Corpus callosum – Agenesis

Corpus callosum – Agenesis

Description

The corpus callosum is a bundle of white matter tracts that decussate between the cerebral hemispheres. The corpus callosum develops from the lamina reuniens of His, forming the rostrum, genu and splenium, respectively, in a rostrocaudal fashion; development is normally complete by the 20th week. Agenesis of the corpus callosum is a developmental disorder of the telencephalon, in which between the 12th and the 20th week, a defective lamina reuniens is created. The massa commissurales (formed from a portion of the lamina reuniens), whose cells are responsible for initiating the interhemispheric connection, never forms or is defective, and so precludes formation of a complete corpus callosum. Agenesis of the corpus callosum may be complete or partial, depending on the stage at which callosal development is arrested. The splenium is usually the affected region in partial agenesis.

Diagnosis

Agenesis of the corpus callosum may be directly observed, or more commonly, inferred from its effect on adjacent structures. Direct demonstration may be optimally made using sagittal views; the characteristic echolucent structure of the corpus callosum is normally seen superior to the cavum septi pellucidi and cavum vergae in the median sagittal plane, running an anteroposterior course. The anterior cerebral artery may be observed in front of the genu. Agenesis results in a loss of this echolucent zone, and depending on the degree of caval agenesis, the anterior cerebral artery will be seen to reside directly on the cavum septi pellucidi in this plane. Coronal views may also be helpful in providing more information. These views are best attained using a transvaginal, transfontanelle approach where a cephalic presentation exists. Even in a dedicated centre, however, a successful detection rate of only 65% may be anticipated. Indirect confirmation of agenesis of the corpus callosum is thus the most common approach, certainly in a routinely screened population. Observations are obtained from the transverse planes, and include the following: Marked separation of the lateral ventricles, with attenuation of the frontal horns and anterior bodies in conjunction with prominence of the atria and occipital horns (colpocephaly). The resultant transventricular view displays a bull’s horn” appearance. Other variants of dilation occur if there is associated obstructive hydrocephaly, cortical heterotopia, or focal loss of brain substance. Dilatation and elevation of the third ventricle, with variable degrees of interposition between the lateral ventricles. This characteristic transthalamic view has a “leaping frog” appearance. Observation of echogenic choroid plexus within the roof of the midline cystic structure aids diagnosis. Associated central nervous system anomalies include the Dandy-Walker and Arnold-Chiari type II malformations, porencephaly, agyria, midline lipomas, encephaloceles, arachnoid cysts, microcephaly, cerebellar hypoplasia, holoprosencephaly, septo-optic dysplasia, and aqueductal stenosis. Agenesis of the corpus callosum can be found in a number of well-defined genetic disorders, especially the Aicardi (XR), Andermann (AR), acrocallosal (AR), lissencephaly (AR, or with 17p13 deletion) and Shapiro (AR) syndromes. Other reports implicate the Apert (AD), Rubenstein-Taybi (16p13.3 deletion) and oral facial-digital I (XD) syndromes, and frontonasal dysplasia. It may also be found in association with trisomies 8, 13, and 18, so it is important that a detailed assessment of the fetal anatomy be carried out in addition to fetal karyotyping. The importance of the accurate diagnosis of isolated agenesis of the corpus callosum is related to its benign prognosis. In fact, once these associated disorders and malformations are excluded, the parents may be reassured and unnecessary obstetric intervention avoided. Prenatal diagnosis of callosal agenesis has been made as early as 20 weeks.”

Differential Diagnosis

Other midline cystic structures, and conditions presenting with mild ventriculomegaly, require exclusion: Cysts of the cavum septi pellucidi and cavum vergae present in a characteristic anterior midline location, and are not associated with the marked separation of the frontal horns of the lateral ventricles, nor with the undue prominence of the posterior horns. Choroid plexus will reside within the roof of the midline cyst in callosal agenesis, confirming the diagnosis of a dilated and elevated third ventricle. A cyst of the cavum velum interpositum is a midline cystic structure which lies superior to the third ventricle, and will demonstrate the high-intensity choroid plexus echoes at its base. The associated lateral ventricular changes will again not be present. The presence of a midline arachnoid cyst, although rare, may present diagnostic difficulty, for it may by its presence and size displace the lateral ventricles, compress the corpus callosum, and even obstruct the atria and occipital horns of the lateral ventricles while compressing the bodies. In such a case, identification of the choroid may lead to the correct diagnosis. Obstructive hydrocephalus may present in a similar fashion, especially in the early stages, and may only be distinguished by sagittal views to discern the presence or otherwise of the corpus callosum. Serial scans will usually demonstrate increasing ventriculomegaly if aqueductal stenosis is present. Adjoining porencephalic cysts may alter the dimensions and appearance of the posterior horns, but are unlikely to produce the clear separation and attenuation of the frontal horns.

Sonographic Features

Dilatation of atria and occipital horns of lateral ventricles.

Increased separation of lateral ventricles.

Disproportionate enlargement of occipital horns (colpocephaly).

Elevation and dilatation of third ventricle may present as an interhemispheric cyst.

Absent or deficient cavum septi pellucidi.

‘Bull’s horn’ appearance of pointed frontal horns and dilated occipital horns, as seen in the transventricular plane.

Leaping frog” appearance of dilated frontal and occipital horns of lateral ventricles seen in the same transventricular plane as the dilated and elevated third ventricle”.

 
 

Associated Syndromes

  • Acrocallosal
  • Agonadism
  • Aicardi
  • Alcohol, prenatal
  • Andermann
  • Apert
  • Basal cell nevus
  • Bertini
  • Brumback
  • Cao
  • Cerebro-facio-thoracic dysplasia
  • Chromosomal abnormalities
  • Cleft lip/palate- ectrodactyly
  • Coffin-Siris
  • Corpus callosum agenesis, isolated
  • Crane
  • Craniotelencephalic dysplasia
  • Da Silva
  • Dellman
  • Diaphragmatic hernia-hypertelorism-deafness
  • DK-phocomelia- encephalocele- urogenital
  • Dwarfism-microcephaly
  • Ectodermal dysplasia-hypothyroidism
  • Edwards
  • FG syndrome
  • Fine
  • Focal dermal hypoplasia
  • Frontonasal dysplasia
  • Frontonasal dysplasia-acromelia
  • Froster-Iskenius
  • G syndrome
  • Hydrolethalus
  • Iris dysplasia-hypertelorism
  • Kallmann
  • Koslowski
  • Leigh
  • Leprechaunism
  • L’Hermitte
  • Lujan
  • Marshall-Smith
  • MASA
  • Meckel-Gruber
  • Menkes: X-linked
  • Moerman
  • Nasopharyngeal teratoma
  • Neu-Laxova
  • Neuroepithelial cysts
  • Neurofibromatosis
  • Nonketotic hyperglycinemia
  • Oculo-pituitary
  • Oro-facio-digital
  • Osteochondro-dysplasia -thrombocytopenia
  • Petersí-plus
  • Polyasplenia
  • Proteus syndrome
  • Proud
  • Rubenstein-Taybi
  • Say
  • Shapiro: hypothermia
  • Siber
  • Smith
  • Smith-Lemli- Opitz type II
  • Thakker-Donnai
  • Thanatophoric dysplasia
  • Toriello
  • Tuberous sclerosis
  • Van Biervliet
  • Vici
  • Walker-Warburg
  • Winter-Wigglesworth
  • X linked pachygyria
  • X linked-unusual facies-callosal agenesis
  • Young
  • Zellweger
  • Zimmer

References

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