Baby Karlos Stephens was likely subject to "significant trauma" that caused his brain to bleed and swell before his death, a forensic pathologist has told a court.
The pathologist says injuries like this could be caused by a "blow to the head" or being shaken with "excessive force or acceleration".
Dr Paul Morrow took the stand during the trial of Shane Claude Roberts, 61, who has pleaded not guilty to the murder of 10-month-old Karlos some time between November 29 and 30, 2014.
His trial began last week in the High Court at Rotorua and the jury is being asked whether Roberts is responsible for the injuries that caused Karlos' death.
The Crown argues Roberts assaulted the baby causing the fatal injuries and was "reckless" as to whether he died or not, but Roberts' lawyer has asked the jury to question whether it was "truly a case of murder" or if Roberts was even the person responsible.
Morrow took the stand this morning saying there was an "accumulation of blood" on both sides of Karlos' brain found upon internal examination after his death and proof of old bleeding in the same area.
He said the brain was "swollen" and there was "congestion in the vessels" but no injuries to his skull.
The "bridging veins" in the brain were damaged and torn and the blood had leaked out of them into the "subdural" space, he said.
This damage could have been caused by injury or "significant trauma" like being shaken or a "blow to the head" and it would have involved "excessive force or acceleration", he said.
He had seen injuries similar to this in people who had fallen from a height or who had been involved in car crashes, he said.
Baby Karlos' brain had swollen and this had put increased pressure on his head, he said.
The pressure was so great it was more than his heart could handle, meaning the blood stopped flowing to the brain. Circulation "slowed down" and the cells in the brain began to die, he said, ultimately causing his death.
He said the swelling would have caused him to "lose consciousness and become unresponsive" before other vital systems began to "shut down".
This could take a "number of hours to develop", he said.
The bleeding found behind Karlos' eyes could have also been caused by "severe head trauma", he said.
It was clear the injury was recent due to the density of the blood found, he said.
He said there were no diseases affecting baby Karlos that would have caused the bridging veins to bleed as they did.
Previously, it had been mentioned that Karlos had been suffering from what was thought to be the flu before his death.
Morrow said there was "inflammation of the airways" found that typically related to cold and flu-like symptoms but these were "mild" and had "nothing to do with his death".
In cross-examination, Roberts' lawyer, Simon Lance, questioned the fragility of infants' bridging veins, to which Morrow agreed they could be different sizes and more susceptible to tearing for some.
Lance asked about "second impact syndrome" which Morrow explained to be when a person suffered an impact to the head then a second, less severe impact a matter of days later. It could cause an "unexpected, severe brain injury" as a result of the two.
However, Morrow said there was no evidence Karlos had suffered from this.
Justice Sarah Katz asked Morrow if it was unusual to see this sort of brain injury without any obvious external injuries and he said it was "unusual" but it had been seen.
She also asked whether this sort of injury could cause vomiting, to which he said swelling of the brain could but it also could be a result of a viral syndrome.
Earlier in the trial, there was evidence that "milky vomit" had been found around the house that Karlos had allegedly stayed at before he died.
Two police officers also took the stand today in the trial.
The trial continues.