In case you're wondering what the heck this airy-fairy acronym is when I mention EPSM or PSSM, here is a good, concise article explaining this disease. This article is a little longer firsthand account but has more information.
And if you think you don't need to know the symptoms, let me urge you to read it anyway. This disease is becoming alarmingly common in many breeds and it is a dominant gene mutation. Sometimes I hear stories of a horse being sold for weight loss, overall crabbiness and bucking going into the lope (or other problems at the lope) and I wonder whether it has EPSM. Sad that through ignorance a horse who could be a great partner with the right treatment is sold out of the family because it hurts and can't work.