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The genetic disorder affects almost exclusively females, causing them to regress neurologically and physically.

The progression of the disease can be roughly divided into four stages.

During the first stage, from about the age of six to 18 months, a baby slows in development, loses interest in play, stops making eye contact, starts walking awkwardly and makes repetitive hand movements.

The second stage, known as ‘rapid destruction’, begins between the ages of one and four.

The child finds it increasingly difficult to communicate and learn and there is often a deterioration in other brain functions.

Symptoms include an inability to control the hands, sudden distress teamed with screaming, unsteadiness, breathing problems, difficulty sleeping, slow head growth and digestive problems.

The third stage, the ‘plateau’, begins between age three and ten.

Limbs become floppy, epilepsy may develop and there may be weight loss and teeth-grinding.

However, many parents say children are less distressed and show more interest in their surroundings.

The final stage can last for decades. Usually severe bending of the spine – scoliosis – develops, as well as losing the ability to walk.

Almost all cases are caused by a mutation in the MECP2 gene which prevents nerve cells in the brain from working properly.

Currently there is no cure and only the symptoms are treated. 

Around three quarters of sufferers will live into their 50s, according to Rett UK. The National Institute of Health says it's not possible to make reliable estimates about life expectancy beyond the age of 40. 

According to Rettsyndrome.org, data from the Natural History Study have determined that a girl with Rett has a 100 per cent chance of reaching age 10, greater than 75 per cent chance of reaching age 30, and a greater than 50 per cent chance of reaching age 50. 

But due to the rarity of Rett, very little has been published about life expectancy.