The 87-year-old actor - who cancelled a public appearance in New York next week due to fatigue and other symptoms - has yet to find a diagnosis for the problems he is suffering, his spokesperson Bob Palmer has said.
And the Mary Poppins star claims he has been undergoing tests for the past seven years but so far everything has come back clear.
He wrote on Twitter: "My head bangs every time I lay down. I've had every test come back that I'm perfectly healthy. Anybody got any ideas?
"It has been going on for 7 years. I've had every test you can think of (CAT Scan, MRI, Spinal tap, etc) Any advice?"
When a follower suggested he undergo a sleep study, he replied: "I've already done that. The only thing they discovered is that I don't snore."
Van Dyke is currently resting at his Malibu home and has been advised not to fly until he is given a definitive diagnosis, which his spokesperson said is "frustrating" for the star.
He said: "This is a guy that never gets sick ... he doesn't smoke, drink or pop pills.
"It's very frustrating that doctors cannot find out what is going on."
Mr. Palmer said Dick is doing "OK" but "the fatigue factor has become acute".
Van Dyke - who is married to Arlene Silver - was due to be honoured in New York with 92Y's first ever lifetime achievement award for bettering humanity through comedy.
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