Since being found unconscious in her bathtub on Saturday, Bobbi Kristina Brown has been in intensive care in a Georgia hospital, breathing with the aid of a ventilator as relatives gather by her side.
A family source told PEOPLE the 21-year-old daughter of Whitney Houston has very little brain activity and her loved ones are "praying, hoping for the best."
Neurologist Eli Zimmerman of Massachusetts General Hospital says her prognosis will depend on "how much oxygen her brain was getting" before Brown was found not breathing, face down in the tub.
"Even minutes [without air] can be grim," Zimmerman says of Brown's chances at a full recovery.
Patients whose brains have been deprived of oxygen are typically sedated and "often kept on medications that slow down brain activity to be able to keep down the amount of energy the brain requires," Zimmerman explains. "In some patients, the body temperature is dropped slightly to reduce energy requirements even more, though this is generally only done for 24 hours."
Cooling pads and blankets are often used to lower body temperature, a procedure typically enacted for cardiac arrest patients. Zimmerman adds that cardiac arrest is the most common cause for a young woman like Brown with no prior known medical conditions to be in an unresponsive state.
"Once things have stabilized, the medications that are keeping the patient sedated are decreased to be able to get an accurate assessment of how the brain is working," says Zimmerman. Positive signs include pupils dilating, eyes blinking and any type of physical reaction.
For those who survive severe oxygen deprivation, "hospitalization is usually pretty long," Zimmerman says, "as is rehabilitation, with recovery often measured in months." In the long term, patients can experience cognitive difficulties, changes in behavior and seizures.
For now, Zimmerman has some advice for Brown's friends and family.
"I always tell families not to make decisions when they're under stress," he says. "It's okay to have doctors or nurses take care of loved ones for a few days."
More on Bobbi Kristina Brown:
Bobbi Kristina Brown in Medically Induced Coma, According to Source
Do Bobbi Kristina Browns's Social Media Accounts Hold Clues About Why She Was Found Unresponsive?
Bobbi Kristina Brown: Inside the Troubled Life of Whitney Houston's Daughter
Bobbi Kristina Brown Hospitalized After Being Found Unresponsive in Tub
Stars Rally Around Bobbi Kristina Brown
Bobbi Kristina's Husband Is Not the Bad Guy, Says Source
Bobby Brown 'Inconsolable' Over Bobbi Kristina's Hospitalization
Bobbi Kristina Brown Recovery Would Be a Miracle, Says Source
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