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).
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.