
I've known three dogs with this; one with the version where opening the mouth is difficult, and two (mother and son) where the muscle wasting on the head occurred for no apparent reason and without any other symptoms, at the age of 8. One of these lived to the age of 14 with no ill-effects from it (other than looking prematurely aged) and the other is still alive and thriving at the age of 12 (again, looking prematurely aged). In their case it seems to have a hereditary element, although no other puppies have been reported as affected.
The other one was treated with steroids and now has no trouble eating - again just has the prematurely aged appearance due to the wasted muscle on the head, which seems to be irreversible.
Information.