Agenesis of the Corpus Callosum (ACC) Normal Life
Agenesis of the corpus callosum may occur as an isolated event or as part of dozens of developmental and dysmorphic syndromes. Although affected patients may exhibit few obvious neurologic functional deficits, the absence of the corpus callosum, a structure responsible for interhemispheric connections, does carry consequences with respect to information processing and language functions. In this article, the author discusses attempts to classify and characterize agenesis of the corpus callosum based on morphologic and molecular findings.
Agenesis of the corpus callosum (ACC) is one of several disorders of the corpus callosum, the structure that connects the two hemispheres (left and right) of the brain. In ACC the corpus callosum is partially or completely absent. It is caused by a disruption of brain cell migration during fetal development. ACC can occur as an isolated condition or in combination with other cerebral abnormalities, including Arnold-Chiari malformation, Dandy-Walker syndrome, schizencephaly (clefts or deep divisions in brain tissue), and holoprosencephaly (failure of the forebrain to divide into lobes.) Girls may have a gender-specific condition called Aicardi syndrome, which causes severe cognitive impairment and developmental delays, seizures, abnormalities in the vertebra of the spine, and lesions on the retina of the eye. ACC can also be associated with malformations in other parts of the body, such …
What is ACC?
The corpus callosum is a structure that connects the right and left sides of the brain. It contains 200 million nerve fibers that pass information back and forth. Agenesis of the corpus callosum (ACC) is a birth defect that occurs when the connections between the right and left sides of a child’s brain don’t form correctly. It occurs in an estimated 1 to 7 out of 4,000 live births.
There are several specific forms of ACC, including:
• partial corpus callosum agenesis
• hypogenesis of the corpus callosum
• hypoplasia of the corpus callosum
• dysgenesis of the corpus callosum
A child that’s born with ACC can survive with the condition. However, it may cause developmental delays, which can be mild or more severe. For example, ACC can cause delays in a child’s development of motor skills, such as sitting, walking, or riding a bike. It can potentially cause difficulties with swallowing and feeding. Poor coordination is also common in children with this condition. A child may also experience some language and speech delays in expressive communication. Although cognitive impairment can occur, many people with ACC have normal intelligence.
• Agenesis of the corpus callosum (AgCC) occurs in approximately 1:4000 live births and can be diagnosed by neuroimaging. It may be part of a broader developmental syndrome or occur in isolation, in which case it is typically accompanied by colpocephaly and Probst bundles.
• Animal models have revealed that the following general mechanisms are important for callosal development: neuronal proliferation and specification, axonal outgrowth and pathfinding, development and signalling at the midline, target recognition and activity-dependent enhancement or pruning.
• AgCC can be caused by a combination of genetic mechanisms. Environmental factors probably also contribute to AgCC. However, most cases of AgCC do not have a known cause at this time.
• The degree of behavioural and neurological impairment in AgCC is diverse, and preliminary findings suggest that other CNS anatomical changes associated with AgCC help determine outcomes.
• AgCC patients do not exhibit the disconnection syndrome seen in split-brain patients. AgCC results in limited interhemispheric transfer of primary visual information, but intact hemispheric transfer of simple conceptual information such as letters.
• Primary AgCC frequently presents with deficient problem solving, impaired linguistic pragmatics and impaired social skills, all of which contribute to difficulty with daily living such as maintaining relationships and a job. Cognitive and social deficits in AgCC may be related to interhemispheric transfer deficits or to other anatomical abnormalities.
• Many neurodevelopmental and psychiatric conditions have been linked to corpus callosum malformation or malfunctioning, including schizophrenia, autism and attention deficit hyperactivity disorder. As such, AgCC may be a good model for examining how callosal and cortico-cortical transfer contribute to these disorders.