This information was written on an internal department form, but not on the consent form given to the mother.

The risks included airway oedema, or swelling, and "loss of airway".

She estimated that it was the practice of 80 per cent of doctors to write the risks involved with anaesthesia on the internal document.

Cheung was placed in an intensive care unit after undergoing surgery for misaligned neck bones on August 4, 2011.

He died there on August 22.

The boy's mother, Cheng Miu-wah, earlier testified that she had been told days prior to the surgery that it was "zero risk".

But one of the doctors in the surgery, Professor Dachling Pang, who was invited from the United States, denied he said that. Pang is chief of the Regional Centre for Paediatric Neurosurgery at Kaiser Permanente hospitals in northern California and paediatric neurosurgery don at the University of California.

Sham said at the inquest the risk of "loss of airway" during the procedure was high as the boy's neck would not be in the optimal position for breathing.

A tube inserted to help him do so could move out of place and need to be reinserted.

The tape fixing the tube in place could also come loose.

Also yesterday, a pathologist said he had concluded that the direct cause of death was hypoxic brain injury, and that an underlying antecedent cause was his condition following the surgery. "Prior to his death, the brain was deprived of oxygen," Pang Chun-yin said.

He wrote in the autopsy report the teenager's brain showed evidence of ischaemia - where an insufficient amount of blood is getting to the brain.

The inquest continues before Coroner Philip Wong Wai-kuen and a jury of five.