What certification is needed for applying for the DTV visa for medical reasons?

What's required is whatever the embassy you apply at says is required. Each embassy sets their own requirement, therefore each embassy requires different things. Check with the embassy you will apply at in order to find out what they require. There have been reports of some people getting the DTV with as little as a single dentist appointment, and some people requiring a full course of treatment for some ailment. There is no answer to this question, as you'll have to check with the embassy.