Thirteen-year-old died suddenly after being discharged from hospital with 'flu'

A 13-year-old boy with flu-like symptoms couldn't walk when he was discharged from a Sydney hospital and suddenly died half a day later, a NSW coronial inquest has heard.

Hours after leaving Concord Hospital on August 10, 2014, Carey Alexander's worried parents took him to Westmead Children's Hospital where he died from cardiac arrest, counsel assisting Maria Gerace told the Glebe Coroners Court today.

An autopsy indicated the normally healthy and active teen likely died from viral myocarditis, she said.

The court heard the Year 8 student fainted during his seven hours at Concord Hospital on the night of August 9.

Nurse Amanda Jacobs said she was walking past the toilets at about 9pm when she heard a "thump".

She said she opened the door and saw Carey on the ground against a garbage bin.

She estimated he lost consciousness for 30 to 60 seconds.

But the teen's father thought he was out for about five minutes, Ms Gerace said.

A doctor, who diagnosed Carey with a viral infection, says he was alert, conscious and feeling better before he was discharged at about 2.30am on August 10, she said.

Yet Ms Gerace said when the teen left the hospital "he was not able to walk".

Carey was given a wheelchair to get from the hospital to the car, she said.

At home Carey's father "had to carry him into the house," she said.

By 7am the teen's hand was purple or red in colour and he complained of sharp pains in his foot.

He was taken to the emergency department of Westmead Children's Hospital later that morning where he suffered a cardiac arrest and was pronounced dead.

The inquest continues before Magistrate Liz Ryan.