I've not found
Detailed statistics that would respond to a quantifiable query on the effectivenessof ECT for this condition have eluded my efforts.
That said, ECT is the 1st or 2nd treatment of choice for this fairly rare condition. Even that is argued to be from 8% - 38% of psychiatric patients. Ect is used by professionals who believe in it and does seem to help people who've not had relief before. Best!