The above question came from one of my Twitter followers (@muekekasimu). Thank you for the question!

So, what brain aeas are affected by dementia?

It is a deceptively simple question but fiendishly difficult to answer. The reason why it is so difficult to answer is that the brain areas affected by dementia differ for each type of dementia. Each of the main subtypes of dementia (Alzheimer’s disease, Vascular dementia, Frontotemporal dementia, Dementia With Lewy Bodies) have their own brain areas affected by the underlying disease. We will explore further down which brain areas are affected by each dementia. But first, we need to understand a second, important aspect.

Not only are different brain areas affected for each type of dementia but the brain areas affected for each dementia also change over the course of the disease. To understand this point we need to grasp that all dementias are progressive. Progressive means that the disease underlying each dementia start in on brain area but once the disease progresses it spreads to other brain areas.

This means that the different types of dementias start in specific brain areas but then spread throughout the whole brain until the end of the disease. This progressive spread through the brain over time explains why in the latter stages of the disease, most people with dementia have similar symptoms regardless of which type of dementia they started with as the disease has by then spread through their whole brain.

Why the disease spreads through the brain is still not completely clear on a scientific level. All we know that is that the proteins which cause the different types of dementia seem to ‘spread’ from one brain area to the next, causing the disease there. See also my previous blog entry on this: xx

Before we come to exploring the brain areas affected by the different types of dementia, we finally need to understand that different brain areas are specialised for different brain functions. For example, some brain areas are more important for our memory, while others are more important for our vision, our language, our behaviour, or even our movement.

If the functions of a particular brain area are disrupted by the dementia disease processes, it causes the function of the brain area to malfunction. This ‘malfunctioning’ of the brain area cause then symptoms in the people with dementia, for example, they have a memory loss. This means that the symptoms in the different types of dementia tell us actually which brain areas are affected by the disease. Pretty amazing, isn’t it?

If this sounds all bit complex, do not worry let’s explore now in more detail which brain areas are affected in the different types of dementia.

Alzheimer’s disease

We start with Alzheimer’s disease, as it is the most common form of dementia, affected ~70% of people with dementia. Because it is the most common type, many people equate dementia with Alzheimer’s disease but we should remember that Alzheimer’s disease is only one type of dementia.

Alzheimer’s disease starts in a region of the brain called the medial temporal lobe – just behind our temple of the head. A key region in the medial temporal lobe, which is affected in Alzheimer’s disease is a brain structure called the hippocampus (from Latin meaning ‘seahorse’; the reason being that when the neuroanatomists in the 19th century described the structure for the first time, it looked to them like a seahorse – and the name stuck!). The medial temporal lobe and the hippocampus are very important in the brain for two functions, memory and spatial orientation.

It should come therefore not as a surprise that the most common symptoms in Alzheimer’s disease are memory problems and spatial disorientation, as the functioning of the hippocampus and medial temporal lobe is affected by the disease.

Not only does it explain the memory symptoms but also the specific memory symptoms in Alzheimer’s disease. One of the most common memory problems seen is that people with Alzheimer’s disease have a poor memory for recent events but generally good memory for events long ago. Families often refer to this as ‘living in the past’, for example, they report that the person cannot remember what they did last weekend but can remember clearly their wedding day many decades ago.

The reason for this particular memory deficit in Alzheimer’s disease is that the hippocampus is actually not the place where we store our memories but it acts as an indexing mechanism which can find the memories stored in other parts of the brain. This indexing mechanism becomes affected by Alzheimer’s disease, which means that indices for new events are not arranged at all or are faulty, resulting in us having no or poor memory for recent events. However, the indices for older memory are very strong and can be still retrieved. This fact explains why people with Alzheimer’s disease – at least in the early disease stages – have still good memory for events long ago while they cannot remember recent events.

The medial temporal lobe and the hippocampus in the brain are therefore critical brain areas for Alzheimer’s disease.

However, for rarer forms of Alzheimer’s disease, other brain regions can be also affected. For example, Posterior Cortical Atrophy (PCA) is a rare form of Alzheimer’s disease and affects more the back of the brain – the occipital and parietal brain areas. These brain areas are very important for our vision and hence people with PCA often present more with visual problems at the beginning of the disease.

Another rare form of Alzheimer’s disease is called frontal variant Alzheimer’s disease, which affects the frontal brain regions, just behind our forehead. The frontal regions of the brain are important for our decision-making and behaviour, which means that people with this rare form have symptoms with decision-making and behaviour while their memory remains largely intact.

In summary, Alzheimer’s disease affects the medial temporal lobe causing memory and spatial orientation difficulties in people with the disease. However, in rarer forms of Alzheimer’s disease other brain regions can be affected by the disease.

Vascular dementia

The next most common type of dementia is vascular dementia, affecting ~10-20% of all people with dementia. Vascular dementia is caused by tiny strokes in the brain, so-called mini-strokes. People commonly do not notice these mini-strokes, hence they can be quite dangerous as they occur without us even noticing them.

The tricky thing with mini-strokes is that they can happen pretty much anywhere in the brain. This means that different brain regions can be affected by vascular dementia depending on where the mini-strokes are occurring. For example, some people with vascular dementia can have quite significant memory problems as their medial temporal lobe is affected – similar to Alzheimer’s disease – while other vascular dementia people can have mini-strokes in their frontal brain areas causing behavioural changes and so on.

This can make it quite hard for people with vascular dementia and their families to predict or adjust to any of those changes as they can be quite different from other vascular dementia people.

Still, there is one very common brain area affected by vascular dementia. In fact, it is not a brain area but the connection between the front and back brain areas. This connection is referred to as white matter tracts, as these are tracts of neurons sending signals to different brain areas. They are called white matter because they look white on most brain scans.

Vascular changes, in particular hypertension (high blood pressure), can cause changes to these white matter tracts, which clinicians refer to as white matter hyperintensities. The hyperintensity means that on brains scans these changes are even brighter than the normal white matter.

The white matter hyperintensity changes cause in particular a slowing of responses and decision-making in people with vascular dementia. They become much slower in switching between tasks or executing them because of these changes.

In summary, in vascular dementia the brain area affected by the disease depends as to where the mini-strokes are happening. However, for many people with hypertension we see changes to the tract connecting different the front and back of the brain – white matter hyperintensities.

Frontotemporal dementia

Frontotemporal dementia is considered to be a rarer form of dementia. It affects – as the name gives away – the frontal and temporal brain regions. We have already mentioned the temporal brain regions in Alzheimer’s disease, however, for frontotemporal dementia a different region of the temporal lobes is affected by the disease.

Instead of affecting the medial temporal lobe, like in Alzheimer’s disease, the more frontal part of the temporal lobe (the anterior temporal lobe) is affected in frontotemporal dementia.

We already know that changes to the frontal lobes can cause behaviour and decision-making changes in dementia, and it is, therefore, no surprise that frontotemporal dementia people often show such symptoms. However, they can also show some more language symptoms, caused by the anterior temporal lobe changes.

The anterior temporal lobe is very important for our semantic knowledge of facts in the world. For example, a cup is a cup regardless of how it looks, or that Paris is the capital of France. Frontotemporal dementia people can therefore often show changes to their naming of objects or even their usage, which is caused by the changes in their anterior temporal lobes.

In summary, the brain areas in frontotemporal dementia are the frontal and temporal lobes. People with this type of dementia show often a mixture of behavioural and language symptoms because the frontal brain regions are important for our behaviour and the anterior temporal regions are important for our semantic knowledge of the world.

Subcortical dementias

Under the term subcortical dementia, I will group together three rarer forms of dementia:

Dementia with Lewy Bodies

Corticobasal degeneration

Progressive Supranuclear Palsy

They are in fact different diseases, however, for the purpose of which brain areas they affect, we can group them all together, as they all affect mostly the deeper regions of the brain – the subcortical regions of the brain.

The previous dementias we discussed affected mostly the surface areas – called cortical regions – of the brain but Dementia with Lewy Bodies, Corticobasal degeneration and Progressive Supranuclear Palsy all affect the deeper, subcortical brain regions.

The subcortical brain regions have various functions with one key function being coordinating our movements. Many of those subcortical dementias are therefore called ‘Parkinsonian’ as they can appear similar to Parkinson’s disease in the movement symptoms they present with. Just to clarify these people do not have Parkinson’s disease but movement symptoms which are similar to Parkinson’s disease.

The exception being Dementia with Lewy Bodies which actually has the same disease process as Parkinson’s disease and therefore Parkinson’s disease with dementia and Dementia with Lewy Bodies can be difficult to distinguish.

One other key aspect to understand with these subcortical dementias is that the subcortical regions are strongly connected to the surface (cortical) regions of the brain. This connectivity means that changes in the subcortex can sometimes also cause ‘cortical’ symptoms, as the disease changes in the subcortex affect the cortical regions as well.

The most common cortical symptoms in these subcortical dementias are related to frontal and parietal lobe functions. This means that people with these dementias can show behavioural and decision-making changes if the subcortical regions connected to the frontal brain are affected. Similarly, they can have visuospatial difficulties, such as having hallucinations when the subcortical regions connected to the parietal cortex are affected. Episodic memory problems are rarer in the subcortical dementias.

In summary, subcortical dementia affects the deeper, subcortical brain regions which are important for our movement. However, the deeper brain region also is strongly connected to some surface regions of the brain and cause therefore symptoms, such as behaviour change or changes to our vision.

Summary

The above is just an overview of the brain regions affected in different types of dementia and the symptoms those brain changes cause in dementia. Clearly it is a generalisation and individual people will show variations of the above for the different dementia subtypes. But it is still important for us to understand that those specific brain areas affected by dementia cause specific symptoms in people having the disease.

The final aspect to remember again is that those brain changes are more distinct in the earliest stages of dementia. Once dementia advances, the disease will have progressed to other brain regions causing symptoms in those other brain areas. For example, someone with Alzheimer’s disease might start with more isolated memory problem but over time might develop behavioural or movement symptoms. This means that people can develop other symptoms when the disease progresses which is important to understand as it will inform our dementia care in the long-term.

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