By Jane Elliott
BBC News health reporter
When Carolyn and Ian Sixsmith lost their eldest son to an unknown degenerative disorder they thought their nightmare was at an end.
Ellie has a degenerative condition
But, just 12 months after they had buried Daniel, they were told that post- mortem tests had finally revealed a cause for his death.
Their relief at finally knowing why their son had died soon turned to despair as doctors revealed he had Niemann Pick Type C, a genetic condition which is similar to Alzheimer's.
Carolyn said that, despite the diagnosis, she had still not realised the full implications for her daughters Katy and Ellie.
"I felt tremendously relieved because we had a diagnosis.
"And because it was a defective gene from both of us, it meant we could feel that no one was to blame.
"But we thought 'what would be the effect on the girls?' I was obviously thinking about any future families they might have - not about any immediate danger to them."
Both girls had a skin biopsy and Carolyn and Ian had a tense three month wait for the results.
When the results came, they revealed the shocking news that Ellie, who was only just three-years-old had the disease. Katy, aged six, did not.
"They said Ellie had it, and for us that was the death knell. We had been through it before and we knew what had happened to Daniel and we knew it was going to happen to Ellie," said Carolyn.
When Ellie turned five, her parents saw that she too was becoming ill.
She started to become more clumsy and her learning regressed, she became incontinent and started to lose the ability to communicate.
There are three forms of Niemann Pick Disease, Type A, B and C - each affecting the body's metabolic process and caused by specific genetic mutations
Types A and B are caused by an enzyme deficiency.
Type A rapidly affects the brain and usually causes death before the age of three.
Type B affects the liver, spleen and respiratory system. Those affected survive into adolescence or adulthood.
Type C is caused by a mutated gene, and causes progressive loss of mental faculties, seizures and dementia. Those affected do not usually survive into adulthood
But then the Sixsmiths were offered a glimmer of hope, a new drug called miglustat or Zavesca, which was being used to treat Gaucher's Disease - an inherited metabolic disorder.
Ellie was deemed unsuitable for the trial, but her consultant thought she might still benefit from using it. So her parents lobbied and finally got funding for the drug.
She has now been taking the drug for a year and her parents say they have seen small signs of improvement.
"She can swallow the drug, which is no mean feat. And it is having results. She swallows better and, from losing her powers of communication, she can now repeat things she has heard on TV."
Her mother said that, although the improvements were small, the drug was allowing them to keep the disease at bay while scientists search for a cure.
"The drug is not a cure, but it gives us hope for the future."
Scientists have recently presented data at the American Society for Haematology Genetics that show a trial carried out in America and at the Royal Manchester Children's Hospital of 29 children and young adults aged 12 and above, had shown positive results.
In Niemann Pick Disease Type C , the symptoms have been traditionally been explained by the storage of cholesterol within cells.
However there is growing evidence that a variety of other lipids accumulate within neurons in the brain causing neurological deterioration.
This extensive neuro-degeneration is typically the cause of premature death in affected people as neurons send and receive messages from the body to the brain and back to the body, controlling functions humans.
In order to treat the disease effectively, the drug needs to cross the so called blood-brain barrier, and this has proved difficult because this barrier between brain blood vessels and brain tissues, is designed to restrict what can pass from the blood into the brain.
It prevents potentially harmful chemicals from the blood accessing the brain, but allowing the entrance of essential nutrients.
However as miglustat is a small molecule, it is able to cross the barrier and access the neurons where the lipids are stored.
Miglustat, is a substrate reduction therapy that reduces the rate of formation of a harmful lipid stored in the neurons.
Jackie Imrie , a nurse specialising in Niemann Pick at the Royal Manchester Children's Hospital said, although there was hope, it must be remembered that it is not a cure.
"There has been some sense of improvements, but this is the first year of results.
"The indication is that it is only going to keep the disease at bay."
Toni Mathieson of the Niemann-Pick Disease Group said the trial offered real hope. "It is a watershed for everyone affected by Niemann Pick C.
"It is not only helping us to learn more about how this dreadful disease works, but also raises the prospect of a treatment that has the potential to offer real benefits to patients for whom currently the outlook is bleak."