Journal of Pediatric Neurosciences
REVIEW ARTICLE
Year
: 2020  |  Volume : 15  |  Issue : 4  |  Page : 352--357

Magnetic resonance imaging findings in fetal corpus callosal developmental abnormalities: A pictorial essay


Cindhya Manor1, Rajeswaran Rangasami1, Indrani Suresh2, Sudarshan Suresh2,  
1 Department of Radiology, Sri Ramachandra Institute of Higher Education and Research (DU), Chennai, Tamil Nadu, India
2 Department of Fetal Medicine, Mediscan Systems, Chennai, Tamil Nadu, India

Correspondence Address:
Dr. Rajeswaran Rangasami
Department of Radiology and Imaging Sciences, Sri Ramachandra Institute of Higher Education and Research, Chennai 600116, Tamil Nadu.
India

Abstract

The corpus callosum is the principal supratentorial cerebral commissure, which connects the two cerebral hemispheres in the midline. It is divided into rostrum, genu, body, and splenium. Affected patients may develop mental retardation, dysmorphic features, spasticity, ataxia, or epilepsy. Corpus callosal abnormalities may be isolated or be associated with other anomalies such as sulcal abnormality, ventriculomegaly, cerebellar hypoplasia or cerebellar vermian hypoplasia. Magnetic resonance imaging (MRI) plays a major role in the diagnosis of fetal corpus callosal developmental abnormalities when they are suspected on sonography. This pictorial essay shows the MRI findings in fetal corpus callosal developmental abnormalities in a very systematic manner.



How to cite this article:
Manor C, Rangasami R, Suresh I, Suresh S. Magnetic resonance imaging findings in fetal corpus callosal developmental abnormalities: A pictorial essay.J Pediatr Neurosci 2020;15:352-357


How to cite this URL:
Manor C, Rangasami R, Suresh I, Suresh S. Magnetic resonance imaging findings in fetal corpus callosal developmental abnormalities: A pictorial essay. J Pediatr Neurosci [serial online] 2020 [cited 2021 Jun 18 ];15:352-357
Available from: https://www.pediatricneurosciences.com/text.asp?2020/15/4/352/307364


Full Text



 Introduction



The corpus callosum (CC) is the principal supratentorial cerebral commissure. Adjacent to the CC, there are two other smaller interhemispheric fissure connections, namely the anterior commissure and the hippocampal commissure.[1] Fetal corpus callosal abnormalities (CCA) may occur due to genetic causes, intrauterine infection, vitamin deficiency, vascular and unknown causes. A number of genetic disorders in humans have been associated with CCA, including several X-linked diseases, metabolic disorders and contiguous gene deletion syndromes.[2] Magnetic resonance imaging (MRI) is the modality of choice in the assessment of fetal CC due to its multiplanar capability and contrast resolution. As there are several CCA, it is important to view the CC in all the three planes on MRI before making a diagnosis.

 Anatomy and Embryology



The CC is the principal supratentorial cerebral commissure made up of white matter tracts tightly packed to connect the two cerebral hemispheres in the midline. It consists of more than 200–250 million myelinated axons that cross the midline in the developing brain to connect the two hemispheres. The CC is divided into four parts (anterior to posterior): (1) rostrum, (2) genu, (3) body, and (4) splenium. The sequence of CC development begins with the differentiation of the commissural plate around six gestation weeks (GW), followed by crossing of pioneer axons.[2] During the 8th to the 14th gestational weeks, callosal precursors and the cortical fibers of bilateral cerebral hemispheres develop to form the CC, it begins with the development of genu anteriorly and proceeds to the splenium posteriorly except for the rostrum, which is the last area that is seen to show crossed fibers.[3],[4] The final shape is reached by 19–20 weeks [Figure 1], though it keeps growing to reach a good volume by 6–9 years. The myelination of CC occurs only after birth and is not complete until adolescence. Owing to defect in embryological development, CCA can be isolated or can occur in combination with other anomalies.{Figure 1}

 Discussion



Terminologies used to discuss CCA are heterogeneous. Some authors just described partial and complete agenesis. Then it was refined as agenesis, hypogenesis, and dysgenesis. Hanna et al.[5] further refined CCA and divided them into four principal classes as follows:

Hypoplasia- Hypoplasia without dysplasia

- Apple core CCA

- Anterior remnant CCA

Dysplasia without hypoplasia

Hypoplasia with dysplasia- Stripe CCA

- Kinked CCA

Complete agenesis

We reviewed the various fetal corpus callosal morphological abnormalities encountered during November 2014 to October 2018 in our institution and have discussed them [Figure 2]. These 10 fetuses did not have any consanguineous background. We have tried to display the various morphological abnormalities that can occur in CC irrespective of the etiology. Of the 10 fetuses, six couples opted for termination and four were managed conservatively. The diagnosis was confirmed by postnatal imaging in the four conservatively managed cases. In the terminated six fetuses, the diagnosis was confirmed by autopsy in four cases. Two couples did not consent for fetal autopsy. The MR images of these two fetuses were read independently by two senior fetal imaging specialists, and the final diagnosis was arrived by consensus.{Figure 2}

Hypoplasia

Hypoplasia consists of uniformly thin or underdeveloped CC in the posterior region [Figure 3]. It has three subtypes: hypoplasia without dysplasia, apple core CCA, and anterior remnant CCA.[5] The normal thickness and measurements of CC at various gestational age have been discussed by Pashaj et al.[6]{Figure 3}

Hypoplasia without dysplasia

In this, the caliber of the CC is reduced, but has all major anatomic features intact, including a distinctive rostrum, genu, corpus, and splenium[5] [Figure 4].{Figure 4}

Apple core CCA

More severe forms of hypoplasia show progressive loss of the more caudal aspects of the CC. This includes a form where the posterior CC is hypoplastic and foreshortened (i.e., corpus or splenium), taking on the appearance of an apple core CCA [Figure 5].{Figure 5}

Anterior remnant CCA

In this subtype, there is agenesis of the posterior CC (i.e., corpus and splenium) called anterior remnant CCA type [Figure 6].{Figure 6}

Dysplasia without hypoplasia

Dysplasia means that the development of the CC has occurred but is malformed.[5] This type encompasses cases in which the CC is morphologically abnormal [Figure 7].{Figure 7}

Hypoplasia with dysplasia

In this class of CCA—hypoplasia with dysplasia—there is disturbance of the overall shape of the CC that is not limited to the posterior region.[5] It has two subtypes—stripe CCA and kinked CCA.

Stripe CCA

This type consists of a uniform thin stripe CCA that lacks an anatomically distinct genu and splenium [Figure 8].{Figure 8}

Kinked CCA

This type is characterized by a CC that is hypoplastic as well as obviously kinked at one or more locations, either anteriorly or posteriorly, and is called kinked CCA [Figure 9].{Figure 9}

Complete agenesis

The next class of CCA is characterized by absent CC as seen on a midline sagittal MR image [Figure 10]. In this class, it is important to distinguish the hippocampal commissure from the CC, the former retained in many complete agenesis cases.[7]{Figure 10}

Agenesis of the CC is one of the common central nervous system malformations. CC agenesis may occur as an isolated lesion. But it is more commonly seen in association with a variety of other central nervous system malformations or malformations of other organ systems. The lack of commissural fibers crossing the midline is a primary feature of this anomaly. There is an ipsilateral migration of the uncrossed fibers, coursing along the superomedial region of the lateral ventricles to form a structure called Probst bundles.[8]

The pathologic changes produced by CC agenesis in the cerebral hemispheres include absent or malformed CC, cingulate gyrus and sulcus. The changes seen in the ventricular system include a high-riding third ventricle, which opens to the interhemispheric fissure superiorly. A radial or “spoke-like” configuration of the sulci and gyri around the third ventricle is seen on the medial hemispheric surface. A dorsal cyst may occasionally be present in this space. There is displacement of the lateral cerebral ventricles superiorly and laterally. In most cases, on axial sections of the brain, a disproportionate dilatation of the occipital horns of the bilateral lateral ventricles called as colpocephaly may be seen.[9]

Mega corpus callosum

Mega corpus callosum (MCC) is extremely rare unlike agenesis and hypoplasia. It may present as an isolated anomaly or be associated with abnormalities such as megalencephaly and polymicrogyria. In this condition, the dimensions of the CC are increased uniformly [Figure 11]. It is postulated that the thickening is due to the persistence of anomalous longitudinal midline supracallosal fibers, which usually disappear during axonal migration.[10]{Figure 11}

CCA with interhemispheric cyst

Interhemispheric cysts are encountered in 7% of the patients diagnosed with CC agenesis.[8] The origin of the interhemispheric cyst in cases of CC agenesis is controversial. Suggested possible causes are arachnoid, neurenteric and ependymal cysts. Barkovich et al.[11] classified interhemispheric cysts, based on morphology into two major types. Type 1 cysts are unilocular and appear to be a diverticulum of the third or the lateral ventricle. Type 2 cysts are multilocular, and there is no communication with the ventricular system [Figure 9].[11]

CCA with pericallosal lipoma

A pericallosal lipoma is relatively common in partial or complete agenesis of the CC [Figure 12]. The size of the lipoma ranges from less than a centimeter to a large mass. Sometimes, it is seen as an ovoid mass, thin streak, or two longitudinal columns showing a central groove.[12] Pericallosal region is one of the most common sites and accounts to about one-third of intracranial lipomas. A size-dependent association of either partial or complete corpus callosal agenesis has been encountered in over 50% of cases with pericallosal lipoma. A larger lipoma may produce mass effect, causing displacement and an alteration in the shape of the CC. In CC agenesis, the lipoma fills in the normal anatomical location of the CC but does not infiltrate it. The region adjacent to the dorsal callosal surface is the most common site of involvement.[13] On MRI, the lipoma of the CC are T1 and T2 hyperintense well-marginated fat signal intensity masses, which show attenuation on fat suppression sequences.{Figure 12}

 Associated Anomalies



The common associated anomalies occurring with CCA are sulcal abnormality, ventriculomegaly, cerebellar hypoplasia, cerebellar vermian hypoplasia [Figure 6], Dandy Walker malformation and holoprosencephaly.[13],[14]

The definitive diagnosis of CCA can be made by ultrasound, if a midsagittal plane parallel to the CC could be obtained. But due to fetal position, it is rarely obtained and hence three-dimensional ultrasound comes handy to provide the plane that is needed once a volume of fetal head is obtained.[15] MRI is a complementary modality to sonography in the assessment of fetal CC due to its multiplanar capability and contrast resolution. It is recommended to view fetal CC in all the three planes to avoid errors in interpretation. MRI is also useful in identifying associated anomalies, which can help in patient counseling.

Acknowledgement

We thank Dr. R. Rajkali for his illustrative diagram.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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