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Background

Agenesis of the corpus callosum (ACC) is an anomaly that may occur in isolation or in association with other CNS or systemic malformations. Because the corpus callosum may be partially or completely absent, the term dysgenesis has also been used to describe the spectrum of callosal anomalies.

Pathophysiology

Dysgenesis of corpus callosum is usually a sporadic occurrence, although the incidence is increased in patients with trisomy 18, trisomy 13, and trisomy 8. Several familial cases have been reported. Organ systems other than the CNS, particularly the musculoskeletal and genitourinary systems, may be affected as well.

Fibers of the corpus callosum arise from the superficial layers of the cerebral cortex and they project to the homotypic region of the contralateral cortex by passing through the corpus callosum while crossing the midline. Disturbance of embryogenesis in the first trimester of gestation by some unknown insult leads to failure of the callosal axons to pass across the midline. These arrested axons form the longitudinally oriented bundles of Probst that are located medial to the lateral ventricles in patients with agenesis.

Spectrum of abnormalities

ACC can be complete, partial, or atypical.

With complete agenesis, the corpus callosum is totally absent.

With partial agenesis (hypoplasia), the anterior portion (posterior genu and anterior body) is formed, but the posterior portion (posterior body and splenium) is not formed. The rostrum and the anterior/inferior genu are also not formed.

An atypical appearance occurs when the anterior to posterior formation is not respected.

In holoprosencephaly, callosal anomalies are atypical, eg, the splenium may be present without a genu or body. In middle interhemispheric fusion, a variety of holoprosencephaly, the genu and splenium may be present without the callosal body.

With pseudo corpus callosum, in conditions of complete or partial agenesis, the hippocampal commissure may become enlarged and appear like the posterior part of the corpus callosum.

Secondary destruction of corpus callosum occurs when the genu and anterior body are destroyed, leaving the posterior portion of the corpus callosum intact. This can occur secondary to porencephaly or schizencephaly, in transcallosal surgical approaches to the lateral and third ventricle, and with hemisection of the callosum for the treatment of seizures.

Other cerebral malformations may coexist with callosal dysgenesis. Examples of these include interhemispheric cysts; intracranial lipomas; and disorders of neuronal migration, such as schizencephaly, neuronal heterotopias, lissencephaly, and pachygyria.

Frequency of abnormalities

The frequencies of some of the more commonly associated anomalies are as follows:

- CNS anomalies (85%)

- Dandy Walker cyst (11%)

- Interhemispheric cysts

- Hydrocephalus (30%)

- Midline lipoma of corpus callosum (10%)

- Arnold-Chiari malformation (7%)

- Midline encephalocele

- Porencephaly

- Holoprosencephaly

- Hypertelorism median cleft syndrome

- Polymicrogyria

- Gray-matter heterotopia

- Cardiovascular, GI, and GU anomalies (62%)

Frequency

In the US: The reported frequency is 0.7-5.3%.

Internationally: The frequency is not known but could be similar to that in the US.

Mortality/Morbidity

ACC may occur as an isolated defect, but it is frequently associated with other malformations, chromosomal abnormalities, and genetic syndromes.

Although ACC has been found in asymptomatic individuals, it is generally considered a potential marker for neurologic impairment.

In children, the prognosis is frequently related to other associated abnormalities.
Sex: ACC is reported to be more common in males than in females.

Age

ACC is a congenital or a developmental anomaly, and therefore, is present at the time of birth. In many cases, agenesis is diagnosed later in infancy or in childhood because of its associated congenital malformations.

Development and anatomy

The corpus callosum develops from the lamina reuniens in the telencephalon, and it begins to appear between the anterior and hippocampal commissures at about 10.5 weeks. The adult form of the corpus callosum is achieved by 17 weeks' gestational age. Initial formation of the corpus callosum occurs in the genu and the body, progressing posteriorly. The anterior genu and rostrum develops last, folding back under the genu. The callosum thickens with increasing myelination.

When the corpus callosum is absent, the third ventricle is often high riding, extending superiorly between the lateral ventricles. On coronal imaging, a candelabra appearance occurs, with the third ventricle forming the central vertical portion and the lateral ventricles the peripheral arms of the candelabra. On axial imaging, the lateral ventricles are parallel.

Medial to the lateral ventricles, longitudinal bundles of white matter are present. These are known as Probst bundles and presumably would have formed a normal corpus callosum. Probst bundles are seen best on coronal or axial T1-weighted MRIs. The occipital horns of the lateral ventricles are dilated in patients with ACC, probably because of a deficiency of peritrigonal white matter fibers. This anatomic finding is known as colpocephaly. When the corpus callosum is absent, the cingulate gyrus is inverted, the normal cingulate sulcus is absent, and the medial cerebral sulci radiate toward the midline in a radial configuration. This finding is especially helpful in evaluating newborns in whom the corpus callosum is normally thin.

The hippocampal formations are frequently hypoplastic in patients with ACC, with resulting mild dilatation of the temporal horns. In partial callosal agenesis, the posterior body, splenium and rostrum are usually absent. Absence of the posterior body and splenium is especially common in patients with a Chiari II malformation. Barkovich has described the unusual absence of the genu or the midbody of the corpus callosum in patients with atypical or mild forms of holoprosencephaly.

Associated midline cysts are noted in some cases. The exact origin and nature of these cysts is controversial. While some of these cysts represent a dilated superiorly migrated third ventricle, others represent true midline cysts that may be lined by ependymal cells or by arachnoid membranes.

Types of midline cyst formation

Raybaud and Girard suggest that there are 3 types of midline cyst formation in association with agenesis or hypogenesis of the corpus callosum.

Type 1 is a large midline cyst that communicates with third ventricle and the lateral ventricles.

Type 2 is similar to type 1, associated cortical anomalies (eg, polymicrogyria, gray matter heterotopia, schizencephaly) are present.

Type 3 involves complex, multilocular cysts that are asymmetric and independent of the ventricles. Cortical malformations are uncommon. With large cysts, the ipsilateral lateral ventricle may be compressed, and the contralateral ventricle may be obstructed and enlarged (hydrocephalus). A CT cystogram may be helpful in identifying the communications between the loculations of the cysts and with the ventricles and in guiding the placement of a ventriculostomy shunt.

Associated anatomic abnormalities

Other anatomic abnormalities in patients with ACC include hydrocephalus; cephaloceles; and neuronal migration disorders such as lissencephaly, schizencephaly, gray matter heterotopias, pachygria, and polymicrogyria.

Clinical Details: The white matter fibers forming the corpus callosum predominantly connect symmetrical regions in the frontal, parietal, temporal, and occipital lobes. Experimental observations indicate that the corpus callosum allows the sharing of learning and memory between the two cerebral hemispheres.

The clinical manifestations of callosal agenesis can be described under 2 headings: nonsyndromic and syndromic.

Nonsyndromic forms are the most common. An unknown, though probably small, proportion of patients are completely asymptomatic, or more commonly, their condition is incidentally discovered during neuroimaging. Patients may present with mental retardation or delayed development, seizures, and cerebral palsy. Macrocephaly may be seen due to hydrocephalus sometimes associated with interhemispheric cysts.

A number of syndromes may be associated with ACC. Some of the more common ones include Dandy-Walker syndrome, Aicardi syndrome, fetal alcohol syndrome, and several of the trisomies.

Preferred Examination

The diagnosis of callosal agenesis depends on neuroimaging. In the newborn prior to closure of the anterior fontanelle, screening ultrasonography (US) can clearly show the absence of the corpus callosum, parallel lateral ventricles, interhemispheric cysts, hydrocephalus, and other related anomalies. US was the first imaging modality to allow direct sagittal imaging of callosal dysgenesis.

Antenatal diagnosis of ACC is possible from about 20 weeks' gestation. Characteristic intrauterine US findings include colpocephaly and parallel ventricular walls. CT findings are also diagnostic of ACC. Parallel lateral ventricles, colpocephaly, and extension of the third ventricle into the interhemispheric fissure are particularly pertinent findings. In patients with ACC and an interhemispheric cyst, the preoperative injection of nonionic water-soluble contrast material into the cystic loculations for CT enables assessment of the ventricular system or of the communication of the cystic components with one another.

MRI is currently the imaging procedure of choice in infants and children with ACC, even in patients who have previously undergone CT and US examinations. The multiplanar capability and high soft-tissue contrast possible with MRI permits confident diagnosis of ACC and its associated anomalies, especially neuronal migration anomalies or atypical forms of holoprosencephaly. These entities may be extremely subtle or indiscernible on CT or US images.

Limitations of Techniques: Both CT and US can depict ACC, but MRI is the preferred imaging modality because of its greater sensitivity for depicting associated cerebral anomalies.

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