SAN FRANCISCO — Mike Matheny had begun power washing his deck last month when the symptoms returned. Headaches. Fogginess. Short-term memory loss.

All were signs the Giants catcher had yet to recover from the concussion that has sidelined him since May 31, and now for the rest of the season, following a series of foul tips that caromed off his mask with such force that they damaged his brain.

Even the lightest bit of exertion — such as climbing stairs — is too much for Matheny, who's known for having played through all sorts of aches and pains during a 13-year career.

This time, he took enough blows in succession to put him on the disabled list at the end of May. More than two months later, his brain still hasn't healed. It's tough for him to watch his teammates on TV, let alone take in a game in person.

"I walk into a room and forget why I went in there," Matheny said, sitting in San Francisco's dugout before a recent game. "I forget things I've never forgotten before. I've lost my wallet and phone I can't tell you how many times.

"Sometimes I shake my head and laugh at myself," he said. "There's nothing I can do about it."

A concussion is a shaking of the brain that causes chemical changes. Athletes suffer an estimated 300,000 of them nationwide each year, according to the Sports Concussion Program at the University of Pittsburgh Medical Center. Most recover fully, but experts believe a person who suffers a concussion is more prone in the future.

Repeated trauma of the brain can result in cognitive and neurological disabilities, even death.

"With a single concussion, you're going to recover," said Dr. Micky Collins, assistant director of the University of Pittsburgh concussion center where Matheny has been treated. "The problem is, when you have a concussion and undergo these chemical changes, you're a lot more vulnerable if you have a second before the first one is recovered."

While comparatively rare in baseball, concussions have ended celebrated careers in other sports. NFL quarterbacks Troy Aikman and Steve Young eventually had to call it quits after too many concussions. So did hockey stars Pat LaFontaine and Mike Richter.

Catchers are not the only baseball players susceptible to concussions.

St. Louis Cardinals center fielder Jim Edmonds, diagnosed with post-concussion syndrome a day earlier, missed his second straight start on Thursday. Edmonds has had bouts of dizziness and blurred vision since crashing into a wall in Chicago on June 21.