Leukodystrophy is a group of rare genetic neurological diseases that cause the sufferer to revert to a child-like state.
In a Channel 4 documentary, The Curious Case of the Clark Brothers, living with the disease is shown through the eyes of Michael and Matthew Clark, who now need 24-hour care.
Tony and Christine, their parents, had moved to Spain after taking early retirement but were forced to return to the UK when they learned that their two adult sons had developed leukodystrophy and were deteriorating rapidly.
They moved into a small one-bedroom flat in order to care for their sons.
In the documentary, Christine says: "I used to wish my sons were small again. I got my wish."
Michael, 42, and Mathew, 39, from Hull, had both lived normal lives until their late 30s. However, they both carried the genetic disease and had become "neurological timebombs".
Michael, a former RAF gunner, is believed to have the mental age of a 10-year-old and giggles constantly, while former factory worker Matthew behaves like a small child.
Matthew married in 1990 but his relationship broke down when he started showing symptoms of the disease. He has a 19-year-old daughter who is expecting a baby.
Tony says of his sons: "Both of them are very childlike now. Matthew went out the other day and bought himself a train set and a Mr Potato Head.
"He also has these awful episodes where he screams and shouts and says 'I don't know what I'm doing'.
"It is like an adult having a toddler's tantrum. It's obviously worse for him but it is terrible for us too. We feel absolutely powerless."
Usually, leukodystrophy is diagnosed in children. The sufferer may lose body tone, movement, gait, speech, the ability to eat, sight and hearing.
It is a metabolic disorder of the nervous system, which results in an accumulation of sulfatide in tissues that manifests as mental deterioration and severe central nervous system disturbances.
Michael and Matthew developing the disease at such a late stage is extremely rare - as is the disease affecting two members of the same family.
Leukodystrophy is estimated to affect just 100 people in the UK.
Lynda Carthy, chief executive officer of the Myelin Project UK, which looks to end suffering for people with demyelinating diseases, said the chance of two people who carry the leukodystrophy gene meeting and becoming partners is one in three billion.
She told the Daily Telegraph: "There is no cure and it will eventually end in death as the brain shuts down."
The documentary shows how the Clark family struggle with day-to-day life and how they are coming to terms with the neurological disease.
The Curious Case of the Clark Brothers is on at 9pm on Channel 4 tonight (Monday).