Experts have warned that a “change in mood” that lasts up to two years could be a warning sign of dementia. This symptom could start “mild” and become more apparent over time.

Dementia is a syndrome, or a group of associated symptoms, linked to the ongoing decline of the brain. Most common in people over the age of 65 it can initially be characterised by problems with memory.

However, there are some lesser-known signs that could alert you to the fact someone has the condition. The Alzheimer’s Society warned: “In the early stages of dementia, a person’s symptoms are often relatively mild and not always easy to notice. 

“Common early-stage symptoms include problems with memory, speed of thought, language or perception.” It continued: “As a very rough guide, the early stage of dementia lasts on average about two years.”

The charity listed some common “early” symptoms will affect someone’s daily life. Among them are changes in mood or emotion.

It explained: “The person may be more anxious, frightened or sad, and so at risk of depression. It is also common to become more irritable – perhaps in frustration at lost abilities – or easily upset. A person can often be more withdrawn, lack self-confidence and lose interest in hobbies or people.”

It is not exactly known what causes depression in dementia patients.

The Alzheimer’s Society said: “For many people, the challenges of living with dementia can lead to feelings of deep sadness or hopelessness. In addition, the diseases that cause dementia may damage parts of the brain involved in emotions and behaviour.”

Other factors that can contribute to depression include:

• A history of traumatic or upsetting events
• Other health conditions or disabilities – particularly heart problems, breathing difficulties, chronic pain or hormonal problems
• Side-effects of medications – including certain sleeping pills, steroids, beta-blockers and drugs used to treat Parkinson’s disease
• Not having enough meaningful things to do, social isolation or a lack of social support
• Not getting enough good-quality sleep
• Bereavement – this can make a person more vulnerable to depression, although grief itself is not normally thought of as a type of depression
• Feeling stressed about issues such as money, relationships or the future
• Drinking too much alcohol.

It added that a person with dementia is “much more likely” to become depressed if they have had depression in the past – particularly if they have had it several times during their life.

According to the charity, symptoms of apathy, depression and anxiety tend to continue into the middle stage of dementia.

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Other early signs of dementia to look for, as listed by the charity include:

• Memory problems
• Difficulties in thinking things through and planning
• Problems with language and communication
• Poor orientation - getting lost easily
• Visual-perceptual difficulties.

If you or someone you know is experiencing symptoms of dementia, you should speak to a doctor.