My husband's father was terminally ill with cancer and we filed an appointment with the local service center. we brought along medical records of his father to show that the situation was critical and that his father did not have much time. The person reviewed the records and asked for clarification of where it said his father was terminal. We pointed to that part of the records and she approved it.
However, this was over a year ago and I understand that the local service centers no longer approve them.
You have to fax a request to the processing center with a letter, documentation and a request that they expedite. A friend of ours did this and they approved it, but even with the approval, it took nearly 3 weeks to receive it.
Good luck.