I just recently went through almost the exact same thing. My GSD also had perianal fistulas. We had been controlling it pretty well with special food and topical steroids.

A little over two months ago she didn't eat anything for a few days. At first this was not thought to be abnormal as she would frequently skip meals. After about a week of not eating much I had our friend (our vet) come over to look at her. The diagnosis was constipation. The weather had been extremely cold so it was thought she had not been getting enough exercise to keep things moving properly. Started giving her a liquid laxative, which seemed to help some. Other than not eating she was acting perfectly normal, good energy, playful, etc.

After about another week she was eating even less. Got to the point where I had to start force feeding her a high caloric food. Then she became incontinent and would pee every time she sat or laid down. The diagnosis was a bladder infection. This went on for about another week. At this time she was still acting normal, following my kids around the yard and vacant lot next to our house and playing fetch (her favorite thing to do),

Then one Saturday she rapidly to get worse. She would go outside to pee and wouldn't come back in even though it was very cold and snowing. She started to be lethargic and was just laying all the time and wouldn't even drink water. I tired to force some water and food into her and she threw up. After that I decided to take her to the emergency room at the vet teaching hospital at CSU. I had to lift her into my truck. She could barely walk into the hospital and she collapsed just inside the doors. I had to pick her up onto a gurney. At this time I was just thinking she was very dehydrated and they would get her on an IV and all would be OK. They took her back while I filled out the paper work. Part of the admission you have to choose what extent you want them to go to if your dog needs life saving efforts. I chose moderate effort, CPR, ventilation, etc. As soon as I completed the paper work the resident Dr came out and said Gretchen was declining rapidly and she may arrest. Maybe 10-15 minutes later she came out again and said she did arrest but they were able to bring her back. They took me back to see her and told me I had a decision to make, she was on a ventilator and had an IV. They said dogs that arrest have serious issues and only a 6%-8% chance of walking out of the hospital. It was also very likely she would arrest again before they could even figure out what was going on. There really wasn't even a decision to make, I would not let her suffer. They got her stable, removed the ventilator, and brought her into a room so I could say goodbye. They administered the drugs and I held her until she was gone.

I still miss her dearly everyday.