"It's not like he can stand on the side and converse," Vikings coach Brad Childress said. "He has to shut down."
Childress said Harvin might be able to rejoin the team Friday.
"Every day is a possibility," Childress said. "We've had good dialogue back and forth."
Migraines are more than just a bad headache. They can knock people out for hours, days or even weeks. Difficult to treat and predict, they often force sufferers to find a dark, quiet room to sleep until symptoms subside. Medication helps, but it's not a cure.
With his toughness as a runner, elusiveness as a pass catcher and straight-ahead speed as a kickoff returner, Harvin has given the Vikings a big lift since he was drafted in the first round out of Florida this year. He's second on the Vikings with 681 yards and six touchdowns receiving, and his 29-yard average on kickoff returns is second in the NFL. He has taken two returns for touchdowns.
Greg Lewis has played in Harvin's slot position in three-receiver sets, and Darius Reynaud is returning kickoffs as well as punts.
"Of course without Percy out there, there is going to be a gap in our offense," tight end Visanthe Shiancoe said. "But we had guys step up and fill in that void. That's one thing about our team: We've got depth, man."
Still, it's not fun to see a teammate endure such pain.
"He has a bright future," Shiancoe said. "Hopefully he'll get over it and heal from this. Our prayers are with him."
The Vikings insist their game plan hasn't changed, though Harvin is so versatile that claim is hard to believe.
"Percy obviously is a dynamic player," quarterback Brett Favre said, "and I think threatens the defense that only a few guys at the slot position can do. He's still light years away from being as good as I think he's going to be, but he's pretty darn good. It's just not the same."
Migraines are more common in women, medical experts say, with 17 percent of the world's female population estimated to suffer from them to varying degrees. Only 6 percent of men have them.
"There is clearly a genetic component to them," said Dr. Mark Mahowald, the chief of neurology at Hennepin County Medical Center, the hospital across the street from the Metrodome. "It used to be thought that they were related to stress and anxiety, and that's undoubtedly not true.
"These are a physiological phenomenon. Stress can trigger them, but only in someone who is genetically predisposed to having a migraine."
There are other known triggers, like red wine for some sufferers. Mahowald, however, said he doesn't believe head injuries are a factor in causing migraines, given Harvin's history playing a violent contact sport.
"It's not a matter of keeping a stiff upper lip and carrying on with what you're doing and toughing it out," said Mahowald, who's also a professor of neurology at the University of Minnesota's medical school. "Migraines are absolutely debilitating and incapacitating. They kind of fell you in your tracks. Once you've seen a few people with really bad migraines, you really appreciate how nice it is not to have them."
Harvin has experienced migraines since he was 10.
Harvin has landed on the Vikings' injury report several times already this season, and as a sophomore at Florida in 2007, his headaches were severe enough to sideline him for two November games and force him to spend five days in the school's health-care center receiving intravenous fluids.
"It just felt like your heart was beating in your head," Harvin told reporters then. "It's black. You can't see anything, and you just have to lay down and be in a dark room."
Harvin also dealt with nausea, vomiting and dizzy spells during that episode.
"I just had to cry myself to sleep some nights," he said then. "It got that bad, to the point where my mom got scared."
The Vikings were aware of Harvin's condition when they drafted him, Childress said, but didn't count on it being this bad.
"Just knew that it said 'migraine headaches' on the scouting report," Childress said. "Just so we're clear, it wouldn't have been a disqualifier or anything like that. But I think anybody that reads the scouting report, I don't know that we look into it and say, 'Oh, boy, to what extent?' Not to the point where he's missed a bunch of games or anything like that.
"We'll be wiser next time. Again, it's just something that we have to learn and deal with."
Copyright 2009 by The Associated Press