Vervoot won gold and silver in the 100m and 200m T52 sprints in London four years ago. Confined to a wheelchair, she said the deterioration meant she would considering euthanasia after returning to Belgium.
The Daily Mail said she was tormented by pain every day and quoted one of her diary entries.
"Fortunately I had a better night even though I had a moment of crisis for about 45 minutes. With me this morning, it felt like I had slept only an hour, but that was not the case," she wrote.
"My body is just exhausted from all that... I let the nurse this morning give me a shot of morphine and this morning went purely on character to the training. I literally knocked all my fears and frustrations out of me."
She competed in the 2007 Hawaii Ironman and also played wheelchair basketball, after a rare disease paralysed her in 2000. Euthenasia - requiring the written consent of three doctors - was legalised in Belgium in 2002.
The story is being widely reported around the world and the Sun quoted her as saying: "I want everyone to have a glass of champagne in hand, and a thought for me. ... There is a chance to medal, but it will be very hard because the competition is very strong. Then we'll see what life brings me, and I try to enjoy the best moments.
"When I sit in my racing chair, everything disappears. I expel all the dark thoughts, I fight fear, sadness, suffering, frustration. That's how I won the gold medals."
The Sun also said she had already made plans for her own funeral. The Games begin tomorrow and her first event is a 400m race.
