Glen Campbell disclosed last year that he has Alzheimer’s disease. As a society, we don’t hide our infirm anymore. Some examples ripped from the headlines: Pat Summitt coached women’s basketball at the University of Tennessee after Alzheimer’s was diagnosed; Dick Clark hosted rock-and-roll telecasts after a stroke.

So when the doctors gave Campbell the bad news, he recorded another record and hit the road. He showed up at the Birchmere on Wednesday as part of what was billed as his “Goodbye Tour.” Campbell is going from city to city, telling adoring crowds who know all about his ailment that he’s the luckiest man alive, sort of like an extended “Lou Gehrig Day.”

Gehrig, however, didn’t go to bat after giving his farewell speech. Campbell, who turns 76 next week, performs all the songs folks love him for.

He wasn’t the same guy everybody in the sold-out house remembered, of course.

It was impossible to ignore Campbell’s condition. Songs were occasionally interrupted by itching fits that caused him to scratch his head with both hands. The set list was rigged with tunes that added, as Spinal Tap would say, too much perspective. “Where’s the Playground, Susie?” had him singing, “You’re the one who’s supposed to know your way around.” “I Can’t Stop Loving You,” made famous by Ray Charles, had Campbell resigned to “live my life in dreams of yesterday.” Reprising his brilliant laid-back anthem “Gentle on My Mind,” Campbell mumbled the line about “the rivers of my memory.”

“We’re in Phoenix?” he asked at one point in the show. His son Shannon, one of three Campbell kids in his touring band, calmly told Dad no, everybody’s in Virginia and “By the Time I Get to Phoenix” was the next song scheduled.

Campbell is a different sort of player these days, too. He began his career as a guitarist for the Wrecking Crew, an esteemed studio session outfit in Southern California, and as part of the house band of “Shindig,” a TV show that introduced pop bands to the masses. (He worked alongside Leon Russell on both ventures.) But even in his prime as a frontman, Campbell never let on that he was aware of how awesome his musical gifts really were.

These days, Campbell is not a precise picker. In his solo on “Galveston,” as on several tunes, he struggled to find the right spot on the fretboard. He hit blizzards of wrong notes all around the song’s timeless melody, then suddenly worked his way to where he’d been trying to get all along. Yet his solos, imperfect as they were, were fascinating. The abstract playing gave his iconic material an aura of jazz or parody.

Best of all, Campbell appeared joyful throughout the night. His demeanor made it easier for fans to live in the moment and celebrate his presence, rather than look forward and feel sad about his prognosis. Campbell changed the last line of “Wichita Lineman” from “still on the line” to “and I’m doing fine,” and as he belted it out, the band stopped playing to add drama. That was an unnecessary move.

McKenna is a freelance writer.