I do a lot of work with TBI patients, use to do more of the longer term and recovery work, now it is mostly acute. Buy a new helmet.
Even "multi impact" helmets should be replaced every few years due to breakdown of the foam inside without any damage to the lid. Like Andy said, you answered it yourself really, but anything beyond a good scrape as far as visible damage warrants a trip to the paddling shop. Other than some of the DH and jumping helmets, one ding and done for bike helmets too. Any dents in the foam weaken them and they will break much easier than intended when undamaged.
Speak of Peace, (but carry a big gun)