There are a lot of different abnormal postures out there, "abnormal posture" being defined as "the way that your body is holding itself, involuntarily, that clues us in that something is not right with you." If you're sitting leaned over to one side at about 45 degrees, that's an abnormal posture. If you're standing, kind of hunched over, with your arms akimbo, that's an abnormal posture. Those aren't as bad, though, as two of the most basic postures, decorticate and decerebrate. We call them "responses" or "poses", by the way, because they show up in response to external stimuli.
Decorticate posturing, also called flexor response or mummy pose, means that you've got fairly substantial damage to a number of structures in the brain that normally keep you from getting stuck in a flexed position. (You want me to be more specific? Fine, but don't start complaining when I start using terms like rubrospinal tract and vestibulospinal extension tracts.) You're lying down with your arms bent and your hands in fists on your chest, like a mummy.
Decorticate posturing is a bad deal, because it usually means damage to the thalamus (without which you really can't get along), the midbrain, and the cerebral hemispheres, which is where all your thinking, speaking, and remembering goes on. Essentially, you could "recover" from the sorts of injuries that would lead you to show decorticate posturing, but you'd not do much more going forward than stare and twitch.
Decerebrate posturing is also called extensor posturing or decerebrate rigidity. Instead of folding your hands onto your chest as you would in decorticate posturing, your hands are straight down by your sides. Your jaws are often clenched. Sometimes this only happens on one side, or only happens in the arms.
Decerebrate posturing is considered even worse than decorticate, for a couple of reasons. First, it shows up in response to a brain stem injury. Brain stem injuries are those that are going to have negative impactfulness, as the government guys say, on your ability to breathe, regulate your own blood pressure, and continue on living. Second, decerebrate posturing that shows up after a person's been doing decorticate posturing for a while indicates that the brain has just herniated down through the brainstem and the game's over.
Now, you can go back and forth from decorticate to decerebrate, but eventually, if you're not treated promptly and things like your intracranial pressure and brain injuries aren't stabilized, you're going to go all decerebrate on your own self and things will get very bad indeed.
Interestingly, I've seen quite a few cases where the patient has gone from decorticate to decerebrate posturing in a fairly short period of time. All of those cases were back when Sunnydale had a run on people with Creutzfeld-Jakob Disease (aka "Mad Cow", but without a cow involved--the normal CJD is a genetic thing). Everybody would start out a little disengaged, a little forgetful, a teeny bit aphasic. They would then progress to decorticate posturing in reponse to stimulus, then on to a really marked decerebrate posturing with the oddest, most eerie catlike meowing noises as a response to even gentle touch.
That last sentence just led me to sit, chin in hand, thinking about that two-year stretch of lots of CJD cases. And to be very glad I'm not doing as much of that any more. And to miss Sunnydale a bit.
Anybody got any fun ideas for Auntie Jo's next Oversimplification of Neuroscience lecture? Leave 'em in the comments!