Either they burst (rupture), or they clot (thrombose).    From Wordnik.com.  [PhysOrg.com - latest science and technology news stories] Reference

I did see my primary doc during this time who said it didn’t look thrombosed, and that external hemorrhoids (which this is) do not thrombose (this threw me for a loop since I’d read all about externals thrombosing).    From Wordnik.com.  [Public announcement for hemorrhoid sufferers | The Blog of Michael R. Eades, M.D.] Reference

"We also want to see that under examination many of the things that we look at to determine if something is going to thrombose are decreased.    From Wordnik.com.  [PhysOrg.com - latest science and technology news stories] Reference