According to the source, Michaels, 47, said the headache that prompted him to call for an ambulance felt "like [getting] hit in the head with a baseball bat over and over again."
Often referred to by doctors as a "thunderclap headache" because of the intensity and suddenness of the pain, such headaches are a tell-tale symptom of the kind of brain hemorrhage that Michaels suffered.
The source close to the situation adds, "Bret is still in ICU. He still has the headache and is heavily sedated."
A statement on his fan site noted that doctors are still searching for the source of the bleeding.
"In some patients, doctors never find the source," says Dr. Keith Siller of New York University's Comprehensive Stroke Care Center. The bleeding could be very small, or the rupture itself can "cure" the source of the problem, says Dr. Siller.
If no source of an aneurysm or an abnormal vein-artery connection known as an AVM is found, "you basically just wait for the symptoms to go away," says Dr. Siller. The patient typically remains under close observation, as Michaels is, so doctors can monitor for possible complications.
But if the patient is able to speak, "they're on the better end of things," says Dr. Siller, noting that physical therapy and speech therapy are often required to make a full recovery.
On TV's Celebrity Apprentice, on which Michaels is a participant, Sunday night's episode carried a post-script. Accompanying slow-motion footage of Bret carrying luggage across a tarmac, a voiceover said, "Everyone here at NBC wishes Bret Michaels a speedy recovery."