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
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Epidemiology
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 paediatric 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 recognised as another risk factor 6.
Associations
Associated anomalies can be frequent and broad which includes:
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aneuploidy syndrome
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non-aneuploidy syndrome
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other CNS associations: often multiple present
Dandy-Walker spectrum (11%)
hydrocephalus (30%): particularly the trigones and posterior horns of lateral ventricles = colpocephaly
intracranial lipoma (10%)
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inborn errors of metabolism 6
pyruvate metabolism disorders
congenital lactic acidosis (due to mitochondrial respiratory chain defects)
Clinical presentation
Isolated partial dysgenesis of the corpus callosum is often asymptomatic. The clinical picture in other cases is dictated by the associated anatomical abnormalities that are frequently found, especially in agenesis 4.
Children with agenesis may have dysmorphic facies, most commonly demonstrating hypertelorism 6. Very rarely, patients may have clinical features in keeping with Shapiro syndrome.
Pathology
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 orientated vertically, separating the lateral ventricles widely, in a racing car sign configuration. These bundles of white matter are known as Probst bundles.
The anterior commissure is usually present and often enlarged. The hippocampal formations are usually hypoplastic, with resultant dilatation of the temporal horns of the lateral ventricles.
Dysgenesis (which may be complete or partial) is a result of encephalomalacia secondary to toxic, ischaemic or traumatic events 2.
Embryology
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. Some studies using MR tractography have 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.
One apparent exception to this rule is holoprosencephaly in which it is the anterior parts of the corpus callosum which are absent 7. This has been termed atypical callosal dysgenesis.
Myelination of the corpus callosum occurs in the opposite direction, from the splenium forwards.
Radiographic features
As with other structural anomalies, all modalities reflect the underlying morphology with variable degrees of sensitivity.
Ultrasound
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third ventricle 5
dilated
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
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lateral ventricles
widely spaced parallel bodies (racing car sign)
small frontal horns
colpocephaly: which can give a "teardrop" configuration on axial scans
septum pellucidum: absent
interhemispheric fissures: widened
gyri: may be seen in a "sunray appearance" on the sagittal plane
colour Doppler study may show an abnormal course of pericallosal arteries
MRI
MRI is the modality of choice in evaluating both the corpus callosum and the frequently associated anomalies. Features include:
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ventricles
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
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cortex
radial gyri (absent cingulate gyrus)
everted cingulate gyrus 10
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limbic system 4
hypoplastic fornices
hypoplastic hippocampi
Angiography (DSA)
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.
Differential diagnosis
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:
interhemispheric arachnoid cyst
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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