This may have its origins in Rome in the 4th century AD. Depressed residents were known to slap
torpedo fish, which generate about 25V, on their heads in an effort to deal with the depression. It is known that the human brain can withstand somwhere around 140V before it completely knuckles under and fries.
As noted by user anomaly above (see last paragraph), ECT has varying degrees of success, ranging from some improvement to "the worst experience of my life." Ernest Hemingway underwent ECT some decades ago. Shortly before committing suicide, he lamented the loss of quite a bit of long-term memory.
Call me opinionated, but it seems pretty obvious to me that creating a circuit path between a person's scalp down through his brain, neck, and chest, and then juicing him up with current, is a really, really, really stupid idea. See also transorbital leukotomy and prefrontal lobotomy. Regardless of anyone's opinions, it is a matter of fact that its results are rather "wildcard" in nature.