The SIADH-EDS Combo and Why Your Brain Freaks Out Faster

If you’re hanging out in the EDS world long enough, you’ve probably heard the acronym “SIADH” whispered (or shouted) in support groups at least once. Syndrome of Inappropriate Antidiuretic Hormone secretion. 

It sounds like a mouthful, but basically it means your body is hoarding water like a camel that misread the weather forecast, even when you’re already over-hydrated. 

And when you have Ehlers-Danlos Syndrome, that water-hoarding party can turn into a full-blown brain emergency way quicker than it does for most people.

Let me break it down in plain English.

What happens in SIADH?

Your kidneys are supposed to get a memo from the hormone ADH that says, “Hey, hold onto water” only when you’re dehydrated. In SIADH, that memo gets sent 24/7, even when you’re drowning in fluids. 

Result? Too much water in the bloodstream → the sodium concentration in your blood gets diluted → hyponatremia (low blood sodium).

Low sodium → water rushes into your brain cells to try to “balance things out” → brain cells swell → hello, cerebral oedema.

For the average person, symptoms usually start when sodium drops below about 125–130 mmol/L: headache, nausea, feeling spacey. Drop it further or faster and you get confusion, vomiting, seizures, or worse.

So why does it feel like EDS brains lose the plot at higher sodium levels?

Because most of us are already playing life on hard mode when it comes to brain resilience. Here’s why the same degree of hyponatremia can hit us like a truck:

1. Dysautonomia / POTS is practically standard issue in hEDS
   The blood pressure and cerebral blood flow regulation is already wonky. Even a small shift in fluid or electrolytes can make the brain freak out.

2. Mast Cell Activation Syndrome (MCAS) loves to tag along
   MCAS can trigger or worsen SIADH all on its own (histamine and other mediators tell the body to retain fluid). Double whammy.

3. Cranio-cervical instability, Chiari, or tethered cord
   If there’s less “extra room” in your skull or spinal canal, even mild brain swelling feels dramatic — instant pressure headaches, visual changes, near-syncope.

4. The connective tissue is… special
   Some researchers think the blood-brain barrier and blood vessels in hEDS might be a bit more “leaky” or elastic, so fluid shifts happen faster / more extremely.

Some patients  start getting neurological symptoms at a sodium of 132–135, numbers that would make a regular hospital doc shrug and say “totally normal.”

The symptom list we all know too well"

- Sudden “I can’t think” brain fog
- Crushing headache that nothing touches
- Nausea that comes out of nowhere
- Feeling like you’re drunk without the fun part
- Wobbly legs, tremor, or even seizures if it drops fast

What to do if you suspect SIADH

1. Get your sodium checked (serum sodium + serum osmolality + urine osmolality/urine sodium). Don’t let anyone brush you off with “it’s just your EDS.”

2. Fluid restriction is usually the first treatment (sometimes down to 500–800 ml a day — yes, it feels awful at first).

3. Some people need salt tablets, demeclocycline, tolvaptan, or other meds.

4. Correct the sodium slowly. Going too fast risks osmotic demyelination syndrome (a different kind of brain injury).

Most importantly: if you have EDS + new or worsening brain symptoms + you’re peeing very little despite drinking tons, go to the ER. Bring a note from your specialist if you have one. 

You don’t always “look sick” until you’re really sick. You’re not dramatic. You’re not “just anxious.” Your brain is literally swimming and it’s trying to send up every flare it has.

Sending gentle, non-swollen-brain hugs to everyone fighting this particular zebra complication right now. You’ve got this, and you’re not alone.♡

P.S. Always work with a doctor who understands the EDS/SIADH overlap. They do exist, and they’re absolute superheroes when you find them.

you can catch early warning signs of SIADH at home with a urine specific gravity sticks (a.k.a. “urinanalysis dipsticks”)

You can buy these on Amazon, eBay or at any pharmacy (search “urine test strips 10-parameter”).