Which side for you?

Most people with cluster headaches notice something strange and unsettling:

Every single attack hits the same side — the same eye, the same temple, the same path of pain.

And it almost never changes.

Here’s why.


1. Cluster headaches come from a one-sided brain circuit

Cluster headaches aren’t caused by random blood vessel changes or tension.

They come from a very specific “loop” that involves:

  • the hypothalamus (your internal clock)

  • the trigeminal nerve (the pain pathway)

  • the parasympathetic system (tearing, runny nose, droopy eyelid)

In cluster headaches, this loop activates on one side only.
Once your brain picks a side, that loop becomes the “default path” for every attack.

That’s why your CH is always left-sided or always right-sided — not both.


2. The hypothalamus fires on one side only

Brain scans of cluster patients during attacks show:

Only ONE side of the posterior hypothalamus lights up.

This is the literal “cluster generator.”

That same side corresponds to:

  • the eye pain

  • the temple drilling

  • the tearing

  • the blocked nostril

  • the flushed or drooping eyelid

So if your attacks are always left-sided, that means:

👉 Your left hypothalamus is the one misfiring.


3. The trigeminal nerve on that side becomes hypersensitive

Cluster cycles leave the trigeminal nerve “primed” even between attacks.

The affected side becomes:

  • quicker to activate

  • more inflamed

  • more reactive to triggers

  • more likely to fire during REM sleep or alcohol exposure

This is why the same pathway lights up every time.


4. Your autonomic system is also one-sided

The parasympathetic nerves behind the eye create symptoms like:

  • tearing

  • nasal congestion

  • facial flushing

  • droopy eyelid

Those nerves are also unilateral, and in CH, they consistently activate on the same side as the pain.

Once they’re sensitized, the side stays locked in.


5. Structural differences play a role

Cluster patients often have tiny anatomical differences between the two sides, such as:

  • slightly different nerve branching

  • variations in sinus pathways

  • different vascular responsiveness

You can’t see these on an MRI, but they change how reactive each side is.

The more reactive side becomes the “cluster side.”


6. Once a side is established, it almost never changes

Cluster headaches are unusual among headache disorders because the lateralisation (side involvement) is incredibly stable.

Statistics:

  • ~85–95% of people stay on the same side for life

  • ~5–15% switch sides occasionally

  • Only 1–2% switch permanently

Your brain essentially learns the pain loop, and it keeps using it.


Quick Summary

Cluster headaches always hit the same side because:

✔ The hypothalamus activates only one side
✔ That side’s trigeminal nerve becomes hypersensitive
✔ The autonomic nerves on that side overreact
✔ The pain/autonomic loop becomes a closed circuit
✔ Your brain “locks in” that side permanently

So if your attacks are always on the left or always on the right —
that is completely normal for cluster headaches.

It’s one of the diagnostic hallmarks of the condition.