A former black-cab driver stabbed his 100-year-old mother in the neck then told his brother: "I've just gone mad," the Old Bailey has been told.
Michael Fitzgibbon, 62, pleaded guilty to manslaughter on the grounds of diminished responsibility and will be sentenced on September 13.
Fitzgibbon killed his mother Hannah at their home in Hainton Close, Stepney, east London, in February.
He was initially charged with murder but prosecutors accepted a plea to the lesser charge because psychiatrists agreed he had depression of "moderate severity" as well as the early stages of Parkinson's disease.
Crispin Aylett QC, prosecuting, said: "This is a case in which the defendant stabbed his mother when she was 100 years old. This occurred at a time when it seems that in fact the defendant's own health was deteriorating at a rather more rapid rate than that of his mother."
Mr Aylett said Fitzgibbon's condition was undiagnosed at the time although "some of the symptoms were apparent to family and friends".
He added: "Neither psychiatrists nor anyone else could think of any rational explanation for why the defendant would have killed his mother other than the one the defendant himself offered in the immediate aftermath.
"He told his brother on the telephone, and later police, 'I've just gone mad'."
Judge Richard Hone ordered pre-sentence reports to decide on whether to jail Fitzgibbon or take an "exceptional" course of giving a supervision order because of the defendant's state of health.
He said: "It is a terrible event but the circumstances have considerable mitigation."