Harvin, a first-round pick in the 2009 NFL draft out of Florida, could be sidelined a second straight game Sunday night when the Vikings play the Carolina Panthers in Charlotte, N.C.
He missed practice Wednesday and tests this week have not provided doctors with answers about reasons for his problem.
Harvin also suffered migraines in college, but the Vikings were unaware of the severity of the problem before they made him with the 22nd overall draft pick.
"I think (doctors) attempt to allay their fears in terms of other things, worse things, by asking questions," Coach Brad Childress told the Minneapolis Star Tribune this week.
Harvin has been a chief target of quarterback Brett Favre with six touchdowns on 48 receptions for 681 yards. He also has two touchdowns on kickoff returns.