Experts described the procedure as generally low risk and almost always having positive results, but the surgery on the 60-year-old leader worried many Argentines, who have struggled to imagine their country with anyone else at its centre.
Fernandez was diagnosed with "chronic subdural hematoma," or fluid trapped between the skull and brain. This can happen when the tiny veins that connect the brain's surface with its outermost covering, or dura, tear and leak blood. As people age, it can happen with a head injury so mild that they don't remember it.
In the president's case, doctors initially prescribed a month's rest, but decided surgery was required after she complained of numbness and weakness in her upper left arm Sunday. A medical team rushed to the presidential residence, confirmed the new and worrying symptoms, and scheduled the surgery at the Fundacion Favaloro, one of Argentina's top cardiology hospitals.
While messages of sympathy poured in, the president's critics were questioning the secrecy that has surrounded her health recently. Her condition was announced in a three-paragraph statement late Saturday after she spent more than nine hours in the hospital, attributing the injury to a blow to her head on Aug. 12. It gave no details on how the injury happened, and government officials declined to comment.
That would have been the day after primary elections showed a significant drop in support for her party's congressional candidates despite her intensive campaigning.
Fernandez, who followed her highly popular husband into the presidency, is the dominant figure in Argentine politics after nearly six years in office, and now she'll be off the campaign trail just three weeks before elections that could loosen her party's hold on Congress.
As she returned to the hospital Monday in preparation for surgery, Vice-President Amado Boudou made no mention of the planned operation. He said in a speech that top officials would run the country as a team "while she gets the rest she deserves."
"What Cristina wants is for us to maintain the administration, and to carry on this project that (her late husband) Nestor Kirchner began and that Cristina has continued," said Boudou, whose popularity has sunk amid ongoing corruption investigations.
There was no official announcement about a transfer of power, but there were reports Boudou signed a document formally assuming control shortly after his speech, and government websites described him as "the vice-president in charge of the executive branch" late Monday.
Argentina's constitution provides for, but does not require, a formal transfer of power in case of health problems, said Daniel Sabsay, a constitutional lawyer. A full medical leave would require congressional approval, but short of that, "she alone decides, according to the problem she faces and her doctors' advice, if she needs to delegate some powers to the vice-president," he told Radio Continental.
The president's critics said the government should be more transparent. Saturday night's statement contradicted earlier claims about her health. During one visit in August that had been described as gynecological, a brain scan was performed that didn't find anything wrong, her doctors revealed over the weekend.
"There needs to be more information to lower the people's anxiety," said Fabian Perechodnik, who directs the Poliarquia political consulting firm.
Experts said it's not unusual for symptoms of a chronic subdural hematoma to take weeks to appear. Many patients don't even recall injuring their heads, according to the Mayo Clinic in the United States. The U.S. National Institutes of Health said symptoms can include confusion, decreased memory, difficulty speaking and walking, drowsiness, headaches, and weakness or numbness in the arms, leg or face.
Associated Press writer Almudena Calatrava contributed to this report.