Studies have clearly shown that there is no proof of ECT causing brain changes. They show no evidence of brain damage. For instance, it would be expected that the levels of certain brain proteins in the blood would go up with brain damage, this has not been seen. This is why a list of studies are not mentioned here. The only practical problem is that in some patients ECT needs to be given as maintenance, because medications take time to act.
Catatonia is a rare syndrome, so it is hard to do studies with adequate amount of patients.
However there is no real controversy over whether it works either. Manic patients usually do not give consent to ECT, hence the limited number of studies in this condition. Also it is sometimes tried when all medications have failed. How to review the studies on ECT on your own. You will get hopelessly confused.