Corpus Callosum, Agenesis Health Article

Definition

Agenesis of the corpus callosum is the complete or partial absence of the corpus callosum, the structure within the brain that connects the two hemispheres.

Description

Agenesis of the corpus callosum is a congenital anomaly occurring during the first trimester of pregnancy. During this time, the fetal brain is developing, and bundles of nerve fibers that create the corpus callosum are forming. This process may be interrupted if the mother is exposed to toxic substances or certain medications during pregnancy. Genetic abnormalities in the fetus may prevent the nerve fibers from growing correctly. In most cases, the exact cause of agenesis of the corpus callosum is unknown.

In some people with agenesis of the corpus callosum, these nerve fibers are formed, but grow from front to back rather than from side to side. When the anomaly forms from front to back, groups of nerve fibers known as bundles of Probst form. These bundles stay within each hemisphere, never crossing the midline, and the two sides of the brain cannot share information. This lack of communication causes the symptoms of agenesis of corpus callosum.

The term agenesis of the corpus callosum usually refers to a complete absence of the corpus callosum. In some individuals, the corpus callosum is partially formed, and the condition is referred to as dysgenesis of the corpus callosum. There are two types of dysgenesis of the corpus callosum, partial and atypical. Partial dysgenesis occurs when the front, or anterior, portion of the corpus callosum does not form. In atypical dysgenesis of the corpus callosum, the rear, or posterior, portion the corpus callosum is not formed.

The corpus callosum is not essential for life or for normal intellectual functioning. Many people who have agenesis of the corpus callosum as an isolated condition have no symptoms at all, and some may only experience mild difficulties with skills that required matching visual patterns. Since the two hemispheres of the brain cannot communicate, images seen by one eye cannot be connected to images processed by the other.

The more severe symptoms experienced by people with agenesis of the corpus callosum are most often caused by other brain malformations, chromosomal abnormalities, and genetic syndromes of which agenesis of the corpus callosum is only one component. Especially in children who have agenesis of the corpus callosum in addition to other brain malformations, the effects can be severe, including mental retardation, seizures, hydrocephalus, and impairment of motor functioning.

Agenesis of the corpus callosum often occurs along with other brain malformations, such as Arnold-Chiari malformation, Dandy-Walker malformation, and other defects of the midbrain. It may also be found in patients with defects of the size and shape of the brain, such as schizencephaly lissencephaly, pachygyria, and hydrocephalus.

Individuals with anomalies of the formation of the forebrain, such as frontal encephalocele and holoprosencephaly, will usually have agenesis of the corpus callosum as well. In this case, the forebrain does not develop into two distinct hemispheres and the corpus callosum does not form. These are severe birth defects and are usually incompatible with life.

Agenesis of the corpus callosum may occur as an isolated birth defect; in such cases, a genetic cause has not been identified. However, it often occurs as part of a syndrome. Agenesis of the corpus callosum has been associated with more than 30 inherited syndromes caused by chromosomal and single-gene anomalies and may result from prenatal exposure to anticoagulant medication during the first trimester of pregnancy.

Agenesis of the corpus callosum may also be known as absent corpus callosum, corpus callosum hypoplasia, ACC, CCA, collasal agenesis, or collasal dysgenesis.