Omdahl stressed that while Harket will have to live with the disease for the rest of his life, medication and brain surgery at the Mayo Clinic in the US have, he said, "softened the impact of his symptoms".
He said the star underwent an advanced neurosurgical procedure - called deep brain stimulation (DBS) - in June last year, in which "electrodes were implanted deep inside the left side of his brain."
In December 2024, Harket underwent a similar procedure on the right side of his brain, he added, which was also successful.
While he can still drive his car, Harket is less sure about his future as a singer.
"The problems with my voice are one of many grounds for uncertainty about my creative future," he said.
His biographer described how, on good days, Harket shows "virtually no sign of many of the most familiar physical symptoms of Parkinson's" but still requires a "round-the-clock effort" to balance medication, signals from the electrodes in his brain, sleep, blood sugar and his mindset - to keep the symptoms at bay.
"This isn't always successful, and is more like a never-ending rollercoaster ride," added Omdahl.
Asked by the writer if he can sing now at all, Harket replied: "I don't really know. I don't feel like singing, and for me that's a sign.
"I'm broadminded in terms of what I think works; I don't expect to be able to achieve full technical control. The question is whether I can express myself with my voice.
"As things stand now, that's out of the question. But I don't know whether I'll be able to manage it at some point in the future."
He added: "I see singing as my responsibility, and at certain moments I think it's absolutely fantastic that I get to do it.
"But I've got other passions too, I have other things that are just as big a part of me, that are just as necessary and true."