The 40-year-old forged a reputation as one of the world's finest scrum-halves during a glittering 89-Test career which saw him secure a World Cup winner's medal in 1995.
Van der Westhuizen was also part of the Springboks side that faced the Lions in 1997. He started all three Test matches in the number nine jersey and scored two tries as the Lions secured a Series win.
He is currently his country's joint-record try scorer with 38 although current Springboks star Bryan Habana has drawn level.
Van der Westhuizen's publicist Bridget van Oerle confirmed the former Springbok has a "serious muscle-related neural disease".
"Joost's doctor, who is also a close family friend, noticed that he was having problems with his right arm and ran a series of tests," said van Oerle.
"He has also consulted two neurologist and has begun immediate treatment.
"The symptoms are serious and vary from person to person. The media is requested to respect his privacy at this time."
In 2009 he was admitted to hospital after suffering a suspected heart attack after watching a Test between South Africa and the British and Irish Lions.