As the disease adversely affects memory, language and judgement, it can have a massive impact on people's lives simply because it affects almost every day-to-day activity - from making a cup of tea to being able to manage social situations.
It can also range from gradual changes over the years to a dramatic deterioration.
Dr Raj Persaud, consultant psychiatrist in Harley Street says: "Dementia is an increasingly common brain disorder for which the most prominent early symptom is memory loss - particularly short term memory, in other words, memory for events which have just happened - as it gets worse more and more gets forgotten or can't be remembered - also new memories are not laid down."
He adds: "The memory loss and poor recognition and naming can get so bad that sometimes eventually sufferers don't even recognised themselves in the mirror - known as the mirror sign."
There are several types of dementia, and Jess Smith, research officer at Alzheimer’s Society says: "Alzheimer’s is the most common accounting for about 60% of dementia cases, vascular dementia is the second most common - which is where there is restriction of blood to the brain and can be brought on by mini strokes. Frontotemporal dementia is rare but tends to most affect people under age of 65."
Dementia is a progressive disease, which means it gets worse over time. To complicate things further, a person may have more than one dementia at the same time - so vascular and Alzheimer's concurrently.
Commonly, people associate poor memory with dementia, but if you are worried you might have dementia because of increasing forgetfulness, the good news is that it actually might be due to other underlying, more treatable health conditions.
Dr Raj adds: "If you suspect someone is suffering from dementia you should get them to a doctor as soon as possible because although some causes are not easily treatable others are - for example some people can appear to suffer from dementia when in fact the underlying cause is some other medical condition such as depression or low thyroid hormone. These latter two are easily treatable and the dementia will be completely reversed if they are treated."
Liz Harmer, head of services at Age UK agrees, adding that stress and urinary tract infections have been known to cause memory problems. "People think, oh my goodness, my short term memory has gotten worse, it must be dementia. It's always best to check in with your GP."
But what signs should you be looking out for? Liz says that some of the key symptoms might be struggling to remember recent events, even though you might have a clear memory of a much older event. "Struggling to follow conversations in groups or forgetting names of friends and everyday objects are key signs. And having difficulty following a story on TV or in a book or in magazine.
"Another one is trouble remembering where things where things are kept." However, I asked, a lot of us might forget where things are kept (I do this on a daily basis), so is there a distinction? She replied: "Some of this is accumulative – we all have times when our memory lets us down but I think it’s about lots of these things starting to come together. Things like feeling very confused even if you’re in a familiar place."
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The key area to look at for clues is definitely the short term memory, and how that's affecting your day-to-day.
Jess says: "Say someone was good at bookkeeping and now is not good at numbers - that there is a clue. Or if they are misplacing things but are accusing people of stealing them. One prominent symptom is also changes in personality. Some dementias affect parts of the brain that control social behaviour so you may find someone who may have been polite is actually quite blunt, or may not pick up on social cues. Someone previously shy may end up being more of an extrovert."
If someone you know has undiagnosed dementia, you may find they get quite prickly when they forget or misplace things because they know something is wrong, but aren't sure what exactly it might be.
Everyone wants to know whether there is a cure for dementia. The bad news, says the NHS, is that most types of dementia can't be cured, but that if it is detected early, there are ways of slowing it down.
Dr Raj adds: "Why some get dementia and others don't is down to a wide variety of factors including poor physical fitness, hereditary problems and so on.
"If you are worried about getting dementia the sooner you start improving your physical fitness, widening your social networks and improving intellectual activity the better. I am personally particularly interested in the idea that high 'purpose in life' - that is, lots of goals you feel passionate about - might be linked with some dementia prevention."
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If you are worried about dementia or would like to know how to prevent it, studies have shown that doing crosswords and brain stimulating puzzles can help. Similarly, you can also improve your diet to include 'brain food'.
Alice Mackintosh, nutritionist at The Food Doctor says: "Many factors can lead to the development of a condition such as dementia, however supporting the brain with beneficial nutrients is a good way to keep the integrity of its structure whilst helping to provide optimal neurological support so it can perform the functions required of it."
Anti-oxidants, which are also great for the heart, are a great addition, she says, as research has revealed that neurons in the brain that are subject to oxidative damage has lead to memory loss. "Anti-oxidants in foods such as blueberries, goji berries and beetroot have been shown to scavenge harmful free radicals, which can help prevent them from damaging brain cells, so ensuring you get plenty of these in the diet is recommended."
No doubt you will have heard about the benefits of oily fish with omega 3 - such as salmon, mackerel and sardines - and for the brain, it is the best food you can give it. "The brain is made up of 60% lipids," she says, "and good fats in the diet have been shown to help support brain structure and function. Foods such as oily fish, walnuts, flaxseed, hempseeds and olive oil should be eaten regularly, and Good quality fish oil supplements can also be taken to help increase beneficial omega 3 levels."
For people who are diagnosed with dementia, Liz says: "People have a real sense of loss because the future you were planning together might now be different. People can be scared of the impact it has on people close to them, Some are worried about how to tell their partners, and some people feel guilt - thinking about whether this is something they could’ve prevented. The fact is that it’s a physical condition, and it's no one’s fault."
On the other hand, Liz adds, while it can be a real shock, some people feel relieved because they finally know what's wrong and can start to move on with the practical details.
"If you do get a diagnosis," she says, "one of the things worth doing – if you can start to work through some of the impact – is to do some of the planning that can be very helpful for further on down the dementia journey. This ranges from putting your affairs in order, sorting out your finances, and having you set up bills by direct debit.
"It's worth sorting out your legal affairs and spending time thinking about powers of attorney, and making sure your will is done. As your capacity declines, some of this becomes much harder to put in place. Understandably after a diagnosis, that can often be something people put off but there are some really good reasons why it's good to take the time while you have capacity. And do let your family know because they will then be give you the support you want."
If you are caring for someone with dementia, the pitfall can be to mollycoddle them and not let them do anything. It obviously depends on the degree of dementia, but if they are capable of getting out and about, it does help to keep them active both mentally and physically.
Jess adds: "Don’t assume they can’t remember that they won’t appreciate a good day out. It’s a mixture of what the person wants to do and what they will respond to. Activities centred around the person work well, and remember they haven't changed so keep doing what they enjoy doing, unless they show obvious signs that they don't like it anymore."