I notice my thumb will spasm for days (constant rapid twitching). Then it goes away but tends to come back every few months.

I only had some minor thumb issues during the acute RSI stages.

Have you read about DeQuervain's Tendonitis ??

http://www.assh.org/Public/HandCondi...endonitis.aspx

http://orthoinfo.aaos.org/topic.cfm?topic=a00007

a whole lot more-
http://www.google.com/#hl=en&expIds=...9141da4f416ce8

Since my resection in 2008, my thumb twitches sometimes too.
Most recently I started doing Feldenkrais sessions in an attempt to help myself since regular physical therapy is a joke for TOS-(no one is educated on proper exersizes for us!)...anyway, when I do certain movements-my thumb twitches like crazy & i feel a slight nerve pull sensation. Needless to say-I stop doing that movement!

When mine flares, it is an activity that I am doing. Like today I wanted to write Holiday cards, I had to use my address book to print the envelopes instead of writing them out.

I even had them done at SAMs Club from a picture file I have and it signed our names. I was able to buy the stamps there too.
So this really limited my hand use. A few clicks printed up everything. I know it is not as personalized, but I need to reserve my hand use.

One time at work I was removing staples from stacks of documents, copying them and restapling them. I developed severe pain, twitching and senovial cyst.

I could have had a cortizone shot, but instead wore a wriste brace, took asprin as an anti inflamatory, or Advil Liquid gels for that localized pain and flare up.

I have had issues with both thumbs. I've had injections in the right thumb when it became a trigger thumb, after having an IV in my left hand -- hard to believe but it happened-left hand IV caused flare in upper body and within three days caused right thumb condition to worsen to trigger thumb. had to have 2 injections before it calmed down. I say it's from the scalenes. My cardiothoracic sugeon agrees.

splints usually helps but didn't for me.