Yes, it is progressive and degenerative - ultimately leading to death in a large percentage of cases. We don't know. Research into the disease is just starting, and it is a highly specialized and small field. Current statistics show that 1 in 4,000 babies are born with this disease to some extent. As doctors learn to diagnose it better, these numbers may change.
Biopsies are currently the most commonly used tool for accurate diagnosis.
These can be skin, muscle or liver. Each has different pros and cons.