Agenesis of the corpus callosum (ACC)
Complete absence of the corpus callosum. No fibres have managed to cross between the two sides of the brain.

Aicardi syndrome
A rare syndrome that occurs only in girls (boys are affected only in extremely rare and specific circumstances). Its main features are developmental brain malformations including ACC, seizures, learning difficulties and eye changes.

Anterior commissure
anterior bundle of nerve fibres (pathways)

Part of the brain at the back and base of the skull that coordinates movement and sensation. It is thought also to be involved in cognition and learning.

Cerebral hemispheres
The main part of the brain that is divided into two halves or hemispheres.

Enlargement of the lateral ventricles (cavities or chambers), particularly at the back of the brain. They are often described as “tear-drop” shaped. This is a common secondary finding in abnormal formation of the corpus callosum.

Computed tomography. This is a well established imaging technique using X-rays. It is less commonly used to look at the corpus callosum than MRI but can be useful in certain circumstances.

Corpus callosum
Corpus means a body or structure. Callosum means a bridge. The corpus callosum describes the fibres that link the left and right hemispheres (the two halves of the brain). It is the largest and most easily visible connection between the two hemispheres.

Dysgenesis of the corpus callosum
Abnormal development of the corpus callosum. This term covers any abnormality in the appearance of the corpus callosum.

Tendency to have recurrent seizures.

Grey matter
The part of the brain containing nerve cell bodies. Much of this can be seen on images as a layer over the surface of the brain.

Hypogenesis of the corpus callosum
Partial formation of the corpus callosum. This is sometimes called partial agenesis. The corpus callosum develops in a front-to-back direction. Therefore we usually see the front portion of the corpus callosum present with the back missing. This can range from just a tiny area to the presence of most of the corpus callosum.

Hypoplasia of the corpus callosum
Underdevelopment of the corpus callosum. Here the corpus callosum generally looks thin but with all of the parts present from front to back. Often all of the white matter of the brain looks underdeveloped as well.

Part of the brain with important roles mainly bodily functions including temperature, hormone function, appetite and sleep among others.

Interhemispheric cyst
A cyst is a fluid-filled space within the body. In this case a cyst may occur between the two cerebral hemispheres either with or without an abnormality of the corpus callosum.

Lateral ventricles
Hollow areas of the brain that are filled with fluid, located in the middle of each hemisphere.

A problem that occurs because of abnormal development of part of the body, for example a cleft lip or a callosal abnormality.

Malformation syndrome
This is sometimes just called a syndrome. A pattern of features, often with a unifying underlying cause, that arises from several different errors during a babys development, for example Aicardi syndrome.

MRI: Magnetic resonance imaging
An imaging technique that uses magnetic signals, rather than X-rays to create image “slices” of the human body.

The insulating coat that covers nerve cells and facilitates transmission of information.

Pituitary gland
A hormone producing gland at the base of the brain which controls the production of other hormones throughout the body. It is directly connected to the hypothalamus.

Probst bundles
In some individuals it is possible to see some of the fibres which would make up the corpus callosum have started to make the journey across but have been interrupted.

A seizure is a sudden disruption of the brain’s normal electrical activity accompanied by an altered conscious level and/or other neurological and behavioural manifestations.

Is a very useful imaging technique using high frequency sound waves. It is most commonly used to look at structures in the abdomen and during pregnancy. It can also be used to look at the brain in a newborn baby through the “soft spot” (fontanelle).

White matter
The part of the brain that contains nerve fibres covered (or which will be covered) in myelin.