She has learned to crib to release the endorphins to COPE. God love her...
I agree. She's probably learned that if she does move the way she would like to, it often hurts. For a horse to teach herself to hold back and shut down would be very stressful. Might be the main reason why she is prone to ulcers and cribs.
I don't quite understand...if she doesn't run around, then why can't you turn her out and allow her a more mentally natural lifestyle? Whether a horse bottles it inside or shows it outwardly, it is stressful to be stuck in a small space without the ability to move around. If she does not move with high intensity or get excited, then there should be no harm in turning her out.
Except for immediately after a severe episode - with muscle damage, the pain will be worse if the muscles are not allowed to move. If she is allowed to move, her body will get rid of any built up waste products from previous damage, and her muscles will function better. Even with horses with chronic RER, the recommendation is not to shut them up and not let them move. Gentle constant movement is the key for almost every condition with horses, and especially muscle myopathies.
Turn-out (if available a big advantage, the more time to blow off steam and move about the better)
Avoid training regimes like holding back at a gallop or intervals that excite the horse
Avoid stall rest or lay-up if possible, providing calm exercise if rested the day before
Everything I've read about muscle myopathies says that turnout is best and avoid stalling.
It makes sense to me now, some of the issues you've had with your mare. Hopping or getting upset when held back when going out with other horses, issues with consistency while dealing with an apparently calm horse, nagging sorenesses that were difficult to pinpoint to just one problem...
This is often the case with things like EPM or muscle myopathies, that horses get diagnosed with SI or stifle or hock issues, when these things are cropping up because of the pain caused by the nerve and muscle issues.
If your horse's condition gets managed to where she is finally feeling better you may find her to have a different personality and a different level of athleticism than before. I'd keep in mind that some horses are unable to have athletic careers with these conditions, but there are some that feel much better and can perform once they are managed correctly.