He has had no further episodes and suffered no long-term effects and although the condition has not been attributed to his participation in rugby, his medical experts have advised against resuming the intense levels of training and playing required for a professional career in rugby union.
Best has represented and captained Ulster, Ireland 'A' and Ireland in a career that also saw him capped at international level for the Ireland Schools, Under-19, Under-21 and Under-23 sides.
Hailing from Poyntzpass in County Armagh and educated at Portadown College and Newcastle University, he began his professional rugby career in 1996 on a semi-professional basis with Newcastle Falcons RFC before joining Ulster in 1999.
In 2005 he took up the role of Ulster Rugby captain, a position he held for two years, leading the team to the Magners League title in the 2005/06 season.
He won his first senior Ireland cap as a replacement against Tonga in June 2003 and made his first start the following August in the Rugby World Cup warm-up game against Wales. He was subsequently selected in the 2003 Rugby World Cup squad.
He remained part of the Ireland squad for the next four years, scoring his first and only try for Ireland against Japan in the first test of the 2005 summer tour.
He captained Ireland in two Tests during their summer tour to Argentina in 2007 and was also selected for his second Rugby World Cup tournament where he made three appearances before being ruled out due to his condition.
Commenting on his retirement from the game, Best said: "It is with great sadness that I am forced to retire from the game at this stage in my career. However, I have no regrets and feel immensely proud and privileged to have represented both Ulster and Ireland for the past nine years.
"I am grateful to have had the opportunity to work with coaches, management and players that I have had the utmost respect for and have formed lifelong friendships with many. I would like to thank the Ulster and Ireland supporters and my friends and family for their loyalty and support throughout my career and during this difficult time.
"I would also like to take this opportunity to thank the IRFU and their medical team for the extremely high standard of care that I have received.
"Finally, having grown up as a supporter of Ulster and Irish rugby I would like to wish the players and management of both squads all the very best for the rest of the season and the coming years. I will continue to support and follow their future successes with great interest," he added.
Commenting on the announcement, IRFU Chief Executive Philip Browne said: "On behalf of Irish rugby, I would like to pass on our best wishes to Simon. Our relief at his recovery following his illness during the Rugby World Cup is tinged now with sadness at this news that he will no longer be able to continue to play.
"He has been a wonderful ambassador for both Ulster and Ireland in his time as a player and we hope that although he will no longer be on the field himself that he will continue to be involved in the game."
Paying his tributes to the player, Ireland coach Eddie O'Sullivan said: "Simon has been an integral member of the Ireland squad since he won his first cap in 2003. He was an incredibly honest, talented and hard-working player in every aspect of his participation in the game and a model professional.
"In addition to that, he was and still is an extremely popular guy with all the players and it is very unfortunate that his career has been cut short by this condition when he still had so much more to offer Irish rugby. I would like to wish Simon and his family the best of luck for the future."
Ulster Rugby Chief Executive Michael Reid added: "It is disappointing that Simon's rugby career has come to a premature end. However, I have no doubt it will be the start of a new and exciting chapter in his life.
"A hard and uncompromising prop forward on the field, he has been the epitomy of the model professional off it and we look forward to seeing Simon regularly at Ravenhill cheering (his brother) Rory and the rest of the boys on."
Click here for Simon Best's Ireland record