● Anatomical considerations
Corpus callosum is the largest commissure of the human brain and is a recent phylogenetic acquisition of placental mammals.
It is about 10 cm long in an adult,
located in the midline in shape of a "C” in medium sagittal sections.
(Fig. 1, Fig. 2) It is subdivided into:
Rostrum- connects orbital surfaces of frontal lobes.
Genu- connects medial surfaces of frontal lobes.
Trunk/body - formed by white fibers of the radiated crown.
Splenium- connects occipital lobes.
- Arterial irrigation:
Mainly supplied by pericallosal arteries (anterior cerebral arteries branches) and posterior pericallosal arteries (posterior cerebral arteries branches).
- Subcallosal and middle callous arteries,
branches of the anterior communicant artery,
are also present in most individuals.
- Development and myelination:
The development of corpus callosum begins at 12 weeks and is completed at 20 weeks of gestational age.
The first segment to be formed is the genu,
and the development continues in a posterior direction. However,
the last segment to be formed is the rostrum.
Myelination,
on the other hand,
follows the opposite path (from posterior to anterior).
● Dysgenesis
Although not so prevalent in the general population,
corpus callosum malformations can be found in up to 1% of central nervous system exams.
They have an estimated prevalence of 1:4000 live births.
The risk factor seems to be maternal alcohol abuse during pregnancy (callosum dysgenesis is one of the possible findings in fetal alcohol spectrum disorders).
The spectrum of malformations includes a total absence of corpus callosum (agenesis) and partial dysgenesis (absence of one or more segments).
Clinical presentation can be extremely variable,
from asymptomatic patients (in cases of partial dysgenesis) to facial dysmorphisms,
seizures,
and developmental impairments.
● Agenesis
Agenesis often results from an insult to the fetus occurring between 8-12 weeks of gestation,
corresponding to an absence of development of the corpus callosum.
In these cases,
there is a constellation of findings:
- Classically,
there is involvement of hippocampal commissure,
which generates enlargement of temporal horns of lateral ventricles (Fig. 3, Fig. 4). In some cases,
there is still involvement of anterior commissure.
- Probst bands: These are white matter fibers that appear parallel to interhemispheric fissure.
They represent fibers that would normally cross the corpus callosum. As a consequence,
they move the medial walls of lateral ventricles,
further separating them. In coronal view,
they generate the appearance of "race car",
"moose head" or "Viking helmet" (Fig. 5,
Fig. 6,
Fig. 7).
- There may be absence or deformity of rotation of the cingulate,
depicting a "sunray appearance" for regional gyri in the sagittal plane,
with a high riding 3rd ventricle (Fig. 8,
Fig. 9).
- In axial view there is usually colpocephaly,
representing an enlargement of posterior horns of lateral ventricles. The explanation for this finding is total agenesis of the fornix,
which would not be incorporated into the Probst bands,
predisposing to the compensatory expansion of occipital horns of lateral ventricles (Fig. 10,
Fig. 11,
Fig. 12,
Fig. 13,
Fig. 14).
Partial dysgenesis
The most classical pattern is agenesis of rostrum and splenium,
explained by their later development. Hippocampal commissure may also be absent,
but the other segments of the corpus callosum and anterior commissure are usually thickened (Fig. 15,
Fig. 16).
Dysgenesis spectrum
As described,
the development of CC occurs in the anteroposterior direction (genu,
trunk,
and splenium).
The last part to form is the rostrum.
When there is dysgenesis, it occurs inversely to the regular order of development, therefore affecting more often the rostrum and the splenium. An exception to this rule occurs in holoprosencephaly,
in which it is possible to observe dysgenesis of anterior segments of the corpus callosum with normal appearance of the remaining ones (Fig. 17).
Lipomas and cysts
Lipomas are often found in association with dysgenesis of the corpus callosum. It is believed that they derive from an abnormal differentiation of meninx primitiva.
Its most common location is the interhemispheric fissure - they have a high association with malformations of the corpus callosum.
They may present with calcified components,
and their typical signal intensity is similar to subcutaneous fat in MRI,
with loss of signal in fat suppression sequences.
Interhemispheric cysts,
on the other hand,
may or may not present ventricular communication,
and present similarly to the cerebrospinal fluid signal (Fig. 18).
Associated syndromes
Some examples of syndromes in which corpus callosum malformations may occur:
• Chiari II
Main findings are meningocele or myelomeningocele,
small posterior fossa, inferior herniation of cerebellar tonsils through the foramen magnum,
and deformity of the brainstem. There may be other associated abnormalities,
such as syringomyelia, Klippel-Feil anomaly,
hydrocephalus, polymicrogyria.
Of patients with Chiari II syndrome,
7% manifest corpus callosum dysgenesis (Fig. 19,
Fig. 20).
• Dandy-Walker
It is the most common malformation of the posterior fossa,
characterized by the triad:
- Hypoplastic vermis;
- Dilation of the IV ventricle with a cystic aspect;
- Expansion of the posterior fossa and cranial displacement of the tentorium.
11% also have associated dysgenesis of the corpus callosum (Fig. 21,
Fig. 22).
• Aicardi syndrome
It is a rare syndrome,
linked to a genetic defect in the X chromosome (lethal in male patients),
that has a classic triad:
- Infantile spasms;
- Dysgenesis of the corpus callosum (Fig. 23);
- Chorioretinal lacunae.
There may be,
however,
several other abnormal findings,
such as gray matter heterotopias,
posterior fossa abnormalities, polymicrogyria,
and coloboma.
● Holoprosencephaly
It is a rare brain malformation that results from the incomplete (or even absent) separation of cerebral hemispheres. It is divided into degrees,
according to the separation of the hemispheres:
1.
Alobar: Most severe form,
in which there is no separation.
2.
Semilobar: Intermediate form.
There is incomplete separation.
3.
Lobar: Mild form,
with developed interhemispheric fissure and total or near total separation of the thalami (Fig. 24).
All types may present some degree of corpus callosum dysgenesis,
and it should be remembered that the typical pattern is a malformation of the most anterior segments in isolation,
unlike the other dysgenesis,
that usually predominates posteriorly.
Differential diagnoses
Some differentials that should be remembered:
• Corpus callosum impingement syndrome
Occurs with supratentorial hydrocephalus,
in which the corpus callosum is stretched (thinner) and displaced by the lateral ventricles (Fig. 25). The group of findings will suggest the disease.
• Thinning due to periventricular leukomalacia
White matter lesions due to hypoxic-ischemic injury associated with prematurity tend to determine the thinning of the corpus callosum. There is usually predominant involvement of CC posterior regions (Fig. 26).
• Post-surgical changes
History is the key in these cases. Some stigmas of surgical manipulation may help (Fig. 27,
Fig. 28).