid (CSF) on every sequence: bright on T2, dark on T1, no enhancement on its own.
- It's almost always a symptom, not a disease. Your real job is hunting the cause upstream: a Chiari malformation, a tumor, prior trauma, or scarring (arachnoiditis).
- The classic clinical fingerprint is a "cape" of lost pain and temperature sensation across the shoulders, because the cavity sits dead center and clobbers the crossing fibers first.
Imagine the spinal cord as a fresh garden hose: solid, springy, doing its job carrying signals down your back. Now imagine someone bored a long thin channel down the inside of that hose and filled it with water. The hose still looks roughly hose-shaped from outside, but the core is hollow. That hollow, fluid-filled cavity is syringomyelia — a syrinx for short. The whole concept lives in that one image: water where solid cord should be.
Wait, isn't there already a tube in the cord?
Yes — and this is the part that trips everyone up. The cord is born with a microscopic central canal running through it, a leftover plumbing line from development. When that native canal dilates, purists call it hydromyelia. When the cavity carves out into the cord tissue itself, that's true syringomyelia. In real life, on a real MRI, you usually can't tell which is which, and most radiologists happily mash them together as "syrinx." I will too. Don't let the vocabulary distract you from the picture.
What it looks like on MRI
Magnetic resonance imaging is the whole ballgame here — a syrinx is a soft-tissue, fluid question, and that's MRI's home turf.
The rule to memorize: a syrinx follows CSF on every sequence. It is the water inside the cord, so it behaves exactly like the water around the cord.
| Sequence | Syrinx looks like | Why |
|---|---|---|
| T2-weighted | Bright (like the surrounding CSF) | Water is bright on T2 |
| T1-weighted | Dark | Water is dark on T1 |
| Post-contrast | No enhancement of the fluid itself | It's just fluid, not tissue |
On a sagittal image it shows up as a long, well-defined bright stripe running up and down the center of the cord, sometimes beaded like a string of sausages. On the axial view it's a round or oval pool sitting smack in the middle, fanning the cord out around it.
If a "syrinx" does NOT follow CSF on all sequences — if it's hazy, if the wall lights up after contrast, if there's a solid bump — stop calling it a syrinx and start thinking tumor. A cord tumor can build its own cysts, and those are imposters.
The cause is the point
Here's the mindset shift that turns a finding into a diagnosis: a syrinx is rarely the primary problem. It's the downstream puddle that forms when CSF flow gets disrupted. Finding the syrinx is step one; finding why it's there is the actual job.
The usual suspects:
- Chiari I malformation — the cerebellar tonsils sag down through the bottom of the skull and gum up CSF flow at the craniocervical junction. This is the most common association, which is why whenever I see a cervical syrinx, my eyes immediately drift up to check the tonsils.
- Tumor — an intramedullary cord tumor can be flanked by a syrinx, or have cystic components masquerading as one.
- Prior trauma — a "post-traumatic syrinx" can form months to years after a cord injury and slowly march upward.
- Arachnoiditis / scarring — inflammation glues the nerve roots and meninges together and throttles CSF flow.
A practical reflex: any time you spot a syrinx, image the whole neuraxis and look upstream. A cervical or thoracic syrinx with no obvious local cause is your cue to scan the brain for a Chiari malformation before you sign the report.
Why the "cape" of numbness?
This is my favorite bit, because the anatomy explains the symptom perfectly. The fibers carrying pain and temperature sensation cross from one side of the body to the other right through the center of the cord — exactly where the syrinx is parked. The cavity expands and squashes those crossing fibers first, before it bothers anything else.
The result is a person who can feel light touch and position just fine, but can't sense pain or temperature across the shoulders and upper arms — the "cape" or "suspended" distribution. The classic, slightly grim teaching anecdote: a patient who keeps burning their hands on the stove without noticing. The fibers for fine touch run in the back of the cord and get spared until much later, which is why the loss is so oddly selective.
Don't get fooled
A thin, smooth, faint central line in the cord — barely there, following CSF, no cord expansion, no symptoms — is often just a prominent central canal and means nothing. A true symptomatic syrinx tends to be wider, expands the cord, and comes with a reason. Don't scare a healthy person with a normal variant.
The other big trap is the one above: a cystic cord tumor wearing a syrinx costume. The tell is enhancement and solid tissue. Fluid that lights up after contrast, or a nodule in the wall, means you owe the patient a closer look — and possibly a very different conversation than "harmless cavity."
So the one thing to walk away with: a syrinx is water in the cord, it follows CSF on MRI, and your job is to find out who put it there.