The brain and dementia

4. Dementia symptoms and areas of the brain

Knowing how different types of dementia affect the brain helps explain why someone with dementia might behave in a certain way. Until recently, seeing changes in the brain relied on studying the brain after the person had died. But modern brain scans may show areas of reduced activity or loss of brain tissue while the person is alive. Doctors can study these brain scans while also looking at the symptoms that the person is experiencing.

The most common types of dementia each start with shrinkage of brain tissue that may be restricted to certain parts of the brain. This means that each type of dementia tends to have particular early symptoms, depending on which part of the brain is affected. Later on, as damage spreads to more areas of the brain, the symptoms across different types of dementia tend to become more similar.

Alzheimer's disease

In Alzheimer's disease, among the areas often damaged first are the hippocampus and its connected structures. This makes it much harder for someone to form new memories or learn new information. A person with Alzheimer's may struggle to remember what they did earlier that day, or what they have just said, meaning they may repeat themselves in conversation.

The hippocampus is needed for retrieval of memories, but retrieving those from longer ago may depend on it less. This is why someone in the earlier stages of Alzheimer's (with a damaged hippocampus but an intact cortex) may remember a childhood holiday but struggle to remember what they ate for breakfast that morning.

In Alzheimer's disease the amygdala is generally affected later than the hippocampus. So a person with Alzheimer's will often recall emotional aspects of something even if they don't recall the factual content. They may therefore respond more according to how they feel about a place or person than in a more logical way.

As Alzheimer's disease damage spreads through the brain, additional areas and lobes become affected. The cortex overall becomes thinner (so memories from longer ago are lost) and the brain gradually shrinks.

Damage to the left hemisphere is linked to problems with semantic memory and language, so someone may struggle to find the right word for something.

Damage to the visual system in the temporal lobes makes recognising familiar faces and objects harder. The person may seem to forget who a familiar person is. However, because the pathways for vision and hearing are separate, they may still know who that person is once they hear them speak. The person with Alzheimer's disease may also respond to someone at an emotional level even if they seem not to recognise them.

If there is damage to the right parietal lobe then the person might have problems with judging distances in three dimensions. Navigating stairs is a common difficulty.

As the damage spreads to the frontal lobes, someone with Alzheimer's may struggle with decision-making, planning or organising (eg family finances). A more complex task with a sequence of steps, such as following a new recipe, might also become much harder.

In contrast to these losses, many abilities are retained, particularly those acquired long ago. Learned skills such as dancing or playing the piano rely on procedural memories, and so are mostly stored deep within the brain. In Alzheimer's disease, these skills are often retained the longest.

'Atypical' Alzheimer's disease

There are rarer forms of Alzheimer's in which the first parts of the brain affected may not be in or near the hippocampus. This means that memory problems are often not the earliest symptoms.

In one form of atypical Alzheimer's - posterior cortical atrophy (PCA) - the early damage is mainly to the occipital lobes and parts of the parietal lobes, which help to process visual information and deal with spatial awareness. The early symptoms of PCA are often problems identifying objects or reading, even if the eyes are healthy. Someone may also struggle to judge distances going down stairs or parking the car. Or they may seem uncoordinated, for example when dressing. For more about atypical Alzheimer's see factsheet 401, What is Alzheimer's disease?

Vascular dementia

Vascular dementia has a wider and more variable range of symptoms than the other types of dementia. It is caused by a range of different diseases of the blood supply to the brain.

Sometimes vascular dementia follows a major stroke, in which a large area of tissue on one side of the brain dies because the blood supply is suddenly cut off. Symptoms are often seen in problems with planning, concentrating and thinking or memory. In addition, the person may be left with weakness down one side of the body or problems with vision or speech. With rehabilitation, some degree of recovery may be possible.

Vascular dementia can also follow several mini-strokes over time. Each mini-stroke creates a small patch of dead brain tissue, called an infarct, in the cortex. Early symptoms can be very specific to where the tissue is lost. For example, problems with episodic memory can be caused by an infarct in the hippocampus, and problems with executive function can be caused by an infarct in the frontal lobe.

A different kind of vascular dementia, called subcortical vascular dementia, follows disease of the small blood vessels deep in the brain. This disease often causes widespread damage to white matter beneath the cortex. These nerve fibres carry signals between different parts of the cortex, including the frontal lobes. A person with subcortical vascular dementia will therefore often have slowed thinking and problems with executive function.

Frontotemporal dementia

In all forms of frontotemporal dementia (FTD), the frontal and/or temporal lobes shrink. The different sub-types of FTD - which affect the person's behaviour and language - reflect different patterns of damage. For more information about these sub-types see factsheet 404, What is frontotemporal dementia?

In behavioural variant FTD, the areas of the brain affected early on are in the frontal lobes. For example, damage to the upper middle surfaces of the frontal cortex is linked to becoming withdrawn and losing motivation. Damage to the front under-surface is linked to losing inhibitions, meaning the person might make inappropriate comments, for example. Damage to the frontal lobes may also mean the person repeats the same word, phrase or action over and over again. It is important to appreciate that none of these things are done by choice.

In semantic dementia, the front of the left temporal lobe, dealing with verbal semantic memory, is damaged first. So the person may have fluent speech but struggle to find the right word for something, or they may ask what a familiar word (eg 'knife') means. Damage to the right temporal lobe leads to problems recognising faces and objects.

Dementia with Lewy bodies

The brain of a person with dementia with Lewy bodies (DLB) often shows less overall shrinkage than the brain of someone with Alzheimer's or FTD. Instead, tiny deposits of protein (Lewy bodies) are seen in the cerebral cortex, limbic system and brain stem.

In DLB, early damage is seen in the visual pathways and - in some studies - also in the frontal lobes. This may explain why problems with vision and attention are common early symptoms of DLB. Similarly, Lewy bodies in the brain stem may be linked to the problems with movement, as also seen in Parkinson's disease.