Tamra thought her daughters were perfectly healthy. Then a teacher asked a surprising question.

“Sometimes it’s confused with just shyness and thought that a child will grow out of it, which usually isn’t the case.”

Isabel seems like a perfectly regular, playful kid at home.

But when the six-year-old’s teacher told her mother that Isabel was completely mute in class, it became clear something was seriously wrong.

“She said ‘well she’s mute, so we [are] wondering if she’s using sign language at home or how she’s able to communicate’,” the girl’s mother Tamra told The Project in a segment screened last night. “And I said to her, I really think you’ve got the wrong parent because my child’s fine.”

But it turned out Isabel wasn’t fine — and nor was her older sister Emilie.

“Apparently she would sit in the corner and she would just cry,” Tamra says of eight-year-old Emilie. “She wasn’t able to participate, she wasn’t able to make friends.”

Last term, Isabel even started to become “hysterical” when Tamra tried to leave.

The girls’ concerned parents took them to a psychologist, where they learned that both children have selective mutism, a complex anxiety disorder characterised by the inability to speak and communicate effectively in certain settings.

“Selective mutism is an anxiety disorder similar to social anxiety or even public speaking anxiety where the child fears how someone is going to react, and because of those fears the children are literally too scared to speak,” Dr Elizabeth Woodcock of Selective Mutism Clinic Sydney told The Project.

“Sometimes it’s confused with just shyness and thought that a child will grow out of it, which usually isn’t the case.”

It’s not clear why school is the girls’ main trigger, but Dr Woodcock tells news.com.au that “trauma or abuse isn’t the cause” of the disorder.

The condition does have a strong genetic component — meaning that usually the child’s family members will have a higher incidence of anxiety disorders.

Isabel and Emilie, along with their four-year-old sister Zoe, will babble happily to each other and their parents during the car trip to school, but as they draw closer to the school building they fall silent.

By the time they’re in the classroom, the girls are too afraid to speak at all, as a segment screened on The Project last night demonstrated.

School is not the only setting in which the girls become frightened, either. The girl’s father Matt also says the girls become introverted “particularly around family like cousins and aunties and uncles, and even the grandparents.”

“Any kind of gathering is hard because they get anxious so they’re always clinging onto us,” Tamra told news.com.au.

While the condition may sound unusual, it’s all too common: almost one in 140 kids aged five to seven have the condition, which is known as elective mutism.

The good news is that it’s a treatable condition, although Dr Woodcock explains that it usually takes one to two years to resolve.

We’re glad to hear that Isabel and Emilie are now in treatment, getting the help they need. We wish them all the best in their journey to overcome the disorder.