What Is Agenesis of the Corpus Callosum?
The corpus callosum is a bundle of nerve fibers that connect the right and left hemispheres of the brain. It is made up of 200 to 300 million fibers of the type known as white matter*, which is a group of nerve fibers covered in a substance called myelin that affects the speed of nerve impulse transmission. The corpus callosum is located in a band right down the midline of the brain. Its function affects the sensory systems, motor function (movement), learning, memory, and thought processes. Different parts of the band have an effect on specific functions.
Agenesis of the corpus callosum (ACC) is a medical term used to describe the partial or complete absence of the corpus callosum during the development of a fetus*. Sometimes the terms hypogenesis or dysgenesis are used instead of agenesis, specifically when the corpus callosum is only partially absent or malformed, respectively. Because the corpus callosum failed to develop normally, the two hemispheres of the brain
are functionally disconnected. The varying degrees of agenesis lead to a wide spectrum of medical consequences. The occurrence of ACC is often associated with other physical abnormalities. The frequency of ACC in
the United States is low at 0.7 to 5.3 percent.
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Causes of Agenesis of the Corpus Callosum
The corpus callosum develops in the fetus around the 10th and 11th
week of pregnancy. If fetal development is disrupted during this period,
the corpus callosum may be negatively affected. The exact cause of ACC is
unknown, but multiple factors may be involved. ACC may be caused by
errors in the chromosomes* or the inheritance of some unknown genetic
factor. Disruption of corpus callosum development may also be due to
prenatal* infections, injuries, or exposure to toxins. The corpus callosum
may be kept from forming properly when blocked by cysts that form in
the developing brain.
Signs and Symptoms of Agenesis of the Corpus Callosum:
Abnormalities of the corpus callosum can occur as in isolation. However,
abnormalities in corpus callosum development often occur with other
medical problems. The number or type of medical complications an
individual has in association with the degree of corpus callosum agenesis
determines what the individual’s symptoms are. There are some medical
signs that appear in most people with ACC, in various degrees of severity.
The range can be from slight to seriously disabling.
Physical and structural defects Many individuals with ACC may
have the following problems: vision defects such as nearsightedness or difficulty
with depth perception; difficulty feeding as a baby, gastric reflux
problems, or problems chewing and swallowing; low muscle tone and weakness;
sleep problems such as insomnia* or bed-wetting; abnormal facial
features; seizures; hearing defects; chronic constipation; low perception of pain. Less often seen are heart problems, defects in the skeletal system, or genetic conditions.
Developmental defects Many children with ACC do not develop
normally or reach standard developmental milestones in the expected time.
Milestones associated with movement are known as motor milestones.
They may be delayed in ACC and include general coordination, walking,
and riding a bike. Other potentially delayed developmental abilities
include speech and toilet training.
Behavioral defects Children with ACC tend to have a lack of selfawareness
and consequently tend to be socially immature. They may have
difficulty interpreting and responding appropriately to other people.
These children may have increasing social difficulty with age, due to the
increasingly complex nature of social interactions. Children with ACC
may be hyperactive and have difficulty paying attention for long periods
of time. Obsessive compulsive behaviors and timidity may also be seen.
Cognitive defects Cognitive defects are problems with communication,
thought processes, language usage, and judgment. Individuals with
ACC may have explicit mental retardation*, or they may seem to have
normal intelligence if the ACC is partial and an isolated medical incident.
However, even those with seemingly normal intelligence are thought to
have some form of more subtle cognitive difficulties. Individuals with
ACC may have difficulty interpreting people’s facial expressions or verbal
cues. Tasks requiring problem-solving skills such as finances or scheduling
events may prove difficult. Unfortunately, individuals with ACC are often
not aware of their limitations and may attempt to perform tasks beyond
their abilities, with poor awareness of the consequences. Abstract language
concepts such as the use of slang or sarcasm may not be understood by
people with ACC. Confabulation*, making up information that is not
true while convincing oneself that it is true may also occur in ACC.
How Is Agenesis of the Corpus Callosum Diagnosed?
ACC can be diagnosed using medical tools that perform brain scans.
Brain scans can be performed using a variety of techniques that provide
an image of the brain for physicians to analyze and search for missing
structures. Prenatal ultrasound* or MRIs* may be performed to look for a
defective corpus callosum. CT scans* are also used to view the structures
of the brain, but MRIs provide the most detailed information.
Treatment and Prognosis of Agenesis of the Corpus Callosum
No medical treatment as of the early 2000s was available to rectify the
defective development involved in ACC. Some treatment may be available
to relieve symptoms associated with ACC, such as constipation or seizures. However, behavioral therapies, educational programs, and support groups
may assist an individual with ACC in leading a more satisfying life.
The prognosis for ACC varies with the type and degree of corpus callosum
abnormality and associated medical conditions. The corpus callosum
cannot regenerate or degenerate, meaning that once formed it cannot
regrow or continue to deteriorate. Some individuals with ACC may seem to
have average intelligence and lead normal lives, but they cope at least with
slight deficiencies in cognition in contrast to individuals without ACC.