Updating CRM when an external source changes

Hello all,

What would I do if I wanted to CRM to update itself whenever the data on an external system (not under my direct control) changes?

I'm integrating CRM with Trello.com, and want to tell CRM to update whenever the information I have in Trello changes. Is there anyway to do this other then constantly querying Trello and comparing the old data with the new data? is this even feasible?

I'm using online CRM2013 and a public Trello account(for now)

Many thanks

Belgy