3-year-old makes a stunning recovery after his parents brought him home to die.

Tobin Chisnall was not expected to live. And if the 3-year-old boy did survive the same rare genetic disorder that left his older sister almost blind and brain-damaged, it was likely he would remain significantly brain-damaged.

So in August, after Tobin succumbed to the immune regulation dysfunction known as acute necrotising encephalopathy of childhood (ANEC), parents Jared and Sarah Chisnall brought their only son home from hospital to die.

But what happened next has left doctors amazed and delighted, and taken the family by complete surprise.

Tobin defied the disorder, which has no cure, making a remarkable recovery.

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"We accepted the situation and thought, 'We have to let him go peacefully' but he has pulled through," Mrs Chisnall told the Herald yesterday. "Every day we are thankful we've got him still."

There are only 250 known cases worldwide of ANEC. It has a high mortality rate and those who do survive usually suffer permanent brain damage, experts say.

However Tobin can walk, talk and see again and is almost completely back to his happy self.

"It's pretty exciting," Mrs Chisnall said. "The doctors can't really explain it either. They can't believe it."

Tobin's miraculous recovery began when he opened his eyes, tried to pull out his feeding tube and said a few words.

Go to tinyurl.com/olke92x to see a video message from Tobin.

The once boisterous preschooler had been in a coma almost a week after contracting swine flu on August 1, which triggered the rare condition.

Starship hospital paediatric neurologist Cynthia Sharpe, who treated the children, said in the disorder a viral infection triggered a catastrophic inflammation in the brain, and that repeated viral illnesses could retrigger it at any time.

In Tobin's older sister Lucia's case, she was a healthy 2-year-old until she caught chickenpox in 2009, when her immune system attacked multiple parts of her brain, killing healthy cells and damaging its ability to send and receive messages.

Though she had almost fully recovered a year later, Lucia contracted swine flu and lost consciousness, suffering irreversible brian damage which left her with the cognitive function of a 2-year-old.

Mrs Chisnall said it was unclear why Tobin has made such an extensive recovery. But because of Lucia's experience, Tobin received treatment including steroids at Starship 12 hours sooner than his sister had.

"And he got immunoglobulin therapy straight away and she didn't. There's no evidence about it but we're continuing with it."

The treatment uses antibodies from healthy blood donors and helps regulate his immune system.

For previous coverage of Tobin's story, go to: tinyurl.com/tobinchisnall

Tobin is now regaining his sight and still speaks slowly but he can run, ride his bike and jump on the trampoline like any other 3-year-old.

Mrs Chisnall said Tobin had frustration meltdowns but this was a small price to pay given his initial prognosis almost three months ago.

Instead their wish that their little boy would "come back to us" came true. More than $75,000 was raised for the family through a Givealittle page and it was being used to pay for a nanny and treatment and rehabilitation costs.

The Chisnalls have two other children, Tobin's twin sister, Milah, and older sister, Darci, 5, who so far are unaffected by the disorder.

Starship clinical director of paediatric oncology Dr Lochie Teague said it was rare for children to defy the odds.

"We might have such cases within our portfolio but it's pretty rare. We can only work on the best information we have and the best evidence and sometimes those are about probabilities."

Q&A

What is acute necrotising encephalopathy of childhood?

ANEC is a devastating and rapidly progressive neurological disorder that occurs in healthy children after common viral infections. It causes the immune system to kill healthy brain cells. There is no cure.

How rare is it?

There are only 250 known cases worldwide. It was first described in Japan in 1995 where research into the extremely rare problem with immune regulation continues.

What are the chances of survival?

ANEC has at least a 30 per cent mortality rate and survivors are usually left with permanent and significant brain injuries. Exact treatments are unknown but steroids are used as well as immunoglobulin therapy. Repeated different viral illnesses can retrigger the disorder. Treatment initially involves steroids and can also include intravenous immunoglobulin therapy, where antibodies from healthy plasma donors are thought to help the immune system of children struck down with ANEC. Ordinarily immunoglobulin therapy is for patients with poor immune systems to help fight infection.