Quite common, although many are never diagnosed. My second Yorkie, Sadie, woke up twice at 4months, with hypersalivation. After some blood work, my vet suspected a shunt. She was going to be spayed at 6months, so I took her to a specialist who could do an exploratory while she was under for her spay. Sure enough, she had an inoperable liver shunt. She immediately went on a low-protein diet (I am blanking on the brand name, but I could only get it from a vet). The surgeon said she could live 6 months or a long, full life. He said many Yorkies probably have shunts that are asymptomatic, therefore never discovered.
Every day with Sadie was a gift and she passed away almost 4 years ago at the age of 11. While I know 11 is young for a Yorkie, our whole family was so grateful for the joy she brought us every day.