The former Springbok skipper sustained the injury during a game between South Africa and Australia and left the field complaining of a stiff neck and a migraine.
Van der Westhuizen returned to Pretoria with former Blue Bulls doctor Henry Kelbrick, where he was submitted to a hospital for tests.
An MRI scan found signs of swelling and although Van der Westhuizen is still under observation, it is believed he will make a full recovery.
Kelbrick said: "I referred him to a neurosurgeon. We did an MRI scan and found swelling on the frontal lobes of the brain."
The former Bok captain retired after the 2003 World Cup in Australia, but has continued playing in veteran invitation events around the world.