Followup to my swollen arm issue...now I'm just confused...r/o

I posted a while ago about my right arm swelling on longer bike rides and promised a follow-up, so here goes.

As mentioned here, my doc suspected Thoracic Outlet Syndrome.  I had a venogram done this morning and it came back normal.  

The doc was very cool.  He was a cyclist himself.  However, he added this caveat - 'You might need to find a different way to ride, ie - no aerobars.'  [<- I honestly thought he was kidding at first, but no] While the test came back normal, something is causing the swelling after an hour or two on the bike and the swelling isn't a good thing. 

The risk would be a blood clot, but there's really no way to gage a true value of the risk (ie - is it a 5% risk, or a 60% risk)  It seems low to me, b/c I have no history of clots and no family history.

So now I'm just dwelling on it, trying to weigh the options.  Giving up Ironman would be incredibly tough, but I'm a pretty risk adverse person. 

Anyone have any thoughts?  Would you quit riding for a 'possibility', even if the tests were normal? 

Thanks