Disorders of the corpus callosum are conditions in which the corpus callosum does not develop in a typical manner. Since these are disorders of brain structure, they can only be diagnosed by brain scan, including:
- Pre/postnatal sonogram (ultrasound)
- Computerized Axial Tomography (CT-scan or CAT scan)
- Magnetic Resonance Imaging (MRI)
In a typical infant brain, the corpus callosum develops between 12 to 16 weeks after conception (near the end of the first trimester). While the entire structure develops prior to birth, the fibers of the corpus callosum continue to become more and more effective and efficient on into adolescence. By the time a child is approximately 12 years of age, the corpus callosum functions essentially as it will in adulthood, allowing rapid interaction between the two sides of the brain. From this age on (and typically earlier) as the corpus callosum becomes increasingly functional in their typically developing peers, children with disorders of the corpus callosum appear to fall behind developmentally because the corpus callosum is absent or impaired.
The disruptions to the development of the corpus callosum occur during the 5th to 16th week of pregnancy. There is no single cause and many different factors can interfere with this development, including:
- Prenatal infections or viruses (for example, rubella)
- Chromosomal (genetic) abnormalities (for example, trisomy 8 and 18, Andermann syndrome, and Aicardi syndrome)
- Toxic metabolic conditions (for example, Fetal Alcohol Syndrome)
- Blockage of the growth of the corpus callosum (for example, cysts)
The following sections clarify the differences between complete agenesis of the corpus callosum (ACC), partial ACC, hypoplasia of the corpus callosum, and dysgenesis of the corpus callosum.
Complete Agenesis of the Corpus Callosum (ACC)
If the nerve fibers don’t cross between the hemispheres during that critical prenatal time, they never will. Agenesis (or absent) of the corpus callosum becomes a permanent feature of the individual’s brain. The callosal fibers may have started to grow, but when unable to cross between the hemispheres, they grow toward the back of the same hemisphere where they began. These fibers form what are called Bundles of Probst. Some smaller connections between the hemispheres develop in most individuals with ACC. These are the anterior commissure, posterior commissure, and hippocampal commissure. However, each of these is at least 40,000 times smaller than the corpus callosum. Thus, they cannot compensate completely for the absence of the corpus callosum.
Partial Agenesis of the Corpus Callosum (P-ACC)
In partial ACC, the corpus callosum began to develop, but something stopped it from continuing. Since the corpus callosum develops from front to back, the part of the corpus callosum that is present in partial ACC usually will be toward the front of the brain, with the back portion missing. Partial ACC includes the entire range of partial absence, from absence of only a small portion of callosal fibers to absence of most of the corpus callosum. In P-ACC, the other smaller commissures usually are present.
Hypoplasia of the Corpus Callosum
Hypoplasia refers to a thin corpus callosum. On a mid-line view of the brain, the structure may extend through the entire area front-to-back as would a typical corpus callosum, but it looks notably thinner. It is unclear in this case if the callosal nerve fibers are fully functional and just limited in number, or if they are both less plentiful and more dysfunctional.
Dysgenesis of the Corpus Callosum
Dysgenesis means that the corpus callosum developed but developed in some incomplete or malformed way. Thus, partial ACC and hypoplasia of the corpus callosum would be forms of dysgenesis, as would any other form of inadequate callosal development. Dysgenesis is a broad term for any malformation of the corpus callosum that is not a complete absence (agenesis).
Parents often ask if the corpus callosum is the only path between the hemispheres of the brain. It isn’t the only path, but it is by far the most important. Some much smaller connections are usually present in DCC. The anterior commissure is the largest and most useful of these other pathways. However, it only has about 50,000 nerve fibers, a far cry from the more than 200 million fibers in the corpus callosum.