Thin Corpus Callosum

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Normal Variant
Immature Brain
Encephalomalacia
Multiple Sclerosis
White Matter Injury of Prematurity
Callosal Dysgenesis
Callosectomy/Callosotomy
Obstructive Hydrocephalus

Less Common

Hypomyelination
Alcoholic Encephalopathy
Injury (Any Cause)

Rare but Important

Susac Syndrome
Holoprosencephaly
Inherited Metabolic Disorders
Hereditary Spastic Paraplegia with Thin Corpus Callosum (HSP-TCC)

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Diffuse corpus callosum (CC) thinning can be normal
- Newborn (immature brain)
Abnormally thin CC can be inherited or acquired
- Seen in many congenital malformations, inherited metabolic disorders
- Check history for trauma, surgery, ischemia-infarction
Thin CC, normal signal hyperintensity
- Normal variant, immature brain
- Secondary to hemispheric white matter (WM) volume loss
- Dysgenesis
Thin CC, abnormal signal intensity
- Hypomyelination or demyelinating disease (chronic MS, Susac syndrome)
- Injury (trauma, ischemia, radiation, toxic-metabolic insult)
- Obstructive hydrocephalus

Helpful Clues for Common Diagnoses

Normal Variant
- Focal thinning of corpus callosum at “isthmus” (junction between posterior body, splenium) is normal
- Sagittal section slightly off-midline can make CC appear mildly thinned
Immature Brain
- Hemispheric WM in newborn unmyelinated, CC thin and hypointense on T1WI
- As myelination progresses, CC thickens, becomes hyperintense on T1WI
  - CC splenium at 4 months
  - CC genu at 6 months
  - By 8 months CC essentially like an adult’s
Encephalomalacia
- Holohemispheric WM volume loss, regardless of etiology, causes diffuse CC thinning
- Focal WM loss can cause focal CC thinning
Multiple Sclerosis
- Look for T2/FLAIR hyperintense lesions along callososeptal interface
- Ependymal “dot-dash” sign along callosoventricular border occurs early
- Long-standing MS with decreased hemispheric WM volume results in thinned CC
White Matter Injury of Prematurity
- CC thinning secondary to periventricular white matter infarction
- Posterior CC disproportionately affected
Callosal Dysgenesis
- Hypoplasia or absence of part or all of CC
- CC remnants vary in size, shape
- Most common abnormality associated with other malformations
  - Chiari 2 malformation
  - Heterotopias
  - Interhemispheric lipoma
  - Cephaloceles
Callosectomy/Callosotomy
- History important!
- Look for surgical changes of craniotomy, ventriculostomy
Obstructive Hydrocephalus
- Obstructive hydrocephalus causes two kinds of CC abnormalities, stretching & intrinsic signal abnormality
- As lateral ventricles enlarge, CC is stretched, appears thinned
  - Look for associated signal abnormality in CC (sagittal T2WI/FLAIR best)
- Post-shunt decompression may show CC thinning, signal abnormality
  - Can appear bizarre, causing horizontal hyperintense “streaks” in CC on axial imaging
  - Can extend into periventricular WM
  - Theories: Impingement of CC against falx cerebri with resulting ischemia or axonal stretch

Helpful Clues for Less Common Diagnoses

Hypomyelination
- Undermyelination, delayed myelin maturation
- Diminished/absent WM myelination
- Can be primary or secondary
Alcoholic Encephalopathy
- Marchiafava-Bignami disease
  - Alcohol toxic to WM
  - Necrosis in middle layers of CC
  - Thinned, hypointense CC seen on T1WI
- Look for other associated abnormalities
  - Superior vermian atrophy
  - Wernicke encephalopathy
Injury (Any Cause)
- Trauma (e.g., axonal injury, radiation-induced leukoencephalopathy)
- Ischemia

Helpful Clues for Rare Diagnoses

Susac Syndrome
- M < F
- Classic triad
  - Encephalopathy (headache, confusion, memory loss)
  - Vision problems (retinal artery occlusions)
  - Hearing loss
- Always involves CC
  - Central > callososeptal interface lesions
  - Middle callosal “holes” (subacute/chronic)
Holoprosencephaly
- Many variants; often affect CC
Inherited Metabolic Disorders
- Focal or diffuse atrophy
  - Focal: X-linked adrenoleukodystrophy
  - Diffuse: Many
Hereditary Spastic Paraplegia with Thin Corpus Callosum (HSP-TCC)
- HSP-TCC is one of many hereditary spastic paraplegias
  - Autosomal recessive with SPG11 gene mutations on chromosome 15
  - Progressive neurodegenerative disorder
- Clinical
  - Slow ↑ spastic paraparesis
  - Adolescent-onset cognitive decline
  - Pseudobulbar dysfunction
- Imaging
  - Thin CC (especially genu, body) with progressive atrophy
  - Cerebral, cerebellar atrophy often associated

Image Gallery

Sagittal T1WI MR in term infant imaged at 2 days of age shows thin corpus callosum

with no discernible myelination. This is the normal appearance of an immature, largely unmyelinated brain.

Axial T1WI MR in 32 week gestation premature infant shows very thin corpus callosum genu

, reflecting total lack of hemispheric myelination.

(Left) Axial DWI MR in a newborn shows extensive diffusion restriction of the left hemisphere following perinatal stroke. Acute axonal degeneration of the corpus callosum