Mayor Bob O'Connor was downgraded to serious condition yesterday after tests revealed seizures and an infection.

The mayor was in the intensive care unit of UPMC Shadyside last night after 30 minutes of surgery in which a drain to remove fluid buildup from his brain was replaced with a new drain coated with antibiotics.

Mr. O'Connor was diagnosed with primary central nervous system T-cell lymphoma last month. His condition was downgraded yesterday afternoon.

Electroencephalogram, or EEG, tests conducted Sunday night and yesterday morning indicated seizure activity. Spinal fluid tests, neurological examination and a CT scan suggest that he has an infection of the spinal fluid and the drain that was inserted three weeks ago to alleviate elevated fluid levels.

Mr. O'Connor was being treated with anti-seizure medications and antibiotics.

A course of intensity-modulated radiation therapy, or IMRT, that he was expected to undergo this week has been put on hold, a hospital spokesman said. From Monday to Friday last week, he received a daily 20-minute treatment, which doctors hoped would shrink lymphoma masses that did not respond to high-dose chemotherapy.

Mr. O'Connor's medical team would not comment about his level of consciousness or the severity of the seizures.

"It doesn't sound good, obviously," said Dr. Michael Robertson, director of the lymphoma program at the Indiana University Cancer Center in Indianapolis, of the latest setback.

But he noted that a drain infection, which is not uncommon, is potentially treatable and the mayor's condition could improve. Certain bacteria associated with medical devices may have suggested to the mayor's doctors that his infection was linked to the drain, Dr. Robertson added.

The mayor's seizure activity could be due to infection, but it might also be related to a worsening of his cancer, he said.

"There's no way to tell at this point what is leading to seizure activity," he said.

"Anybody who has a brain tumor is at risk of having seizures," said Dr. Marshall Lichtman, executive vice president of research and medical programs at the Leukemia and Lymphoma Society and a professor of medicine at the University of Rochester. "That wouldn't be a surprise."

That said, the sudden presence of seizures and the development of an infection represent "a very complicated situation," he said. Doctors will want to address the infection very quickly and minimize any pause in radiation therapy, he said, so the tumors don't get a chance to grow significantly.

Seizures alone aren't necessarily a grave sign, said Dr. Virginia Stark-Vance, a neuro-oncologist in the Dallas-Fort Worth area who has treated and researched central nervous system lymphoma. But to her, the trajectory of the mayor's treatment, from the failed chemotherapy regimen to radiation, to the insertion and replacement of a shunt to drain fluid, is troubling.

"I would be concerned with the number of complications he's suffered," she said.

Infection and surgery to remove a shunt can delay radiation treatment, Dr. Stark-Vance said. "It's much harder to heal a surgical wound when you are also conducting radiation therapy."

In addition, the IMRT the mayor has been receiving isn't likely to cure the lymphoma. Because the beams are so focused, there's "a lot you're not treating" in the rest of the brain and the spine, she said. "There are going to be other tumors that pop up in the future."

There are still other treatments the mayor's doctors haven't tried, Dr. Stark-Vance said, so there's hope. If she were treating the mayor, she said, "I would still try to be as aggressive as we could."