Former Springbok skipper and Rugby World Cup-winning scrum-half Joost van der Westhuizen has been diagnosed with a serious muscle-related neural disease and is on treatment, his publicist said.

The diagnosis was made after the 40-year-old's doctor noticed that the 1995 champions squad member was having problems with his arm and ran a series of tests, said Bridget van Oerle in a statement.

"Van der Westhuizen also consulted two neurologists and has begun immediate treatment. The symptoms are serious and vary from person to person," she said.

Doctor Henry Kelbrick told SuperSport.com that the disease affected the entire body and that few treatment options were available but said the ex-player was "functioning at 100 per cent at the moment and is in good spirits".

Afrikaans daily Die Beeld on Friday quoted Kelbrick as saying he had noticed the arm weakness over the Easter weekend when at a holiday resort with the ex-player.

"I said nothing, but kept a close eye on him. On the Thursday afternoon I realised his tongue was slurring a bit and he also mentioned to me that it sometimes happens out of the blue. That's when I realised there's trouble," he said.

"It still remains a mystery in the medical field as to why people get this type of disease. It is not drug, stress, or alcohol-related," Kelbrick told the newspaper.

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