It is a form of treatment that sends electric currents to the brain that actually induces a seizure. It was used way back in the day for people who were deemed "crazy," and was used pretty widespread.
Today, the electric currents, to my understanding are not as strong and are more monitored and calculated. It is still extremely controversial and has some intense side effects. Of course, most docs use it as a last resort - if at all. However, you then have the school of thought that states it is very safe, is a quick remedy and has been used for years. It is essentially "shocking" and resetting the brain. It is used for SEVERE cases of depression (usually people with STRONG suidcidal intent, plan, prior attempts) and severe depression that has accompanying psychosis. There is also literature on it being used for severe mania that is unable to be controlled with medication (however, keep in mind - meds that control mania are VERY strong and have a long history - Lithium for one). Schizophrenia, which also is unable to be treated/controlled with medication. Schizpphrenia is psychotic disorder in which people hear and see visual and command auditory hallucinations. So we are not talking run of the mill Mood D/O that ECT is used to treat. So I guess to answer the tweet - ECT could be used in a person with Bipolar - as mania and depression are what make up the D/O. Again, what is the severity and ECT is always a last resort.