Margaret Moran cannot be found guilty of any criminal offence because she is too ill to be able to defend herself, the jurors trying her were told.
Mr Justice Saunders told the jury at Southwark Crown Court they would be hearing "an unusual form of trial" in which the defendant would be absent and they would only be asked to deliberate on findings of fact.
The judge said the jury would not be asked to decide whether or not Mrs Moran was guilty, but only whether "the prosecution has proved that she did the acts alleged".
He added: "She is not fit mentally to defend herself against the charges that she faces. It would therefore be unfair for her to be found guilty when she has had no proper opportunity to defend herself."
He said that if the jury decided the prosecution case was proven: "That will not result in punishment as such, but will ensure that the court makes sure Mrs Moran gets appropriate treatment."
Instead of swearing to try the defendant according to the evidence, the jurors instead swore to "try whether the defendant did the acts charged against her".
The case is being heard under the provisions of the Criminal Procedure (Insanity) Act 1964.
Peter Wright QC, for the prosecution, said psychiatrists who had examined Mrs Moran both for the prosecution and for the defence had agreed that she was suffering from "a severe depressive disorder" and was unfit to be interviewed or to stand trial.
A spokesman for the Crown Prosecution Service said the case was a finding of fact and could not result in a criminal conviction.