Etiquette when unable to attend a conference
Just tell the convener that you will be unable to present due to unforeseen circumstances.  That is enough and will be understood — there can be many reasons and it won't be a first time.  He/she will be glad that you inform him/her at all — it happens all to often that people simply don't turn up without giving any information at all.  The convener shouldn't need a doctor's note.

That being said, is any co-author travelling to the conference, or perhaps a colleague familiar with your work?  If yes, you could ask one of them if they are willing to take over your presentation.  I've done this for colleagues and although I wasn't able to answer detailed questions from the audience, it's still beneficial both for me (visibility to experts in the field) and to the first author.