If a head injury is received death can occur quite some time later.
The S-76 occurrence I mentioned above the aircraft came came to rest on its left side, the captain was able to exit and helped the attending folk get the trapped FO out of the left seat. Nothing was noted wrong with captain until he dropped dead 45 minutes after the crash.
Wear a helmet guys, your fixed wing friends might think you're a pussy, but they don't fly an aircraft with a broom cupboard to slam your head against, or run the risk of having a rotor blade wanting to share cockpit space with you.