Frances Inglis is alleged to have killed her son at his Hertfordshire care home
A woman accused of murdering her brain-damaged son wrote of her fears he was in a "dark and terrifying place", the Old Bailey has heard.
Frances Inglis, 57, of Dagenham, denies injecting son Thomas with a lethal dose of heroin at a care home in Hertfordshire in November 2008.
Mr Inglis, 22, had suffered "extremely serious" head injuries when he fell out of an ambulance in 2007.
The court heard Ms Inglis was "off her head" when she wrote of her fears.
Ms Inglis was on police bail for attempting to kill her son using a similar method when she allegedly carried out the fatal attack.
She denies both the murder and attempted murder of Mr Inglis.
Detectives investigating the first incident in September 2007 found notes stored under the stairs at her home in Dagenham, east London, the Old Bailey heard.
One said: "People keep saying Tom isn't suffering. How do they know? Can they know the terror of knowing you cannot control anything anymore?
"Can they know the agony of being denied pain relief just to see his reaction?"
Ms Inglis later told police: "When I wrote this I was sort of off my head really," the court heard.
Another typed note addressed to her sister Jean said: "What if he is in a dark and terrifying place?"
Jurors have heard her son was left helpless and unable to speak after the injuries he suffered falling from the ambulance.
In a police interview after the first alleged attempt to kill her son in 2007, Ms Inglis was asked if she had anything to do with her son being given a "high amount" of drugs, which she denied.
The jury heard she told detectives: "I want my son to live. I want my son to get better. I want my son to recover."
The trial continues.