Jo Cameron only realises her skin is burning when she smells singed flesh. She often burns her arms on the oven, but feels no pain to warn her.
It wasn't until she was 65 she realised she was different - when doctors couldn't believe she didn't need painkillers after a serious operation.
When she felt nothing, her anaesthetist - Dr Devjit Srivastava - sent her to pain geneticists at University College London (UCL) and Oxford University.
Jo, from Whitebridge, near Inverness, told the BBC Scotland news website that doctors didn't believe her when she said she wouldn't need pain relief after surgery.
She said: "Looking back, I realise I hadn't needed painkillers, but if you don't need them you don't question why you don't.
"You are what you are, until someone points it out you don't question it. I was just a happy soul who didn't realise there was anything different about me."