What is cortical thinning?

Cortical Thinning: Definition and Clinical Significance

Cortical thinning refers to the reduction in thickness of the cerebral cortex, which can occur as part of normal aging or as a pathological process in various neurological conditions.

What is Cortical Thinning?

Cortical thinning describes the progressive decrease in the thickness of the cerebral cortex, which is the outer layer of neural tissue of the cerebrum. According to neuroimaging studies:

• The normal human neocortex occupies 90% of the overall cerebral cortical surface and consists of a compact, mostly six-layered ribbon of grey matter 1
• Normal cortical thickness typically ranges from 1 to 4.5 mm, being thinner in the depth of the sulci and thicker at the crown of the gyri 1
• Cortical thickness measurements should not be used as strict cut-off values as they vary with:
  • Age
  • Myelination status
  • Gender
  • Head size
  • Brain cortical area
  • Secondary phenomena (acquired lesions or seizures) 1

Normal Age-Related Cortical Thinning

Cortical thinning occurs naturally as part of the aging process:

• In healthy middle-aged adults (44-49 years), longitudinal studies have measured a mean annual decrease in cortical thickness of 0.26% in the left hemisphere and 0.17% in the right hemisphere 2
• Age-related thinning predominantly affects the frontal and cingulate cortices, while medial and lateral temporal regions are generally spared 2
• The normal external cortical surface and cortico-subcortical junction should always be smooth and sharply delineated despite age-related thinning 1

Pathological Cortical Thinning

Accelerated or abnormal patterns of cortical thinning can be indicative of various neurological conditions:

Neurodegenerative Disorders

• Alzheimer's Disease (AD):

  • Shows significant cortical thinning in bilateral frontal, parietal, temporal, and occipital lobes 3, 4
  • Particularly affects the medial temporal lobe in early stages 3
  • Cognitive impairment negatively correlates with cortical thickness in frontal, parietal, and temporal lobes 4

• Mild Cognitive Impairment (MCI):

  • Shows thinning mainly in the medial temporal lobe and some regions of the frontal and parietal cortices 3
  • Represents a potential pre-clinical stage of Alzheimer's disease 3

• Frontotemporal Dementia (FTD):

  • Exhibits a different pattern than AD with thinning primarily in bilateral frontal and temporal regions 4
  • Also shows some thinning in inferior parietal regions and posterior cingulate 4
  • Compared to AD, FTD patients have relatively preserved thickness in parietal and precuneus regions 4

Inflammatory Conditions

• Multiple Sclerosis (MS):
  • Shows both diffuse and focal cortical thinning 5
  • Mean overall thickness of the cortical ribbon is reduced in MS patients compared to controls 5
  • Focal cortical thinning in frontal and temporal brain regions can occur even early in the disease or in patients with mild disability 5
  • Patients with longstanding disease or severe disability present with additional focal thinning of the motor cortex area 5

Genetic Influences on Cortical Thinning

• Cortical thinning during development is highly heritable 6
• Different genetic factors influence various areas of the brain during development 6
• Specific genetic factors affect thinning in language-related areas (Broca's and Wernicke's areas), sensory-motor regions, and frontal poles 6

Imaging Techniques for Measuring Cortical Thickness

• MRI is the primary modality for measuring cortical thickness:

  • High-resolution structural MRI with automated surface reconstruction techniques can yield accurate measurements 5, 4
  • Cortical thickness measurement is considered more reliable than volume for detecting atrophy due to the low variability in the cytoarchitectural structure of grey matter 3

• Advanced MRI techniques:

  • Diffusion Tensor Imaging (DTI) can identify post-traumatic microstructural changes in white matter, though data in children remains limited 1
  • MR Spectroscopy may detect metabolic changes in the brain following trauma 1
  • Functional MRI (fMRI) may show changes in connectivity that correlate with post-concussion symptoms 1

Clinical Implications

• Patterns of cortical thinning can serve as biomarkers for specific neurological conditions
• Measurement of cortical thickness can help differentiate between normal aging and pathological processes
• Cortical thickness measurements can improve the classification of different types of dementia when combined with neuropsychological testing 4
• Studying regions that are specifically vulnerable to or spared from healthy age-related cortical thinning may be important for the early identification of neurodegeneration 2

Cortical thinning analysis provides valuable insights into both normal brain development and pathological processes, offering potential for early detection and monitoring of various neurological conditions.