The 40-year-old made his name as one of the world's best scrum-halves during an 89-Test career that saw him secure a World Cup winners' medal in 1995.
He is also the Springboks' joint-record try scorer alongside Bryan Habana with 38 and captained his country at the 1999 World Cup, where they reached the semi-finals, before playing his final Test in the quarter-final loss to New Zealand at the 2003 tournament.
The diagnosis was made after Van der Westhuizen complained to his doctor of a pain in his arm, and he was sent for a series of tests. He has since seen two neurologists in order for treatment to begin.
A statement from his publicist Bridget van Oerle on Supersport.com read: "Sports personality Joost van der Westhuizen has been diagnosed with a serious muscle-related neural disease.
"Van der Westhuizen's doctor Dr Kelbrick, who is also a close family friend, noticed that Joost was having problems with his right arm and ran a series of tests.
"Van der Westhuizen also consulted two neurologists and has began immediate treatment.
"The symptoms are serious and vary from person to person.
"The press is requested to respect his privacy at this time and note that neither he nor his family will be available for interviews."
Van der Westhuizen has endured some difficult times during the last few years.
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, while his marriage came to an end in 2010.
His former South Africa team-mate Andre Venter has also suffered with a form of motor neurone disease, which has recently seen him confined to a wheelchair, while former London Irish wing Jarrod Cunningham died from the disease aged 38 in 2007.
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