A Japanese court on Tuesday acquitted a former assistant nurse who was convicted of murder and spent 12 years in prison after the 2003 death of a patient, saying at the conclusion of her retrial it is highly likely the man died of natural causes.

Mika Nishiyama, 40, was exonerated by the Otsu District Court, with presiding Judge Naoki Onishi saying there was "no foul play" in the incident and that the 72-year-old patient could have suffered "a fatal irregular heartbeat or a lack of oxygen after not having his sputum sucked out."

"I'm very happy. My parents shed tears of joy," Nishiyama said at a news conference in Otsu, the capital of western Japan's Shiga Prefecture. She celebrated the ruling by taking the hands of her 69-year-old mother Reiko, who was seen sobbing at the courthouse.