Dementia affects the brain in several ways, causing not just one or more structural changes but also, triggering a myriad of cognitive, functional, emotional, behavioural and physical changes as well. If you or a loved one was diagnosed with dementia, it may help to better understand what changes are happening within this vital organ as well as what outward changes to expect as the disease progresses.
A healthy human brain contains tens of billions of specialized cells called neurons. Neurons use chemical and electrical signals to process and transmit information, and to send messages between different parts of the brain and from the brain to various muscles and organs throughout the body. Dementia disrupts the communication among neurons, which ultimately results in loss of cell function and eventual death.
To better understand how dementia affects the brain, it may first help to understand what the various parts of a neuron do. Most neurons have three main components: a cell body, dendrites and an axon.
The very function and survival of brain cells depends on their ability to carry out the key biological processes for which nature intended them. The most critical processes are as follows:
Though neurons are the star players in the central nervous system, the brain contains other types of cells that are critical to healthy brain function. One such cell is the glial cell, which outnumbers neurons by 10 to 1.
Glial cells are not all created equal, with various types floating about the brain. Common forms of glial cells include astrocytes, microglial and oligodendrocytes. These cells are responsible for protecting and supporting the health and function of neurons by cleansing the brain of cellular debris and foreign substances. To carry out their jobs, glial cells must collaborate with blood vessels in the brain. Together, blood vessels and the cells maintain a delicate balance within the brain and ensure it functions at optimal capacity.
Though the brain does shrink with age, it is not normal for it to lose brain cells in large numbers. Yet, this is precisely what happens in dementia. With dementia, cellular damage becomes so widespread that neurons lose connections, cease to function entirely and eventually die. This is because dementia interferes with essential neuron function and the networks they maintain, including communication, repair and metabolism.
At first, and in most cases of dementia, the disease destroys neurons in just one part of the brain. Typically, the first parts of the brain to be affected are those that control memory. However, dementia eventually spreads to areas of the brain associated with reasoning, social behaviour and language. As time wears on, other areas of the brain become affected, gradually causing loss of motor control, senses, spatial awareness and more. As a result, dementia patients often become unable to live or function independently. For patients who would otherwise outlive the deterioration, the disease is fatal.
Though researchers understand what happens to the brain with dementia — widespread cell death — they still struggle to understand what causes it. Observations show that several changes take place within the brain of a person with dementia. However, researchers are still unclear as to whether these changes take place because of dementia or are what cause it.
That said, the fact is that changes to brain tissue do happen. Those changes are major and are what ultimately contribute to rapid cognitive decline. Below are a few of the most significant changes in brain tissue that researchers notice.
As the naturally occurring amyloid precursor protein breaks down, it forms several types of beta-amyloid proteins, each of which collect between the neurons. One form, beta-amyloid 42, is particularly toxic. As high levels of this protein build up in the brain, the molecules clump together, forming plaques that collect between the neurons and disrupt cell function. Though researchers are unclear as to how or at what stage of the disease beta-amyloid plaques influence Alzheimer’s, they are certain they do play a significant role.
Tau is another naturally occurring protein within the brain. In a normal functioning brain, structures called microtubules guide molecules and nutrients from cell bodies to axons and dendrites, thereby contributing to neuron health. Tau’s role is to bind to and stabilize microtubules for optimal functioning.
In a brain plagued by dementia, however, tau detaches from microtubules and begins to stick to other tau molecules, forming neurofibrillary tangles within neurons. These tangles block neurons’ transport systems and, in the process, hinder synaptic connections between brain cells.
Emerging evidence suggests that as beta-amyloid plaques build up and reach a tipping point, tau rapidly spreads throughout the brain. Though tau can build up in any part of the brain, research suggests that abnormal accumulation is most common in areas of the organ that affect memory.
In a healthy brain, glial cells consume and destroy waste that float through the brain. In a dementia brain, however, glial cells fail to perform this critical function, leading to a build-up of waste, debris, protein and beta-amyloid plaques. This build-up causes chronic inflammation, which contributes to the development and/or exacerbation of dementia.
Though researchers are still unsure as to why glial cells fail to perform their vital roles, recent studies focus on the TREM2 gene. Typically, this gene tells microglial cells to cleanse the brain of beta-amyloid plaques and fight inflammation. In brains where this gene does not function normally, however, plaques build-up between neurons. This build-up triggers the release of another type of glial cell called an astrocyte, the job of which is to clear plaque build-up and any cellular debris the microglial cells left behind. What ends up happening, though, is that the microglial cells and astrocytes collect around the neurons without ever cleansing the brain. As they continue to accumulate without performing their jobs, they release an inflammation-causing chemical that causes further damage to the neurons they were meant to protect.
It is rare for the brains of dementia patients to only show signs of Alzheimer’s-related changes. In most dementia patients, vascular issues — such as those that affect the blood vessels — play a role to some extent. In many instances, vascular issues lead to reduced blood flow and oxygen to the brain, which in turn triggers the breakdown of the blood-brain barrier. The blood-brain barrier is what protects the tissue from harmful substances while allowing glucose and other necessary agents entry. In persons whose blood-brain barrier is not intact, glucose cannot reach the brain, which then inhibits the brain’s ability to clear away toxic tau and beta-amyloid proteins. Again, an accumulation of these proteins leads to inflammation, which contributes to problems within the brain.
Researchers are certain that vascular conditions both contribute to and exacerbate brain troubles. For this reason, much research regarding treatment is focused on fixing or preventing vascular issues within the brain so as to stop the destructive cycle they cause.
A final and obvious change that researchers note in the brains of dementia patients is the loss of neural connections and cell death. As neuron connections disintegrate, cell death becomes more widespread, causing actual shrinkage of the brain. In the final stages of Alzheimer’s disease, brain atrophy has become so rampant that the brain volume of any one patient has diminished considerably.
The effects of dementia on the brain discussed above are those that researchers typically only notice after death, or that doctors can only witness with brain scans and imagery. However, the effects of dementia are obvious to many afflicted persons (at first) and their loved ones. These effects depend largely on which part(s) of the brain are afflicted and fall into four distinct categories: cognitive, emotional, physical and behavioural.
In persons with dementia, people close to them typically notice declines in cognitive abilities and functioning. Some changes that may become increasingly obvious include the following:
These changes typically occur when dementia affects the temporal lobe or cerebrum.
Many people with dementia in the frontal lobe experience drastic changes in emotions or moods. Because this lobe largely dictates social judgment and behaviour, some common changes associated with this type of dementia are as follows:
The parietal lobe is responsible for body sense and spatial awareness, meaning it helps us to know where our arms and legs are in relation to the rest of our body as well as helps us locate and recognize objects in 3D. When this lobe begins to deteriorate, you may notice loss of body control, unusual clothing choices or the lack of an ability to follow a logical sequence. Some other changes you may notice are as follows:
The frontal lobe is typically one of the first affected by dementia. It also happens to regulate emotions, behaviour and personality. As a result, some of the first signs of dementia fall into the behavioural category and may include the following:
Though these changes may appear in stages or gradually, if left unmanaged, dementia almost always progresses to include most of them.
Dementia is a scary disease, in part because it is so life-changing but also, because it is not well understood. Yet, as researchers learn more about how dementia affects the brain, they get closer to devising ways to manage it and, hopefully, reverse its effects.