On Monday, her mum Rosanne Stuart sent a message to her fans.
"Maddy went to surgery three hours ago," she wrote alongside an image of her daughter in her hospital gown.
"She is with an amazing team and hopefully will be out of surgery in about 4 to 5 hours. Thank you all for your beautiful messages. We will update you when all is over."
Maddy seemed to be in bright spirits, sharing a sweet snap the day prior, dressed in Christmas-themed PJs.
"Unfortunately I have gone into heart failure and my heart has enlarged to a point where if they do not operate it will not be able to be fixed," she shared on Instagram from her hospital bed on Sunday evening.
'"Tomorrow morning I will have open heart surgery to hopefully repair my Mitral valve that has reached a four out of four defect."
"I am sorry I have not shared this news sooner but it has been a very stressful time. I know you will all be praying for me and I am so very grateful for that. I love you all."
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Maddy seemed to be doing well.
At the beginning of the year she shared that she that her heart "was the healthiest it has ever been."
"At my last check-up my doctor said it had not enlarged, as it is no longer working so hard to maintain the three holes and leaky Mitral valve," she said at the time.