Kooyong MP Monique Ryan has told an inquiry into Kathleen Folbigg's convictions for killing her four children one of her sons may have died from an undiagnosed neurogenetic disorder.
Dr Ryan, a paediatric neurologist before entering parliament in 2022, said Patrick Folbigg's February 1991 death at 36 weeks could be linked to epileptic seizures.
"More likely than not he had an as-yet-uncharacterised epileptic encephalopathy," she told the inquiry on Tuesday.
Patrick had been considered a normal and healthy baby before presenting to hospital following an apnoeic episode in October 1990.
"He had actually undergone more investigations than most because of the family history of what was thought to be SIDS," Dr Ryan said.
Patrick's brother Caleb died suddenly in February 1989.
Folbigg was in 2003 convicted of manslaughter over his death and murder of her other three children, who were aged between 19 days and 18 months old when they died over a period of more than a decade.
Folbigg and her two daughters, Laura and Sarah, were later found to carry a rare genetic variant known as CALM2-G114R, leading to the new inquiry.
It came after a 2019 inquiry concluded her guilt was "even more certain".
The inquiry has heard the genetic variant found in Sarah and Laura could be a likely cause of infant deaths.
Dr Ryan gave evidence at the 2019 inquiry, when she similarly said Patrick's October 1990 hospital presentation was atypical of an infant who had been subjected to an inflicted injury and some aspects were more consistent with an undiagnosed neurogenetic disorder.
Patrick was only reacting to painful stimuli and a description of him arching his back carried a contemporaneous note he "always does this", which Dr Ryan found unusual.
"I would be concerned at that point the baby has something going on from a neurological point of view," she said.
She said Patrick developed recurrent seizures after presenting to hospital, some of which were epileptic seizures.
Patrick died before any definitive diagnosis, but Dr Ryan said it was not uncommon for one to be made years after an infant's death.
"Very often we don't have sufficient time during baby's lifetimes to make a specific diagnosis," she said.
Dr Ryan agreed it was possible but did not believe Patrick sustained a hypoxic-ischemic episode due to a lack of oxygen that resulted in his later neurological decline, because he was well enough to not be in ICU and had made an early recovery before deteriorating.
Blood gas testing that would have confirmed whether or not he was hypoxic was not conducted.
Brain imaging showed initially normal results before abnormalities progressively evolved, Dr Ryan said.
Patrick also experienced episodes including "oculogyric crises", where the eyeballs fix in an upward position, which are not seen after hypoxic-ischaemic brain injuries.
They can occur in some rare neurogenetic disorders, which were not able to be diagnostically tested in 1990, Dr Ryan wrote in her supplementary report to the inquiry.
Folbigg was originally sentenced to 40 years in prison, which was reduced on appeal to 30 years with a non-parole period of 25.
She is not eligible for parole until 2028.
