"Evander (Sno) had a cardiac arrest which was followed by a shock from his internal defibrillator," Feyenoord doctor Casper van Eijck said.
"And the shock is what he felt and that terrified him, which I could see in his eyes on the pitch.
"He was transported to a hospital because the defibrillator caused arrhythmia (abnormal heart beat) so he has to stay there for the night."
Sno had the defibrillator fitted after suffering a cardiac arrest playing for Ajax Amsterdam's reserves in September 2010 when he was revived by the use of an external defibrillator.
The 25-year-old Dutchman, who started his career at Feyenoord before spells with NAC Breda, Celtic, Ajax, Bristol City and RKC Waalwijk, was substituted after 34 minutes of Saturday's Dutch league match which Nijmegen lost 5-1.