Further tests showed that starting the diet early was more beneficial, leading the researchers to conclude that in mice "treatment should begin early in life and continue into adulthood".

This study was only in mice, meaning it is not known if there would be a similar effect in people - or if there would, how early treatment would have to start.

The authors of the report said: "Dietary cholesterol does not cure PMD, but has a striking potential to relieve defects."

It is thought the cholesterol frees up a "traffic jam" inside cells in the brain. The disease is caused by producing too much of a protein needed in myelin, which then becomes stuck inside the cells. It is thought the extra cholesterol helps to free up the protein.