What kind of dentist uses peanut butter instead of toothpaste? Or strokes the side of your face with the toothbrush rather than put it in your mouth?

Wendy Bellis is not your bog-standard dentist, but then neither are her patients - she is one of the world’s foremost experts on treating children with autism.

“No child likes having their teeth brushed, but imagine what it’s like going to the dentist if you have extreme oral hypersensitivity, or you are terrified of new things?” said Ms Bellis, the dentist for TreeHouse, the charity The Times is supporting in this year’s Christmas appeal.

In other words, autistic children, while different from the rest of us, share a universal human condition: they hate going to the dentist - just they really hate it.

It was, until Ms Bellis started her pioneering work, an overlooked but very distressing side effect of the condition.

“Toothbrushing can be such a struggle, for parents already under a lot of stress, often they give up, understandably so,” she said.

“They can’t go to a dentist on the high street - a lot of dentists don’t like treating children at all, let alone a child who’s unpredictable and going to make a noise,” she said.

Autistic children may also be receiving sweets as rewards from their tutors, all the while developing serious toothache - with no way of telling anyone.

“That is a great worry for parents, that they might miss it.”

That means the only way to get even a check-up is by general anaesthetic - a risky procedure.

“When we examined one boy under anaesthetic we found a dead tooth on the top of his mouth, and the gums around it totally inflamed. His behaviour improved tremendously afterwards.”

So Ms Bellis decided something must be done. With 30 years specialising in special needs children, when the Treehouse school opened she approached them with an innovative idea. What about a mock dentists office, right in the school?

This looks in every way like a the real thing, complete with chair, light, and offputtingly clinical smell.

“We don’t treat them here, we just aim to reach the gold standard: being able to sit in the chair, and open their mouth to have their teeth brushed.”

After that, they can “graduate” to a dentist proper. Sounds simple enough, but here are two examples of her caseload during a visit to the school one morning last week: first, a 14-year old boy who had not been to the dentist in seven years, because he was too scared to leave the house. He could not even sit down.

And second, a bouncy teenager who, when passing her door (it has the sign for “dentist” on it in the pictorial code children are taught), pops his head around to give her the thumbs up.

“He loves the dentist - although at first he couldn’t put a spoon in his mouth, he’d feed himself with his hands, he was so horrified by the thought of an object in his mouth.”

How does she get from one to the other? Through an imaginative series of tricks and rewards she has devised, and a commitment to the long term, “one boy took a year to open his mouth for me, another girl I had to sit with in the back her mother’s car, as she wouldn’t come in.”

All depends on working out exactly what is scaring the - possibly non-verbal - child. If it’s the texture of the brush, she gradually brushes it closer and closer along their body until it reaches their face.

“The things I’ve done to get that brush in their mouth - I’ve covered it with peanut butter, or cream cheese. I wouldn’t put jam on of course, I’m a dentist, I’m genetically programmed not to. Still, people would think I was mad if they saw me. But it’s a means to an end.”

One parent, Virginia Bovell, said that her son 14-year-old Danny never went to the dentist “the level of fear mean it was a complete non-starter - any meaningful dental treatment had to be done under general anaesthetic.”

After “a gradual painstaking process” from Ms Bellis, using a lot of crisps as rewards along the way, he is now a toothbrushing, dentist-loving teenager.

In fact, in his visit to Ms Bellis at Treehouse, he revels in every moment, asking for the light to be angled more on his face, before flashing Ms Bellis a toothy, happy smile.

— The Pears Foundation will match every donation to TreeHouse made by Times readers pound for pound