What can't treat deep depression?

Nothing.: Depression is caused, and treated, in several ways. Therefore any of the treatments for mild depression can be of use in treating more severe depression, don't give up hope! however that said, the most effective treatments for severe, refractory, depression include ECT and some of the older antidepressant medications whereas the newer medications or therapy may be less helpful.

Depends: 15% of depressions are treatment-resistant. This really means that the suffering patient has not yet tried the right treatment to address their specific depressive syndrome. For patients who have not responded to talk therapy or medication, transcranial magnetic stimulation (tms) should be considered. If suffering from rapidly progressing symptoms, then ECT may need to be considered.