Atrophy
Also known as: brain atrophy, cortical atrophy, cerebral atrophy, neurodegeneration
The loss of brain tissue volume or cortical thickness over time, reflecting a reduction in the size or number of neurons and their supporting cells.
Atrophy refers to the progressive reduction in brain tissue. On an MRI scan, it appears as thinning of the cortex, shrinkage of subcortical structures, or expansion of the fluid-filled spaces (ventricles and sulci) that fill the gaps left by lost tissue.
Some degree of atrophy is a normal feature of ageing — the brain loses roughly 0.2–0.5% of its volume per year in healthy adults over 60. The rate is not uniform: regions like the hippocampus, prefrontal cortex, and entorhinal cortex tend to thin faster than primary sensory areas.
What drives atrophy
Atrophy is not a single process. It reflects the net result of several overlapping mechanisms:
- Neuronal death — loss of nerve cell bodies
- Synaptic pruning — reduction in the density of connections
- Axonal degeneration — white matter tract breakdown
- Glial changes — alterations in the supporting cell population
- Vascular injury — micro-infarcts, white matter lesions reducing tissue viability
- Reduced metabolic activity — "use it or lose it" effects at the circuit level
Atrophy and brain age
Brain age models are largely detecting atrophy — specifically, whether the regional pattern of tissue loss matches what is expected for a person's chronological age. A larger brain age gap often reflects accelerated or regionally atypical atrophy.
Focal vs. diffuse atrophy
- Diffuse atrophy involves relatively uniform loss across the cortex. It is the typical pattern of normal ageing and is what most brain age models are calibrated against.
- Focal atrophy is concentrated in specific regions and is more likely to signal a pathological process. For example, hippocampal atrophy is a well-established early marker in Alzheimer's disease; frontotemporal atrophy is characteristic of FTD.
Reversibility
Most structural atrophy is not reversible in the conventional sense — lost neurons are not replaced. However, some apparent atrophy on MRI is partially reversible: heavy alcohol use causes brain shrinkage that partially recovers with prolonged abstinence; dehydration and certain medications can also transiently affect apparent brain volume.
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Related terms
Cortical thickness
The thickness of the brain's outer grey-matter ribbon (the cortex), measured millimetre-by-millimetre across its surface from a structural MRI.
Brain age gap
The difference between brain age and chronological age. A positive gap means the brain looks older than it should; a negative gap, younger.
Dementia / Alzheimer's disease
Dementia is a syndrome of progressive cognitive decline severe enough to impair daily life; Alzheimer's disease is its most common cause, characterised by amyloid plaques, tau tangles, and gradual neurodegeneration.
MRI
A non-invasive imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the brain's soft tissue structure.
FreeSurfer / FastSurfer
Software pipelines that automatically segment the brain from an MRI scan and extract morphometric measurements such as cortical thickness and regional volume.