What is Sudden Death Syndrome that jailed Russian opposition leader and Vladimir Putin critic Alexei Navalny died of?

The mother of Russian opposition leader Alexei Navalny was informed on Saturday that her son had died from 'sudden death syndrome' and that his body would not be returned to his family until an inquiry was finished, according to his team.

Former lawyer Navalny, 47, was serving a three-decade term at the 'Polar Wolf' penal colony in Kharp, roughly 1,900 km (1,200 miles) northeast of Moscow, when he fell unconscious and passed away on Friday, according to the prison service.

"When Alexei's lawyer and mother arrived at the colony this morning, they were told that the cause of Navalny's death was sudden death syndrome," Ivan Zhdanov, who runs the Russian opposition leader's Anti-Corruption Foundation, said on social media platform X.

"Sudden death syndrome" is a vague term for different cardiac syndromes that cause sudden cardiac arrest and death.

It was also unclear where Navalny's body was, his team said. His mother had been told that the body had been taken to Salekhard, the town near the prison complex, but when she arrived at the morgue it was closed.

Sudden death syndrome

According to Medical News Today: "Sudden death syndrome (SDS) is an umbrella term for many biological scenarios leading to quick-onset and unforeseen mortality. The person involved often has little to no warning signs of illness. Even after death, an autopsy may not reveal obvious abnormalities."

(With inputs from REUTERS)