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Actor Bruce Willis has been diagnosed with frontotemporal dementia, a group of brain disorders caused by progressive nerve cell loss in the brain.

The 67-year-old was previously diagnosed with aphasia - which causes difficulties with speech - but this has progressed, and he has been given a more specific diagnosis, his family said.

Frontotemporal dementia (FTD) is a disease. It is caused by the progressive loss of nerve cells in the brain's frontal and temporal lobes. As those areas shrink it can lead to changes in a person's personality, behaviour and language.

This is not quite the same as "dementia", which is a general term that describes the symptoms of a large number of different brain diseases, including Alzheimer's.

FTD is unusual as it largely affects people in mid-life, between the ages of 45 and 65 - most other forms are found in older ages. There are different types of FTD, but estimates suggest between 50,000 and 60,000 have some form of the disease in the US,

Symptoms include

Those can include inappropriate social behaviour, lack of judgement and being easily distracted, as well as motor problems such as muscle weakness and tremors.

Because FTD affects people's personalities and behaviours, the disorder can be misdiagnosed as a psychiatric issue, according to the .

There is no cure or way to slow down the disease, although there are treatments for some of the symptoms. As the disease progresses, patients eventually need full-time care. On average, people live eight to 10 years after diagnosis, but some people live much longer.

There is no specific test, so a diagnosis is usually made after running tests and eliminating other conditions.

If you are noticing subtle changes in behaviour or language, contact a healthcare provider for evaluation.

The only known risk factor is a family history of the disorder.