What is Corpus Callosum Agenesis?
Agenesis of corpus callosum (ACC) is a common birth defect of the brain, characterized by a partial or complete absence of the corpus callosum. This condition does not directly result in functional or behavioral abnormalities like autism. The corpus callosum is the main structure linking the two halves of the brain, consisting of 200 million nerve fibers. The formation of the corpus callosum starts around the 74th day of pregnancy and it is usually fully formed by the 20th week. The development starts at the front of the brain and finishes at the back.
What Causes Corpus Callosum Agenesis?
The corpus callosum, a part of the brain, can sometimes not form correctly. This is known as agenesis of the corpus callosum. It can occur on its own or it can happen alongside other brain abnormalities. This condition can be linked to several factors:
Moms who drink alcohol when they’re pregnant, moms who have a disorder called phenylketonuria, and a problem with a part of the brain called Chiari II could all possibly lead to agenesis of the corpus callosum. Certain genetic factors can also contribute to this condition. In fact, about 20% of people with agenesis of the corpus callosum have some kind of chromosomal abnormality, mainly trisomy 18 and 13.
There are also different syndromes that are often linked with agenesis of the corpus callosum. These include Apert syndrome, basal cell nevus syndrome, and Mowat-Wilson syndrome, all of which are passed down through families. Joubert syndrome and Lyon syndrome are inherited in a different way and also may cause agenesis of the corpus callosum.
Some other causes can be linked to genetic abnormalities such as Aicardi syndrome, X-linked aqueductal stenosis or hydrocephalus, and certain chromosomal rearrangements. Recent studies also suggest that mutations in a specific gene, referred to as the disrupted-in-schizophrenia 1 (DISC1) gene, could be associated with agenesis of the corpus callosum.
Risk Factors and Frequency for Corpus Callosum Agenesis
Agenesis of the corpus callosum is a frequently occurring brain malformation. The number of instances can vary from 0.5 to 70 in 10000 people, and is often found in children with developmental disabilities, with a prevalence rate of 2.3%. This condition also makes an appearance in about 1 out of every 19000 autopsies. It should be noted that agenesis of the corpus callosum is more commonly found in males compared to females.
Signs and Symptoms of Corpus Callosum Agenesis
People with the condition known as agenesis of corpus callosum often don’t show symptoms. However, if they do, it usually happens within the first two years of life. Symptoms can vary, but there are several common signs that medical professionals look for.
In what medical professionals call the “core syndrome” of corpus callosum agenesis, people usually experience:
- Lower ability to transfer sensory-motor information between the brain’s hemispheres
- Slow cognitive processing
- Difficulties in analyzing complex information and unfamiliar tasks, with an increased sensitivity to higher cognitive demands
However, there are other common symptoms of this condition:
- Intellectual disability (in 60% of cases)
- Vision problems (33% of cases)
- Developmental delay in speech (29% of cases)
- Seizures (25% of cases)
- Feeding problems (20% of cases)
- Hand-eye coordination problems
- Behavioral issues, such as attention-deficit-hyperactivity disorder (ADHD) and psychosis
Note though that in some cases where the corpus callosum is missing, the person may have a normal intelligence quotient. However, they may still exhibit some cognitive deficits, much like those seen in the above-mentioned “core syndrome”.
Testing for Corpus Callosum Agenesis
An ultrasound of a baby in the womb can detect if a part of the baby’s brain called the corpus callosum is missing as early as the 16th week of pregnancy. However, it’s important to note that this test can sometimes give false positives, meaning it might suggest the corpus callosum is missing when it’s actually there. This happens anywhere from 0 to 20% of the time.
In an ultrasound, certain findings might suggest that the corpus callosum is missing. These can include a third ventricle that’s larger and higher than usual, abnormally spaced brain structures, a “teardrop” shape seen in some scans, a missing brain structure called the septum pellucidum, or distorted or missing arteries. However, these findings are only suggestive and further tests are necessary for a definitive diagnosis.
The best test to confirm if the corpus callosum is missing is a brain scan called an MRI. Certain signs in an MRI can suggest the corpus callosum is missing. For instance, certain parts of the brain might look like a racing car or a moose head. Additionally, the brain ridges might appear to fan out from the ventricle, or the ventricles might appear parallel and non-converging. The scan may also reveal a condition called colpocephaly, which involves the enlargement of certain brain structures, or a higher than normal third ventricle. Sometimes, the MRI will show the presence of a structure known as the Probst bundles.
Finally, it is crucial for all patients suspected to have a missing corpus callosum to undergo a neuropsychological evaluation, which is an assessment of how one’s brain functions and how it affects their behavior.
Treatment Options for Corpus Callosum Agenesis
The treatment for this condition primarily focuses on managing its symptoms and providing support. Here is the main course of treatment:
1. Medications to control seizures
2. Specialized educational programs
3. Physical therapy
4. Placement of a ventriculoperitoneal shunt if there’s a build-up of fluid in the brain (hydrocephalus)
5. Genetic counseling for family members
6. Surgeries to correct physical abnormalities associated with syndromes
7. Speech and vision therapy
8. Mental health therapies as needed.
What else can Corpus Callosum Agenesis be?
When doctors are trying to diagnose issues with the brain, they often look at image scans. Two conditions that can look similar and thus are often considered together are holoprosencephaly and septo-optic dysplasia.
The main clue that doctors look for to tell these two conditions apart is the presence of something called the septum pellucidum and the corpus callosum, which are structures within the brain. Specifically, they’ll examine whether the frontal “horns” of the brain appear to be joined or communicating with each other and whether the corpus callosum looks unusually thin.
Holoprosencephaly can be distinguished from other conditions by several features: the joining of the frontal horns, an unusual anterior cerebral artery, and the absence of a normally developed “interhemispheric fissure” at the front part of the brain.
Sometimes, the absence of the corpus callosum is associated with midline interhemispheric cysts. The different types of these cysts include:
- Cavum septum pellucidum
- Cavum vergae
- Cavum velum interpositum
- Interhemispheric arachnoid cyst
These conditions might make a diagnosis more difficult, as a widened area in the brain known as the third ventricle with a dorsal cyst can look similar, too. However, if there is a choroid plexus – a structure in the brain – in the top part of the third ventricle, it can help distinguish between these conditions.
What to expect with Corpus Callosum Agenesis
The likely outcome primarily depends on the presence of other abnormalities.
Agenesis of the corpus callosum, a condition where a part of the brain doesn’t develop properly, is often linked with several other brain anomalies. These are, in order of their frequency:
1. Interhemispheric cyst with fluid buildup in the brain
2. Dandy-Walker malformation, a congenital brain malformation
3. Neuronal migration disorder, where nerve cells don’t move to their correct locations in the brain
4. Absence of the lower part of the cerebellum (an area of the brain)
5. Encephalocele, a sac-like protrusion of the brain from the skull
6. Lipoma of the interhemispheric fissure, or a fatty tissue growth in the brain
Children who only have agenesis of the corpus callosum without any significant neurological after-effects tend to fare the best.
On the other hand, children who have agenesis of the corpus callosum along with neuronal migration disorder, with or without Dandy-Walker malformation, could have a worse prognosis.
Possible Complications When Diagnosed with Corpus Callosum Agenesis
Complications related to associated disorders might occur more frequently. Such conditions could include:
- Aspiration pneumonia
- Seizures
- Schizophrenia
- Cerebral palsy
Preventing Corpus Callosum Agenesis
People who have developmental delays or intellectual disabilities can often gain a lot by getting help early on. This help can come in the form of early intervention services, specialized education, and other kinds of therapy. These means of support can make a significant difference in their lives.