Houston Astros Manager Larry Dierker will undergo surgery this morning to remove blood vessels from his brain after he suffered a seizure during Sunday's game with the San Diego Padres at the Astrodome.

Gerry Hunsicker, the Astros' general manager, said during a news conference yesterday that tests indicated a blood vessel mass in the brain rather than a tumor. "His prognosis is excellent," Hunsicker said after speaking with doctors in Houston. "We have every reason to believe he will return."

Dierker, 52, collapsed and shook violently in the Astros' dugout during the eighth inning of the game. Paramedics rushed to assist him; players from both teams were stunned. The game was suspended with the Astros leading, 4-1, and will be resumed July 23 when the Padres return to Houston.

It is unknown how long Dierker will be away from the team. Bench coach Matt Galante was named interim manager.

Dierker is the third major league manager this season to suffer a serious ailment. The New York Yankees' Joe Torre underwent surgery for prostate cancer and the Florida Marlins' John Boles had back surgery. Both have returned to their teams.

"This is a serious situation," said Kevin McGrail of Georgetown University's Department of Neurosurgery in describing generally the kind of setback Dierker suffered. "If treatments are successful, the patient can return to full activity." McGrail said that hemorrhaging can be a concern in such a case but can be prevented by surgery.

The condition afflicting Dierker stems from abnormal connections of arteries with veins in the brain. Known as arteriovenous malformation (AVM), it affects about 2 percent of Americans.

Large AVMs may require a procedure before surgery called embolization. Using a catheter, doctors snake inside the blood vessels feeding the mass and inject a glue to close part of the AVM, so that surgery to remove the mass is easier.

Some doctors also are using "stereotactic radiosurgery," also known as a "gamma knife" surgery, to focus radiation directly onto the AVM and thus destroy it.

Dierker, the National League's 1998 manager of the year, was in his third season as Houston's manager. He had been an Astros announcer for 17 seasons before he was hired in October 1996. He led the Astros to Central Division titles in both 1997 and 1998, but both times the team was eliminated in the division playoffs.

Dierker told Hunsicker that he didn't remember anything after the second inning of the game, including a grand slam by Derek Bell in the sixth inning. "He said he was sorry he missed the grand slam," Hunsicker said to illustrate Dierker's upbeat attitude.

But Hunsicker also related that Dierker said he had been suffering "severe headaches" for several days, which was unusual for him. The general manager said doctors told him that on a scale of one to five, with one being the least serious, Dierker's condition was a one. "But everything is unpredictable at this point," Hunsicker cautioned. "Until they go in and do the surgery we don't really know."