B767 Captain experiences debilitating vertigo on a domestic flight and elects to turn over his seat to a deadheading First Officer. Flight continues to destination after declaring a medical emergency.

Narrative

I was the Captain and pilot not flying (PNF) for this leg. Prior to departure I had the very; very slightest onset of a sensation of vertigo. I was able to walk normally and there was no impairment to my ability to function as a pilot. After takeoff; the vertigo escalated at a truly amazing rate. After some twenty minutes; I handed off the PNF duties to the First Officer thinking that I was incurring some form of food poisoning. At a certain point; the vertigo was so extreme that when I went to stow my headset to my immediate left; I was 'thrown' hard to the left. My vision became jittery owing to the inner ear/vision conflict. I was soon using oxygen with my forehead braced against the glare shield. The sensation of vertigo was now extreme; like the worst ride in a vertigo trainer times five. Approximately an hour and a half from destination; I declared myself incapacitated. Utilizing the help of my number one Flight Attendant; I was able to shift from the left seat to the right jump seat. I then summoned a B767 First Officer who was in the cabin as a passenger and installed him in the left seat. I made a few constructive suggestions to the crew and let them proceed. A medical emergency was declared at some point. I urged the crew to proceed to destination as other than the vertigo; I had no other symptoms. On descent; I incurred rather violent vomiting. Utilizing my Captain's authority; I admitted a nurse to the flight deck. She took my vitals and administered a cold compress to my neck; which helped alleviate the discomfort an appreciable amount. The nurse then left the cockpit. I urged the crew to coordinate a tow-in at the gate in the interests of safety. Although the tug was not waiting; little time was lost getting us to the gate and paramedics removed me from the flight deck. Several points come to mind looking back on this event. First; never having experienced vertigo; I had no idea a medical event like the one I experienced could occur. Had I even the slightest idea; we would not have departed. I might have declared myself incapacitated earlier than I did. I don't believe anything bad became of it; but the crew would have had perhaps thirty more minutes to organize themselves. As it was; they did very well with the time (approximately 1 hour 20 minutes) allotted to them. The role of the number one Flight Attendant cannot be understated. He was instrumental in getting me out of the left seat and into the jump seat; summoning medical help; providing a walk-around bottle; etc. Good CRM of the entire crew was paramount to the positive outcome of this event.