Fears over the health of Simon Best eased on Thursday night after it was confirmed the Ireland prop should be able to return home within the next two days.
Specialists at Haut Leveque - one of France's leading cardiological hospitals - have eliminated neurological problems as the source of his condition.
They now believe that the symptoms are related to a cardiac issue, allaying concerns he had suffered a stroke, but an exact diagnosis has yet to be determined.
Best, who has been ruled out of the remainder of the World Cup, was rushed to Bordeaux's Pellegrin Hospital on Wednesday evening after developing a loss of sensation down his right side.
The 29-year-old Ulster front row also complained of a mild headache and a difficulty in speaking and underwent a series of tests, including an MRI scan, an angiogram and a CT.
Best, who transferred hospitals this morning, is comfortable and his symptoms have settled. He has been joined by his wife Katie, who arrived in Bordeaux on Thursday afternoon.
He will remain in hospital for further tests and doctors hope that he will return to Belfast within the next 24 to 48 hours.
Ireland Coach Eddie O'Sullivan said: 'This is very positive for Simon and we will continue to monitor his progress carefully. The care he has received so far has been excellent and team doctor Gary O'Driscoll has been with him throughout.'
Thursday's encouraging update has allowed a stunned Ireland camp to breath an almighty sigh of relief.
O'Sullivan, his voice cracking with emotion, was visibly shaken by Best's condition as he answered questions from the assembled media at lunchtime.
Speaking before doctors released Thursday evening's news, O'Sullivan said: 'Simon has been around for a long time. He's one of these quietly-spoken guys that everybody loves. It's a shock for everybody that something like this can happen.
'Simon was in downtown Bordeaux with his good friend Paddy Wallace, just strolling around the city centre, when he felt this coming on.
'He called the team doctor Gary O'Driscoll, we got him back to our hotel immediately and then we moved him to the hospital straight away. He's been in hospital since.'
Best, who has 23 caps, led Ireland's summer tour to Argentina and spent two seasons as Ulster skipper before brother Rory succeeded him as captain for this season.
A regular for the province since 1999, Best was also part of the 2003 World Cup squad and has been a virtual ever-present in the Test squad since.
Only a broken leg prevented him from touring New Zealand and Australia last summer.
Munster prop Tony Buckley has been called up to replace Best, who was named on the bench for Sunday's World Cup clash with Argentina in Paris.
Buckley has departed for France with Best's Ulster team-mate Bryan Young filling the vacancy on the bench against the Pumas.
Thursday evening's developments will ease tension as Ireland continue preparations for the critical showdown against Argentina.
They must beat the in-form Pumas by at least seven points and score a minimum of four tries if they are to progress to the quarter-finals.
O'Sullivan said: 'We have to get on with our preparations because the game is going to go ahead regardless of what happens.
'It's just about getting that balance right in our heads. We've got to also deal with the fact that we've to get ready for a game on Sunday.'