"I was doing a 2 somersault on the vault and over-rotated it and landed square on my head," Hadlow, who lives in Queenstown, told the Herald on Sunday.
"I opted out of surgery. I was in a halo brace for six months and in an Aspen brace for another three. They wanted to fuse [the vertebrae] but ... had they fused them, there was no way I was doing any kind of sport.
"I've been hurt a lot more outside the ring than inside it."
Hadlow said he had just sparred 24 rounds in camp with Christchurch super welterweight Bowyn Morgan, who is fighting on the card at Horncastle Arena, and was travelling back to Queenstown when he was told about the aneurysm.
"I was training well in the gym and everything was going perfectly," he said. "I completely respect the process [to screen all fighters].
"We don't know when it has occurred - I may have had it my whole life.
"I don't think this is the end of it," he said of his boxing career.
Hadlow said he will attend the event to support his fellow fighters. He also works as a personal trainer and part-time youth worker.
"I help people make the right choices with the wisdom gained from making a few wrong ones when I was younger," he said.
Hadlow's aneurysm was found because of Duco Events' insistence that all fighters on the Parker v Flores card in Christchurch undergo an MRI scan due to the recent deaths and severe head injuries that have blighted the sport.
Last month, Christchurch man Kain Parsons died after a corporate fight in the city.
Duco's David Higgins was reluctant to say he felt vindicated by the aneurysm discovery but added: "To maximise safety was an obvious step. It would have been nice if no issues had been picked up but I tell you what, I'm relieved we have taken this step as we could have had another tragedy on our hands."