The condition, called hyponatremia, is a side effect of a brain hemorrhage that has left the Poison frontman in intensive care at an undisclosed hospital.
The tests also found the 47-year-old Michaels suffered a type of stroke that causes bleeding in the fluid-filled spaces around the base of the brain.
"It presents itself suddenly as the sound of a loud gunshot or thunderclap at the back of the head causing severe cranial pain and muscle spasms," the Facebook post reads. "Many people are speculating Michaels head injury suffered from a prop striking the singer at the Tony awards last June is the cause. Additional studies are planned throughout the week to hopefully detect the exact cause of the rupture. Coupled with the fact that Michaels is a lifelong Type 1 diabetic and has recently undergone emergency appendectomy surgery while on tour in San Antonio, he will remain monitored closely by his medical team to make sure no complications occur from the diabetes."
Michaels' New York-based publicist Joann Mignano provided a written statement confirming the Web posting.
The message said doctors are hopeful Michaels will gradually improve as the blood surrounding the brain dissolves and is reabsorbed into his system.