Kyle Fisher was due to have an operation on his eye socket
A babysitter convicted of murdering her neighbour's two-year-old son was the victim of a serious miscarriage of justice, the Court of Appeal has heard.
Suzanne Holdsworth, 37, of Hartlepool, was jailed for life after a court heard she repeatedly battered Kyle Fisher's head against a wooden banister.
However, her barrister at the appeal in London said new medical evidence now made her conviction unsafe.
He said evidence pointed to Kyle's death resulting from an epileptic fit.
Holdsworth was accused of having snapped while minding the boy at her home in Millpool Close, while the child's 19-year-old mother was on a night out.
Suzanne Holdsworth was sentenced in April 2005
Holdsworth consistently denied injuring the child and claimed he had suffered a fit as they sat watching television.
But a jury at Teesside Crown Court in 2005 found her guilty of murder.
The court had heard that the impact on the child's head was similar to being thrown from a car at 60mph.
Kyle was rushed to hospital after the incident at his home in Troutpool Close, Hartlepool, in August 2004, but died two days later from severe brain swelling.
Henry Blaxland QC said new expert evidence suggested his death may have been caused by a prolonged epileptic seizure brought on by an eye injury suffered in an accident a year before his death.
He said bruising and swelling to his head could have been caused by another accident up to 10 days before the seizure.
This contradicted evidence given at the trial that he must have been injured in the 75 minutes he was with the babysitter, said Mr Blaxland.
He said doctors who gave evidence at the trial "got it wrong" and "collectively failed to diagnose" that the child had three abnormalities of the brain, two of which in particular predisposed him to epilepsy.
The crown insists the epilepsy theory is not credible.
The appeal hearing is expected to last for two or three days.