A Montreal naturopath has been found guilty of involuntary manslaughter in connection with the death of one of her patients, after the Quebec Court of Appeal overturned her 2015 acquittal by a Quebec court judge.

On Thursday, Quebec's highest court found Mitra Javanmardi guilty of involuntary manslaughter and ordered her to undergo a new trial on a second charge of criminal negligence causing death.

The charges stem from the 2008 death of one of Javanmardi's patients, 84-year-old Roger Matern.

The lower court erred in its legal interpretation of what is required to get an involuntary manslaughter conviction, Court of Appeal Justice Claude Gagnon wrote in the decision on behalf of the panel of three justices.

Gagnon said "exceptional circumstances allow [the court] to substitute the acquittal with a guilty verdict."

The court also ordered the lower court to determine an appropriate sentence for the involuntary manslaughter conviction.

Acquitted in 2015 after lengthy trial

Javanmardi already been sanctioned by the province's College of Physicians in the case, before the criminal case went to court.

She was acquitted of both charges in 2015 after a 39-day trial that began four years earlier.

She was accused of causing the death of Matern, who died in 2008 shortly after being injected with magnesium.

Matern was suffering from breathing problems because of fluid in his lungs following heart surgery when he sought treatment from Javanmardi.

Naturopaths in Quebec are not legally allowed to administer treatments intravenously.

Javanmardi, who trained as a naturopath in Portland, Ore., and practised for 30 years, had treated between 4,000 and 5,000 patients at her Westmount clinic.

At the time of her acquittal, Matern's daughter, Gabrielle, said the family was saddened and surprised by the verdict, calling it a miscarriage of justice.

"My father died 16 hours after leaving her office. She performed illegal acts. How can she be totally acquitted? We just don't understand," she said.