Coronation Street spoilers follow.
Coronation Street has aired emotional scenes this month, as Steve McDonald and Leanne Battersby's young son Oliver has been admitted to intensive care after a serious seizure.
The worried parents have been told that Oliver may have mitochondrial disease – a diagnosis which will be confirmed officially in upcoming episodes.
So far, the doctors haven't given Steve and Leanne the full details of the implications for their three-year-old child, as they still need to conduct more tests.
Here's what we know about the condition and Coronation Street's future plans for the storyline.
What is mitochondrial disease?
Mitochondrial disease is a genetic condition which is present from birth, although symptoms may not develop until later in life.
Mitochondria are present in nearly every cell in the human body and are responsible for producing the energy we need to function.
Mitochondrial disease occurs when the mitochondria in a person's cells are not producing enough energy. Without this energy, tissues and organs in the body can stop working properly.
The NHS's Rare Mitochondrial Disorders Service explains: "There is huge variety in the symptoms and severity of mitochondrial disease. It depends on how many cells are affected and where they are in the body.
"Every person with mitochondrial disease is affected differently. Each individual affected will have a different combination of mitochondria that are working and not working within each cell.
"The parts of the body commonly affected are those that have the highest energy demands, such as brain, muscle, liver, heart and kidney. When these systems are affected, mitochondrial disease is usually progressive."
Due to the complexities involved, the term 'mitochondrial disease' actually covers a group of different medical disorders.
Other known symptoms include fatigue, vision loss, hearing loss, poor growth and respiratory problems.
What is the treatment for mitochondrial disease?
Currently, there is no known cure for mitochondrial disease. Doctors can help to treat the symptoms which develop (such as Oliver's seizures), or slow the progression of the disease, but it is not currently possible to treat the underlying cause.
Mitochondrial disease is a progressive condition, meaning that it will get worse over time. The prognosis depends on the individual and the severity of the condition, but Coronation Street has already confirmed that Oliver's illness is life-limiting.
"The rate of progression can be variable and unpredictable, but most patients will eventually develop involvement of several organs.
"Although there are currently no real treatments or a cure at present, there is now money being invested in research into mitochondrial disorders and many exciting discoveries are being published that we hope will lead to treatments to slow the progression of these diseases for future cases."
How has Coronation Street researched the storyline?
Coronation Street has been working with The Lily Foundation while developing this storyline. The foundation – the UK's leading mitochondrial disease charity – was set up by Liz Curtis in 2007 in memory of her daughter Lily, who sadly died from mitochondrial disease at just eight months old.
Curtis recently said: "All of us at The Lily Foundation are excited to be working with Coronation Street on a storyline about a child with a mitochondrial disorder, and grateful to the show for highlighting an issue that affects hundreds of families in the UK.
"For everyone who has worked hard for years to raise awareness about mitochondrial diseases, in particular for families living with a diagnosis and those who have lost a child, having their story told on one of the nation's most popular soap operas is truly momentous news.
"There is currently no cure for mitochondrial diseases, so those diagnosed face an uncertain future. We have been impressed with how sensitively the show's researchers and script writers have handled this, listening to those who have been affected by the disease and the doctors who support them."
Professor Robert McFarland, from the Wellcome Centre for Mitochondrial Research in Newcastle, also worked with the Coronation Street team to ensure that scripts were accurate.
Jane Danson, who plays Leanne, has said: "We've worked closely with Liz Curtis at The Lily Foundation. It was harrowing hearing the story of what happened to her daughter Lily, but also really amazing to hear how people come through this, how they support each other and learn to live again.
"It's almost too much to comprehend, but I came away from the meeting bowled over by her bravery and how amazing she is as a human being. She shared with me how she felt emotionally, how she got through her days, how people rallied around her.
"I've also read a lot of literature about how families cope around their children's diagnosis with life limiting illnesses, looking at the human elements to their stories amidst all the medical speak and hoping I can get it right.
"It is quite overwhelming, I’ve been so lucky to have so many stories with Leanne over the last 20 odd years but this one feels different. This one could really break her and it feels like it's the one where I've got the most responsibility to get it right."
Why is Coronation Street telling the storyline?
In a recent interview with Digital Spy and other media, Coronation Street's producer Iain MacLeod revealed that this storyline will closely follow parents Steve and Leanne along with their partners Tracy and Nick.
Asked why the show has taken on the story, MacLeod explained: "Firstly, we felt that it was a really interesting story to tell with a modern, blended family.
"The story features Steve, Leanne, Tracy and Nick – drawing in Gail, Emma and Amy. This big clan have to overcome the worst possible news.
"The other reason for wanting to tell the story is that the condition that Oliver has is something that most people don't really know about much. I certainly didn't know much about it, but it is relatively common and mostly underfunded from a research point of view.
"We wanted to draw a bit of attention to families in the real world who are going through a tough time and need a bit of support. Hopefully the extra exposure will lead to more charity revenue and hopefully find better treatments for this type of illness."
Coronation Street currently airs on Mondays, Wednesdays and Fridays at 7.30pm on ITV.
You can find more information about mitochondrial disease at The Lily Foundation's website.
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