Thin Cortex

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Aging Brain
Prematurity
Obstructive Hydrocephalus
Cerebral Infarction, Chronic
Encephalomalacia, General

Less Common

Multiple Sclerosis
Alzheimer Dementia
Multi-Infarct Dementia
Frontotemporal Dementia

Rare but Important

Microcephaly
Subcortical Laminar Heterotopic Gray Matter
Inborn Errors of Metabolism (Gray Matter Disorders)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Is cortical thinning focal (typical for encephalomalacia) or generalized?
Is cortex thin but normal signal intensity?
- If abnormal, consider infection, infarction, trauma, etc.
Child vs. adult
- Child: History important
  - Prematurity, family history of inborn error of metabolism
  - Seizures (heterotopias, encephalomalacia)
- Adult: Normal cognitive function or demented?

Helpful Clues for Common Diagnoses

Aging Brain
- White matter (WM), not gray matter (GM) volume loss predominates in normal “successfully aging” brain
  - Posterior vermis, cerebellum > cerebral hemispheres
  - Cortical thinning minimal
- “Black line” in visual, motor/sensory cortex common in normal older patients
Prematurity
- Hemispheric WM almost completely unmyelinated (“wet brain”)
- Cortex always appears thin
  - Pre- and post-central gyri myelinate early
  - Hyperintensity on T1WI, hypointensity on T2WI normal
- Note: White matter injury of prematurity spares GM
  - Undulating ventricular borders, ventriculomegaly
  - Generalized volume loss due to ↓ WM
Obstructive Hydrocephalus
- “Maximal” hydrocephalus thins cortical mantle
- May be difficult to distinguish from hydranencephaly on NECT
  - MR diagnostic
Cerebral Infarction, Chronic
- Usually wedge-shaped, involves both cortex & underlying WM
- “Hierarchy” of vulnerability to territorial or hypotensive ischemia
  - CA1 hippocampus most sensitive
  - GM generally more vulnerable than WM
- Collateral flow across pial watershed (border zones) may permit cortex within ischemic penumbra to survive
- Thin rim of cortex may persist adjacent to densely ischemic core of infarct
- Often hyperintense on T2/FLAIR, reflecting spongiosis/gliosis
Encephalomalacia, General
- Trauma, infection, toxic-metabolic insults
- May primarily affect GM, WM, or both
- Can be generalized (e.g., following global hypoperfusion) or focal

Helpful Clues for Less Common Diagnoses

Multiple Sclerosis
- Multiple T2/FLAIR hyperintensities perpendicular to callososeptal interface
- Chronic, severe multiple sclerosis (MS) causes variable brain atrophy
  - WM > > GM
  - But normal-appearing GM may have abnormal metabolic profile with ↓ NAA
  - Cortical loss in secondary-progressive MS common
Alzheimer Dementia
- Alzheimer dementia (AD) is most common of all dementias
- Best diagnostic clue = temporoparietal cortical atrophy + disproportionate hippocampal volume loss
  - Perihippocampal fissures widen
  - Hippocampal, entorhinal cortex thins
  - Temporal horns enlarge
  - Perfusion MR, FDG, & PET can identify hypometabolic areas
Multi-Infarct Dementia
- Also known as “vascular” dementia
- Second most common dementia after AD
  - 10-30% of all dementing disorders
- Imaging findings vary
  - Generalized, diffuse atrophy
  - Large ventricles, superficial sulci
  - Generalized cortical thinning
  - Focal territorial &/or lacunar infarcts
  - Subcortical WM T2/FLAIR hyperintensities
  - Diffuse bilateral, confluent deep WM hyperintensity secondary to arteriolosclerosis
Frontotemporal Dementia
- One of several tauopathies, also known as Pick disease
- Frontotemporal dementia (FTD) causes disproportionate frontotemporal atrophy
- “Knife-like” gyri with very thin cortex
- Subcortical WM usually hyperintense
- Parietal, occipital lobes relatively spared

Helpful Clues for Rare Diagnoses

Microcephaly
- Small head size, ↓ craniofacial ratio
- Sutural overlap common
- Simplified gyri with thin cortex
- Shallow sulci
- Many causes
  - Primary (genetic) microcephaly (e.g., § microlissencephaly, many syndromes)
  - Secondary (nongenetic) microcephaly (e.g., TORCH infection, fetal alcohol syndrome)
Subcortical Laminar Heterotopic Gray Matter
- “Band” heterotopia (“double cortex”): LIS1 or LISX1
  - Thick inner band of dysplastic GM in subcortical WM =’
  - Overlying cortex thin (not all neurons “arrive”) §
- Classic lissencephaly: (LIS1)
  - Shallow sylvian fissure (“hourglass” configuration of hemispheres)
  - Thin outer layer of GM
  - “Cell sparse” WM zone
  - Thick inner band of GM
Inborn Errors of Metabolism (Gray Matter Disorders)
- Includes inborn errors of metabolism that affect WM > > GM
- Many “poliodystrophies”; all uncommon
- All have similar imaging appearance
  - Generalized atrophy with ↑ sulci, thinned cortex
  - Cortical signal generally normal
  - BUT WM often hyperintense due to secondary axonal degeneration
- Lysosomal (example: Neuronal ceroid lipofuscinosis) clue
  - Hypointense thalami (best seen on standard T2WI, not FSE T2WI)

Image Gallery

Axial FLAIR MR in an intellectually normal 65 year old shows mild ventricular, sulcal enlargement. Thin rim of periventricular hyperintensity

is normal. Very mild cortical thinning

is present.

Axial T2WI MR in an elderly demented patient with subcortical arteriosclerotic leukoencephalopathy shows diffuse confluent hyperintensity in hemispheric white matter but only mild cortical thinning

(Left) Sagittal T2WI MR in a normal 28 week premature infant shows thin cortical ribbon

. The brain is smooth, and only the central

, calcarine

, and parietooccipital

fissures are present. (Right) Axial T2WI MR in the same patient shows age-appropriate, undersulcated brain. The shallow, “squared” sylvian fissures are normal, as is the very thin cortical mantle overlying almost completely unmyelinated hemispheric white matter.

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