Thanks for writing. I'm sorry, but without being able to examine your rhododendron in person, there's no way I can give your question a good answer.
Rhododendrons do have a lifespan, and if yours if extremely old, it may be coming to its natural end. If this is the case, there's really not anything you can do.
But if the problem is something environmental, so the rhododendron no longer likes the spot it's growing in, you may be able to help it along by figuring out what the issue is (too much or not enough sun, not enough water, exposure to too much wind in winter, etc.) and addressing it.
If you're afraid it may be disease, I'd bring in a sample of the foliage to the folks at your county extension office; they should be able to help diagnose any diseases if they can see the leaves in person.
---Justin, Senior Garden Editor, BHG.com