Overview

  • ACC refers to complete or partial absence of the corpus callosum, one of the most common fetal neurological diagnoses.
  • Increasing use of fetal ultrasound and MRI has improved understanding of incidence, associated findings, and prognosis.
  • Can be:
    • Complete (entire corpus callosum absent).
    • Partial (hypogenesis or dysgenesis): incomplete formation or abnormal structure of parts of the corpus callosum.

Embryological Background

  • Corpus callosum formation involves multiple midline components.
  • Traditionally thought that partial ACC involved failure of posterior corpus callosum (splenium); however, partial ACC may involve failure of any callosal region.
  • Corpus callosum formed between weeks 12–20 gestation:
    • Fully formed by about 20 weeks gestation.
    • Diagnosis of partial ACC difficult before 20 weeks.

Diagnosis

Ultrasound Challenges:

  • Fetal ultrasound detection challenging due to fetal movement.
  • High false-positive rate (~20%) with ultrasound alone.
  • Limited sensitivity to associated anomalies.

Fetal MRI (Gold standard):

  • Detects additional anomalies in >20% of cases initially diagnosed as isolated ACC.
  • Postnatal MRI can detect further anomalies in ~15% of cases.

Imaging features supporting ACC diagnosis:

  • Absence of cavum septi pellucidi.
  • “High-riding” third ventricle.
  • Colpocephaly: enlarged occipital horns of lateral ventricles.
  • Parallel orientation of lateral ventricles ("steer-horn" sign due to Probst bundles).
  • Radial ("sunburst") arrangement of medial hemispheric sulci.
  • Abnormal vertical trajectory of pericallosal arteries on Doppler ultrasound.
  • Possible presence of midline cysts/lipomas.

Associated Anomalies & Syndromes

  • ACC is classified as:
    • Isolated (no other anomalies).
    • Complex (associated with other malformations/syndromes).
  • Complex forms are common (50–90% of cases).

Cerebral Anomalies Often Associated with Complex ACC:

  • Cortical malformations (in ~50%):
    • Polymicrogyria
    • Lissencephaly
    • Pachygyria
    • Schizencephaly
  • Posterior fossa anomalies (Dandy-Walker malformation in up to 1/3 of cases).
  • Associated with neurological syndromes:
    • Aicardi syndrome
    • Walker-Warburg syndrome
    • Myelomeningocele
  • Associated chromosomal abnormalities (~20% cases overall):
    • Trisomy 13, 18, and other chromosomal deletions/duplications.
  • Metabolic disorders (rare):
    • Non-ketotic hyperglycinemia
    • Pyruvate dehydrogenase deficiency

Prognosis & Outcome

  • Prognosis depends heavily on associated anomalies:
    • Truly isolated ACC has a highly variable outcome but generally better prognosis:
      • Normal outcome: 40–75%
      • Mild-moderate developmental issues: ~15–50%
      • Severe abnormality: ~10%
    • Complex ACC (with additional anomalies):
      • Nearly universal significant neurodevelopmental impairment.
  • ACC often detected around fetal anomaly scan (~18–20 weeks), complicating counseling and prognosis due to cortical development still ongoing.

Specific Clinical Entities

1. Aicardi Syndrome

  • X-linked dominant, lethal in males (almost exclusively affects females).
  • Features (in addition to ACC):
    • Infantile spasms
    • Ocular anomalies (retinal lacunae, coloboma)
    • Markedly asymmetrical ventriculomegaly
    • Cortical dysplasias (polymicrogyria, heterotopias)
    • Posterior fossa anomalies (cerebellar cysts/hypoplasia)
    • Subependymal cysts
    • Vertebral and costal anomalies
  • Prognosis: uniformly poor.

2. ACC with Midline Meningeal Dysplasia (Midline Cysts/Lipomas)

  • Midline cysts classified as:
    • Type 1 (communicating): single cyst, may have ventricular communication
      • Type 1A: Macrocephaly/hydrocephalus
      • Type 1B: Type 1A + ventricular obstruction (e.g., thalamic hamartoma)
      • Type 1C: Microcephaly
    • Type 2 (non-communicating): multiloculated cysts, distinct from ventricles due to meningeal dysplasia
      • Type 2A: Hydrocephalus alone
      • Type 2B: Aicardi syndrome
      • Type 2C: Associated cortical dysgenesis and heterotopias
  • Midline lipomas: result from abnormal meninx primitiva differentiation.

Partial ACC

  • Not just an incomplete version of complete ACC; may involve selective loss or underdevelopment of any segment (rostrum, genu, body, isthmus, splenium).
  • Often due to late developmental failure of neocortical fibers (particularly frontal cortex).
  • MRI: Pericallosal arteries typically curve upwards at posterior limit of residual corpus callosum.
  • Frequently associated with genetic/chromosomal syndromes.
  • Prognosis highly variable; associated anomalies strongly predict outcomes.

Clinical Management Considerations

  • Accurate prenatal counseling based on detailed MRI findings crucial.
  • Postnatal MRI recommended even if prenatal imaging suggests isolated ACC.
  • Multidisciplinary follow-up required: developmental pediatrics, neurology, genetics.