Goodness, this can be a dramatic or dark element in somebody's fic:

Locked in syndrome is due to interruption of bloodflow to the hindbrain, midbrain, and brainstem either due to a stroke, an interruption of flow from the vertebrobasilar artery system (they run through little apertures in your neck bones on the way up to neural structures).

So how about it? Your main character is paralyzed, able to breathe but not speak, able to have sensation and proprioception (often) but can't move? Absolutely frightening, if you are in the clutches of someone apathetic to you or meaning to do you harm...

Causes? Check it out: (Thank you Wikipedia for all of your hard work )
Unlike persistent vegetative state, in which the upper portions of the brain are damaged and the lower portions are spared, locked-in syndrome is caused by damage to specific portions of the lower brain and brainstem, with no damage to the upper brain.

Possible causes of locked-in syndrome include:

• Poisoning cases – More frequently from a krait bite and other neurotoxic venoms, as they cannot, usually, cross the blood–brain barrier
• Brainstem stroke
• Diseases of the circulatory system
• Medication overdose ^{[examples needed]}.
• Damage to nerve cells, particularly destruction of the myelin sheath, caused by disease or osmotic demyelination syndrome (formerly designated central pontine myelinolysis) secondary to excessively rapid correction of hyponatremia [>1 mEq/L/h])^[10]
• A stroke or brain hemorrhage, usually of the basilar artery
• Traumatic brain injury
• Result from lesion of the brain-stem

So... Easy to believe a poisonous snake from Equestria could bestow "Locked-in Syndrome" on your pony/character. Stuff that keeps you awake at night.

That is all. Carry on!

6546487 Our brain can initiate the locked-in syndrome all by themselves. They do it every night before we fall asleep. It's to prevent acting out in RL when we dream. 40% of people experienced the locked-in syndrome even after they woke up. It usually only lasts for a couple of seconds, though. I, myself, have experienced it many times. It's quite dreadful, I can attest to that.

6546512
Oh

6546512
Huh. Never knew that, and haven't experienced it (obviously) I have startled quite bad from a "sudden fall" during the beginning of a dream as I am slipping into sleep. That can be pretty funny, until you nearly break your toe on your dresser. Fortunately I've never kicked my wife

6547058

6546512
Fairly sure it's called sleep paralysis.

6690597 The symptoms are exactly the same. You can't move any part of your body.