The mother of a boy with Angelman Syndrome has said his diagnosis was discovered thanks to an "on the ball" consultant.

Joan Johnston's son, Leo, has the syndrome and is one in 15,000 to 20,000 children affected by the neurogenetic condition.

It is characterised by profound intellectual disability and severe developmental delay.

Most children with Angelman Syndrome do learn to walk, but this may not happen until they are aged five.

There are also feeding difficulties, and some 90% of children will develop epilepsy at some stage.

Joan Johnston and her son Leo | Image: Supplied

Joan told Newstalk Breakfast with Susan Keogh: "One of the best things about Angelman Syndrome - cause you always have to look at the positives - is that they have a very happy disposition.

"When we got the diagnosis about Leo, the hardest thing for me was finding out that Leo is unlikely to ever learn to speak - that maybe he might be able to speak five words.

"But Leo is unbelievably emotionally available to us - he is the happiest child in the world.

"He has a smile, irrespective of whether he's in the hospital and he's feeling rubbish or if he's at home enjoying life with his sister.

"And I suppose that made the diagnosis a little bit easier."

She said her consultant examined Leo and said his features "fit with a cerebral palsy-type diagnosis"

Asked if Leo was always this happy, Joan said he was - and that led to testing for Angelman Syndrome.

"I'd never heard of it".

"To be honest, we're very lucky that he was so on the ball - very often Angelman Syndrome won't be diagnosed until the children are a little bit older.

"They get a cerebral palsy diagnosis, or perhaps another diagnosis, and because its rare the screen wouldn't be sent until they'd no longer fit that diagnosis profile".

Joan Johnston and her son Leo | Image: Supplied

"It's been a grieving process - you grieve for the loss of future that you'd envisaged for your child with special needs, you grieve for the future of your other child - who when my husband and I pass away Ella, his three year old sister, is going to become his guardian and carer.

"That's a lot to put on a three year old".

She said her marriage and life suffered, but the diagnosis helped.

"I was in the house, I barely left because I didn't know what he was going to be like - I think I remember I twice made two trips out with Leo and both times he screamed inconsolably, there was nothing I could do to calm him.

"I was hugely anxious, I stayed in my living room, I barely went anywhere else in the house."

She said before the diagnosis, she did not know what to do: "My relationship with Leo suffered, he couldn't feed properly and there seemed to be no apparent reason.

"I longed for the connection of breastfeeding, but he just couldn't - and he would never be able to learn to latch on, because he has problems controlling his tongue and his oral motor control is poor as part of his condition.

"But without a diagnosis, you think you're the problem - or you think your baby's just being a pain".

You can listen back to the full interview here

Joan keeps a blog through her Facebook page, Leo the Angel Lion