Bret Michaels Update

(CNN) -- The feeling of blood hitting the brain's sensitive covering can give a person the worst headache of his life, doctors say.

That's what happens in a subarachnoid hemorrhage, and that's what singer and reality star Bret Michaels, 47, might have felt before he was taken to the emergency room Thursday. A source close to the situation said Michael described it as "like [getting] hit in the head with a baseball bat over and over again," according to People magazine.

In a subarachnoid hemorrhage, an individual bleeds into the space between the brain and its transparent, web-like tissue covering. This is also the place where spinal fluid is located.

Doctors are still searching for the cause of Michaels' hemorrhage, his road manager posted on his website Monday.

"As of now, Bret remains in ICU under 24 hour surveillance," she wrote. "Further testing this week will help locate the source of the bleeding."

About 65,000 of these hemorrhages occur in the United States every year, said Dr. M. Sean Grady, chairman of neurosurgery at the Hospital of the University of Pennsylvania. Grady was not involved with treating Michaels.

Michaels' type of hemorrhage has a wide range of outcomes: Some patients die almost instantly, some will have difficulty speaking and other neurological dysfunction and some will walk away without long-term complications.

"Sometimes the patients will experience the onset of [the] absolute worst headache of their life, and can be wide awake and alert," said Dr. Daniel Barrow, chief of neurosurgery service at Emory University Hospital, who did not treat the singer. These are often called thunderclap headaches.

In about 10 to 15 percent of cases, the cause is never found. This scenario often has the most promising survival outcome, as these patients often make good recoveries and don't tend to have a recurrence, Barrow said.

The most common cause is physical injury, Barrow said. Car crashes can lead to a subarachnoid hemorrhage in young people, according to the National Institutes of Health. In the elderly, people who fall and hit their head may experience this kind of bleeding.

For spontaneous subarachnoid hemorrhages, an aneurysm is the likely cause, experts say. An aneurysm is a bulging of a blood vessel in the brain. About half of patients with a ruptured aneurysm die from the burst itself, Grady said.

Patients with hemorrhages undergo a procedure called an angiogram, a test used to visualize blood vessels. Sometimes the initial angiogram does not pick up an aneurysm because when the bleeding stops, the clotting fills the aneurysm. That is one possibility for Michaels' case, Barrow said, but he could also fall into the category of unknown cause.

Blood thinners and blood disorders can cause this type of bleed, Barrow said, as can cocaine.

Michaels had undergone an emergency appendectomy on April 12, less than two weeks before suffering the hemorrhage Thursday. The events are likely unrelated, Grady and Barrow said.

The singer's type I diabetes also does not put him at any greater risk of a subarachnoid hemorrhage, Grady said.

The brain's covering is pain-sensitive, and blood coming into contact with it hurts, resulting in a severe headache, Barrow said.

Other symptoms may include sudden loss of consciousness or diminished consciousness, nausea and vomiting, seizure, stiff neck, vision problems and mood changes, according to the National Institutes of Health.

If there is one, it's important to identify the aneurysm and treat it because it will bleed again, Barrow said. One treatment is to surgically clip the aneurysm, excluding it from the normal circulation. An alternative is to put tiny coils of soft platinum metal in the aneurysm.

Someone in Michaels' position would most likely have his blood pressure and neurological function monitored carefully, Grady said. Doctors would watch the arterial blood flow into the brain to make sure the vessels are working as they should, he said.