There is a medical term used by veterinarians that is easily misunderstood: 'early onset'. It means the disease started early in the dog's life. Some people mistake this term to mean the disease is in early stage.
Having a clear understanding what 'early onset' means when your cavalier has been diagnosed with progressive diseases like MVD and SM is important.
I know of someone who assumed that early onset SM meant the disease was in an early stage and because of this misunderstanding neglected to get the dog the appropriate level of care, assuming there would be lots of time to deal with the SM in the future. In reality, the dog was diagnosed at a very young age and the disease had progressed to the point of extreme pain and severe deficits while the dog was still young.
I'm hoping this info will help others, so they won't have to go through a similar experience.