Enkianthus campanulatus in trouble?

I have a mature Enkianthus campanulatus that is not looking good. It's about 15' tall and it was a beautiful specimen until about two years ago. It suddenly lost a lot of leaves, mostly on the top half of the plant. The result is that the tree looks quite dead on the upper half and quite normal on the lower half. My first instinct is to prune out the 'dead' wood, only it's not completely dead. There are still a few green leaves here and there on most of the limbs of the upper portion of the plant. I've been leaving them be, hoping that the plant will bounce back. It just isn't bouncing back. Any ideas for what might be wrong with it and / or how to remedy it? I really don't want to lose one of my favourite small trees.