Partial agenesis of corpus callosum

Partial agenesis of the corpus callosum (pACC) means that the corpus callosum is incompletely formed, with some segments present and others absent.

It is a spectrum between hypoplasia/dysplasia and complete agenesis, and prognosis is driven by which segments are missing and what else is abnormal.

The corpus callosum develops antero-posteriorly (rostrum → genu → body → splenium).

Partial agenesis = arrest of development

  • Common pattern: absent splenium ± posterior body
  • Less common: anterior segments absent with posterior preserved

This is different from:

  • Hypoplasia (thin but continuous)
  • Dysplasia (malformed but continuous)

Embryology and etiology

Timing

  • CC development: ~8–20 weeks
  • Partial agenesis reflects interruption during formation

Etiologic categories

  • Genetic
  • Associated CNS malformations
  • Vascular disruption
  • Environmental / acquired insults

Ultrasound findings

Direct signs (mid-sagittal plane)

  • Shortened corpus callosum
  • Absent splenium or rostrum
  • Abrupt termination of CC
  • Discontinuity rather than thinning

Indirect signs

  • Absent or abnormal CSP
  • Colpocephaly (disproportionate occipital horn dilatation)
  • Parallel lateral ventricles
  • Elevated or enlarged third ventricle
  • Teardrop ventricles

Doppler clue

  • Absent pericallosal artery branch in the missing segment

Pericallosal artery

Normal

  • Continuous artery running over the CC from genu to splenium

Partial agenesis patterns

CC segment Pericallosal artery
Posterior CC absent Posterior pericallosal artery absent
Anterior CC absent Anterior pericallosal artery absent
Hypoplasia Artery present but thin
Dysplasia Artery present but tortuous

Vascular absence confirms true agenesis, not delayed maturation.

MRI findings

  • Confirms which segments are absent
  • Shows radial sulcal pattern posteriorly
  • Identifies associated cortical malformations
  • Evaluates posterior fossa and optic pathways

Genetic associations

Chromosomal abnormalities

  • Trisomy 8, 13, 18
  • Pathogenic CNVs (1p36, 8p, 15q, 22q11)

Single-gene disorders (common and relevant)

  • DCC – partial agenesis, mirror movements, variable outcome
  • L1CAM – X-linked, often with ventriculomegaly
  • ARX – severe neurodevelopmental outcome
  • KDM5B, KMT2D
  • DYNC1H1
  • TUBB3 / TUBA1A (if cortical malformations present)

Syndromic associations

  • Acrocallosal syndrome
  • Mowat–Wilson syndrome
  • Aicardi syndrome (rarely partial)
  • Joubert spectrum (with posterior fossa findings)

Prognosis

Isolated partial agenesis

  • Often better than complete agenesis
  • Many children have:
    • Normal intelligence
    • Mild learning or coordination difficulties
  • Epilepsy risk is low but not zero

Non-isolated partial agenesis

Outcome depends on:

  • Cortical malformations
  • Ventriculomegaly progression
  • Posterior fossa anomalies
  • Genetic diagnosis

These carry a high risk of intellectual disability and seizures.

Scenario Outcome
Isolated posterior pACC Usually favorable
Isolated anterior pACC Variable
pACC + ventriculomegaly Guarded
pACC + cortical malformations Poor
Monogenic / syndromic Usually poor

Counseling

  • Partial agenesis is not the same as complete absence
  • Prognosis depends more on associated findings than on the callosum alone
  • MRI and genetics refine risk substantially
  • Normal early milestones do not exclude later learning issues

Postnatal follow-up

  • Neonatal MRI
  • Developmental surveillance
  • Vision and hearing screening

Take-home messages

  • Pericallosal artery anatomy is decisive
  • MRI determines prognosis
  • Isolated pACC often does well
  • Genetics matter most when pACC is non-isolated