A son stabbed his 100-year-old mother in the neck then told his brother "I've just gone mad," a court has heard.
Michael Fitzgibbon, 62, admitted the manslaughter of Hannah Fitzgibbon at the Old Bailey.
She died from a stab wound to her neck at the terraced home they shared in Stepney, east London in February.
Fitzgibbon denied murder and the prosecution accepted his manslaughter plea on the grounds of diminished responsibility.
The unemployed former black cab driver was initially charged with murder.
But prosecutors accepted a plea to the lesser charge after psychiatrists agreed he was suffering 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 of the killing, 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 action in giving a supervision order because of the state of his health.
He was remanded in custody to be sentenced on 13 September.