Last Updated April 27, 2007 8:35PM

After recovering from an emergency appendectomy two weeks ago, Poison front man and reality star Bret Michaels is in critical condition from a "massive subarachnoid hemorrhage."

Late Thursday night, Michaels complained of an "excruciating headache" and was rushed to an undisclosed hospital, where he remains while undergoing tests to determine the cause of the bleeding.

Though much is still unknown about the "Celebrity Apprentice" star's condition, Donald Trump told TMZ Monday that Michaels is in "big, big trouble, and the prognosis is not great."

Conflicting reports circulated Tuesday on Michaels' condition with US Magazine reporting that Michaels was in stable condition and improving, but Entertainment Tonight reporting that his condition was worsening and that the rocker/reality star was headed to surgery to relieve swelling around his brain.

Tests results indicate Michaels has a lack of sodium in his body that leads to seizures, according to a message posted Tuesday on his website.

The condition, called hyponatremia, is a side effect of a brain hemorrhage. 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, the Associated Press reported.

Dr. Jeffrey Thomas, a San Francisco-based neurosurgeon and spokesman for the American Association of Neurological Surgeons, said that because the cause of the bleeding is unknown, Michaels' diagnosis is only partial at this point, but he believes Michaels' bleeding could be the result of one of three conditions:

-- a ruptured cerebral aneurysm.
-- a dissection of a cerebral blood vessel.
-- bleeding of a vein.

"The most benign one of those three is bleeding from vein," Thomas said. "This is called a perimesencephalic venous hemorrhage, and if that's all it is, usually the patient gets entirely well and it never happens again. The most important job for the doctor right now is to make sure something arterial has not bled."

A tear or dissection on a cerebral blood vessel could have far worse consequences. This is because the torn blood vessel may cause the cerebral artery to narrow and even close off entirely. If this occurs, it could cause a stroke.

Thomas explained that the body's arterial system is the blood system that is under pressure from the heart to pump blood throughout the body via arteries to vital organs. Throw a wrench in that system, and it can cause the body as a whole to stop functioning properly.

Aneurysms are another worrisome condition because they can cause arterial bleeding.

"When arteries bleed, they bleed under great pressure, and pressure is not good for the brain," Thomas said. "When an aneurysm involves arterial bleeding, the death rate in patients is exceedingly high. It's a matter of urgency to know if the patient does not have an aneurysm or a dissection."

Thomas said he would assume Michaels has undergone an angiogram in hopes of finding the cause of the bleeding, also noting that the initial test sometimes doesn't show small dissections and aneurysms, so doctors will often perform the test again a few days later.

"Conventional wisdom is that a tiny aneurysm will not bleed as much as a larger one, but all neurosurgeons, including myself, would tell you they have seen very tiny aneurysms rupture," he warned.

As far as Michaels' outcome, Thomas stressed that because so much is still unknown, there are many different scenarios that could play out. If it is discovered that the celebrity has a cerebral aneurysm, he has surgical options -- traditional neurosurgery that opens a patient's skull or a procedure called coiling that involves a pinhole-size incision in the femoral artery and the insertion of a catheter that extends up into the brain.

"First thing you worry about is re-bleeding," Thomas said of potential concerns for Michaels.

The neurosurgeon also stressed the importance of a patient's clinical condition when he or she first comes into the hospital.

"If they are walking and talking and alert, that's usually a good sign; however, a high number of patients with cerebral aneurysms may appear normal at first," said Thomas.

It's been reported that Michaels has suffered from some dizziness, blurred vision and slurred speech since the onset of his bleeding.

"Anyone with blood in his head will have those types of symptoms," Thomas said." The doctors have seen the scans, so I'd assume [they] have some idea on the source of the problem."

The rocker is getting some much-needed support from many people hoping for his speedy recovery. Friends have been by his hospital bedside, and celebs are using the Internet to express their concern.

Over the weekend, Selita Ebanks took to Twitter, saying of Michaels, "Well, he's an amazing guy and very strong and positive. Let's keep the positive energy going."

Ashton Kutcher tweeted this morning, "Everybody pray for Bret."