CHAPTER 2: CENTRAL NERVOUS SYSTEM

2.1 AGENESIS OF THE CORPUS CALLOSUM

EPIDEMIOLOGY/GENETICS

Definition Complete or partial agenesis of the corpus callosum (ACC) is a failure of the callosal commissural fibers to cross in the midline and form the corpus callosum between the two cerebral hemispheres. Partial agenesis may also be called hypogenesis, or dysgenesis, of the corpus callosum.

Epidemiology Incidence may be as high as 1%, with many asymptomatic individuals.

Embryology The corpus callosum develops between the 12th and the 22^nd week of gestation. Vascular disruption or failure of formation may cause complete or partial agenesis. Associated abnormalities include hydrocephalus, microcephaly, pachygyria, and lissencephaly. More than 80 sporadic, genetic, and chromosomal syndromes have been described with ACC, including trisomies 13 and 18. The incidence of chromosomal abnormalities is approximately 10%. Agenesis of the corpus callosum, combined with intracranial cysts or eye anomalies, should suggest Aicardi syndrome, an X-linked dominant disorder with male lethality.

Inheritance Patterns Most isolated defects are sporadic. Autosomal dominant, recessive, and X-linked syndromes have been described.

Teratogens None known.

Prognosis Isolated ACC may be asymptomatic unless associated with other brain abnormalities. Typically, complete agenesis will have a worse prognosis than partial ACC. Associated brain, or extracranial, abnormalities suggest a guarded prognosis.

SONOGRAPHY

FREQUENT FINDINGS

• Absence of the cavum septum pellucidum

• Colpocephaly (teardrop-shaped lateral ventricle)

• Widened interhemispheric fissure

• Midline interhemispheric cyst

• Absence of the corpus callosum

• Absent pericallosal artery

• Atypical radiating appearance of the median sulci, which converge toward the third ventricle

LESS FREQUENT

• Medial intracranial lipoma in the third trimester

KEY FINDINGS/PITFALLS

• If ACC is diagnosed, a detailed anatomic survey is required for complete evaluation.

• The pericallosal artery follows the superior surface of the corpus callosum. Complete and partial absence of the corpus callosum can be inferred by evaluating the course of the pericallosal artery.

• Three-dimensional (3-D) multiplanar imaging can be employed to obtain appropriate sagittal and coronal images of the corpus callosum.

• Fetal magnetic resonance imaging (MRI) should be considered as an integral part of the assessment of ACC, particularly when isolated ACC is diagnosed. Fetal MRI may detect additional cerebral anomalies (i.e., heterotopias and migration anomalies).

DIFFERENTIAL DIAGNOSIS

• Isolated absence of the cavum septum pellucidum: There is fusion of the frontal horns of the lateral ventricles with inferior pointing. The corpus callosum is present.

• Lobar holoprosencephaly (HPE): There is absence of both the cavum septum pellucidum and the normal interhemispheric fissure.

• Septo-optic dysplasia must be distinguished from isolated absence of the cavum septum pellucidum. MRI or 3-D ultrasound can be employed to look at the optic chiasm and nerves. Atrophy of the optic chiasm would indicate a diagnosis of septo-optic dysplasia rather than isolated ...