Dysgenesis of the corpus callosum
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Dysgenesis of the corpus callosum may be complete (agenesis) or partial (dysgenesis) and represents an in utero developmental anomaly. It can be divided into:
- primary agenesis: corpus callosum never forms
- secondary dysgenesis: corpus callosum forms normally and is subsequently destroyed
A true estimate of incidence is difficult to establish as many isolated cases are asymptomatic. It may be as uncommon as 1:20,000 according to autopsy series 6. It should be noted however that in pediatric neuroradiology practice it may be seen in as many as 1% of referrals, clearly representing a significant selection bias, attributable to a large number of symptomatic associated abnormalities.
There appears to be a male predilection (M:F ~2:1) 6. Maternal alcohol consumption during pregnancy has been recognized as another risk factor 6.
Associated anomalies can be frequent and broad which includes:
- aneuploidy syndrome
- non-aneuploidy syndrome
other CNS associations: often multiple present
- Chiari II malformation (7%)
- Dandy-Walker spectrum (11%)
- grey matter heterotopia
- hydrocephalus (30%): particularly the trigones and posterior horns of lateral ventricles = colpocephaly
- interhemispheric cysts
- intracranial lipoma (10%)
- schizencephaly 11
- inborn errors of metabolism 6
Isolated partial dysgenesis of the corpus callosum is often asymptomatic. The clinical picture in other cases is dictated by the associated abnormalities that are frequently found, especially in agenesis 4.
Children with agenesis may have dysmorphic facies, most commonly demonstrating hypertelorism 6.
Agenesis is a result of an insult occurring at approximately 8-12 weeks gestation 2,4 resulting in failure to form the corpus callosum. The white matter tracts which usually cross the midline, instead are oriented vertically, separating the lateral ventricles widely, in a racing car sign configuration. These bundles of white matter are known as Probst bundles.
Dysgenesis (which may be complete or partial) is a result of encephalomalacia secondary to toxic, ischemic or traumatic events 2.
The development of the corpus callosum occurs between the 12th and 16-20th weeks of gestation 2,4.
Traditionally, it was believed that development begins in the genu and progresses posteriorly with the rostrum appearing last.
More recent studies, including using MR tractography, cast some doubt on this assertion, instead suggesting that the anterior body develops first and then continues bidirectionally, with the anterior portions (genu) developing earlier/more prominently than the posterior portions (splenium) 12,13.
In primary dysgenesis parts of the corpus callosum which form before the insult will be present whereas later parts will be absent. The presence of the rostrum essentially excludes primary agenesis.
Myelination of the corpus callosum occurs in the opposite direction, from the splenium forwards.
As with other structural anomalies, all modalities reflect the underlying morphology, with a greater lesser degree of sensitivity.
- third ventricle 5
- can be elevated or dorsally displaced 8
- may communicate with the interhemispheric cistern
- may project superiorly as a dorsal cyst
- choroid may be seen as an echogenic structure in the roof of the cyst
- lateral ventricles
- septum pellucidum: absent
- interhemispheric fissures: widened
- gyri: may be seen in a "sunray appearance" on the sagittal plane
- color Doppler study may show an abnormal course of pericallosal arteries
MRI is the modality of choice in evaluating both the corpus callosum and the frequently associated anomalies. Features include:
- run parallel rather than the normal "bow-tie" configuration giving a racing car appearance on axial imaging
- colpocephaly (dilatation of the trigones and occipital horns) gives a characteristic "Texas longhorn"/moose head/viking helmet appearance on coronal imaging
- dilated high-riding 3rd ventricle appears to communicate with the interhemispheric cistern or project superiorly as a dorsal cyst
- bundles of Probst
- radial gyri (absent cingulate gyrus)
- everted cingulate gyrus 10
- limbic system 4
Angiography no longer has a role in diagnosis, however, if performed for other reasons may demonstrate an abnormal course of the anterior cerebral artery, passing directly posterosuperiorly with widely spaced pericallosal arteries, and absent pericallosal moustache. The azygos ACA is commonly present in most cases.
Treatment and prognosis
The overall prognosis can be highly variable depending on the presence of other associated anomalies.
High-quality MRI essentially eliminates differentials due to the exquisite imaging of the corpus callosum.
In the setting of an antenatal ultrasound demonstrating an interhemispheric cyst then the following should be considered 5:
The key to distinguishing these entities from a dilated third ventricle with dorsal cyst is identifying the choroid plexus, which demarcates the roof of the third ventricle 5.
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