The IRFU has revealed that Ulster flanker Chris Henry missed the win over South Africa due to a "temporary blockage of a small blood vessel in his brain".
Henry had been named to start against the Springboks but was replaced on the morning of the game by Rhys Ruddock, who scored a try in the 29-15 victory.
He was initially believed to be suffering from a virus or migraine but further tests have revealed the blood vessel issue, which the IRFU says will require further exploration.
"Chris Henry suffered some weakness on the morning of the South Africa game and a viral illness was suspected, he recovered quickly and the initial diagnosis was of severe migraine," a statement said.
"Further tests however have shown he suffered a temporary blockage of a small blood vessel in his brain. He is at home now and is well but needs further investigation and specialist opinion."
The 30-year-old has played 16 times for Ireland and was a key contributor to last year's successful Six Nations campaign.
Meanwhile, lock Mike McCarthy reported some symptoms of concussion after Sunday's win over over Georgia and is undergoing the return to play protocols.
Jared Payne, who made his debut against the Springboks, is continuing his rehabilitation from a sprained ankle and is due to run with the medical team later today
Rory Best is set to train fully this week and is available for selection after missing the games against South Africa and Georgia.
