Brain scans are often used as part of the assessment process for dementia. Looking at these images can raise an interesting question.
As dementia progresses, scans frequently show increasing amounts of fluid-filled space around and within the brain. If the brain is shrinking, where does all of this extra fluid come from?
The answer lies in understanding how the brain is protected and what happens when brain cells are gradually lost over time.
The Brain Is Not Packed Tightly Inside the Skull
Many people imagine the brain sitting snugly inside the skull, rather like an object inside a box. In reality, the brain is surrounded by a clear fluid called cerebrospinal fluid (CSF).
This fluid cushions and protects the brain and spinal cord. It helps absorb shocks, carries nutrients, removes waste products and supports the healthy functioning of the nervous system.
Cerebrospinal fluid is not something that appears because a person develops dementia. It is present throughout life and plays an essential role in keeping the brain healthy. The body continuously produces fresh cerebrospinal fluid and reabsorbs older fluid as part of a natural cycle.
Where Is the Fluid Made?
Cerebrospinal fluid is produced inside the brain by specialised structures called the choroid plexuses. The fluid circulates through a series of spaces known as ventricles before flowing around the brain and spinal cord and eventually being reabsorbed into the bloodstream.
These fluid-filled spaces are present in every healthy brain. The difference is that they become more noticeable when the brain begins to lose volume.
What Happens to the Brain in Dementia?
Dementia is caused by diseases that damage and destroy brain cells. As brain cells die, the brain gradually loses volume — a process known as atrophy.
Brain atrophy does not happen overnight and it does not affect every area equally. Different types of dementia damage different parts of the brain. Alzheimer’s disease often affects memory-related areas first. Frontotemporal dementia primarily affects the frontal and temporal lobes. Vascular dementia results from damage caused by problems with blood supply to the brain. Dementia with Lewy bodies affects networks involved in attention, movement and visual processing.
Research has shown that in Alzheimer’s disease, the brain loses on average around 2.4% of its total volume each year — significantly more than the 0.5% seen in normal ageing. Over the course of the condition, the cumulative effect of that loss can be substantial.
PET scan research from G. Small at UCLA School of Medicine, featured in Positive Approach to Care training, illustrates this clearly. Images comparing normal brains with those in late-stage dementia show not only a significant reduction in overall size, but a dramatic decrease in brain activity — with late-stage dementia brain scans bearing a closer resemblance in both size and function to a child’s brain than to a healthy adult brain.
Why Does the Fluid Look Like It Is Increasing?
The skull is a fixed size.
As brain tissue shrinks, it cannot leave a vacuum behind. The space that was once occupied by healthy brain tissue is gradually filled by the fluid-filled spaces that already exist within and around the brain. The ventricles become larger. The grooves and folds on the surface of the brain become wider. The cerebrospinal fluid becomes more visible on scans.
This can create the impression that the body is producing large amounts of additional fluid. In reality, the fluid is simply occupying space that was previously filled by functioning brain tissue.
A helpful way to think about it is to imagine a grape slowly becoming a raisin. The skin remains, but the contents shrink — and as they do, the space inside becomes more apparent. The principle is similar for the brain: the overall space within the skull remains the same while the amount of tissue within it gradually decreases.
Does Dementia Cause the Body to Make More Fluid?
Usually, no.
In most cases, the increased visibility of fluid on a scan reflects the loss of brain tissue rather than excessive production of cerebrospinal fluid. The fluid has always been there. The spaces containing it have simply become larger.
This distinction matters because some medical conditions do involve abnormal fluid accumulation. In dementia-related brain atrophy, however, the primary change is the loss of brain tissue rather than the overproduction of fluid.
What Do Different Brain Scans Show?
Several types of scans may be used when investigating dementia, and each provides different information.
CT scans show the structure of the brain and can help identify strokes, bleeding, tumours and visible patterns of shrinkage. MRI scans provide even greater detail and can often reveal subtle patterns of atrophy associated with different types of dementia. PET scans take a different approach altogether — rather than showing structure, they show how the brain is functioning. Some PET scans show how different areas of the brain are using glucose for energy; others can identify specific proteins associated with diseases such as Alzheimer’s. This means PET scans can sometimes detect changes in brain function before significant shrinkage becomes visible on other types of scan.
Why Understanding Brain Shrinkage Matters
Brain scans can help explain why someone may be experiencing increasing difficulties with memory, communication, problem-solving, visual processing or everyday activities.
However, scans only tell part of the story. Two people with apparently similar scan results may have very different abilities, personalities, coping strategies and support needs. This is one of the reasons why dementia care should never focus solely on what a scan shows.
Understanding the person — their history, their preferences and their retained abilities — remains just as important as understanding the changes occurring within the brain itself. No scan can capture that. It comes from time, attention and a genuine commitment to seeing the whole person.