It is defined as the combination of a plasma-cell proliferative disorder (typically myeloma), polyneuropathy, and effects on many other organ systems.
It was about 2003 when Mr Garvan - who grew up at Elephant Hill, near Waimate, and emigrated to Canada in 1985 - noticed some "weird and wonderful things" were starting to happen to his body.
He believed 20 years of living in North America had got to him: eating the wrong foods, working all the time training and shoeing horses, and a lifestyle that was "just go, go, go".
"I was 6ft tall and bulletproof, and nothing was going to happen to me," he told the Otago Daily Times during a visit to New Zealand.
His body shut down, and he spent a year and a-half in a wheelchair.
His weight plummeted to 65kg and, after coming out of radiation treatment, he was so weak he could just manage to lift 0.9kg.
He had gone from "hero to zero - like boom".
Mr Garvan was told the survival rate was anywhere from 145 days to nine years, but the tenacious New Zealander was not going to give up. Along with not wanting to leave his partner Lynne alone, he wanted to ride his polo pony Russell again.
"I had to ride again. I knew I hadn't finished. There was still work to be done."
Dr Norman Schachar, a professor of surgery at the University of Calgary, who treated him, in 2006, told the Calgary Herald Mr Garvan was a "stubborn, pig-headed Kiwi who made up his mind that he was going to get better, and that always helps, no matter how bad you are. I tell patients never to give up hope and he never did".
He had never seen anything like Mr Garvan's recovery.
"As far as I am concerned ... this is miraculous," Dr Schachar said.
Mr Garvan began walking again in 2006, initially with the aid of a walker. And he got back on Russell. Each year, he has made improvements, although he has severe nerve damage from his elbows to his hands and from his mid-thighs to his feet.
During his illness, which he tackled with his own "Monty Python"-style sense of humour, he said he learned much about the deficiencies in the medical profession.
While he acknowledged he could have despaired during those dark times, he said he had remained positive. Becoming bitter "doesn't get you anywhere".
"It's been a blessing, really. It's taken me to a level I would have never been [otherwise]. I'm enjoying life because every day is special."
Mr Garvan has become passionate about health and has also become an advocate for alternative medicine. He urged people to be vigilant about their health.
"We don't do anything about our health until we crash and burn. It happens for a reason. The body does tell us what's going down; it really does."
Getting back in the saddle had also proved to be therapy for him and he believed equine therapy was the most underrated and underused tool for rehabilitating people.
Mr Garvan, who lives at Okotoks in Alberta, just south of Calgary, keeps busy coaching polo and developing a stockmanship programme, saying people have lost the art of looking after stock.
