Agenesis Of Corpus Callosum

Agenesis of Corpus Callosum or ACC is amongst the most frequently occuring malformations of the human brain, with an incidence of 0.5 to 70 in 10,000. It is more prevalent among males than females.

Corpus callosum refers to the midline structure which connects the two hemispheres of the brain. Consisting of about 200 million axons, it is the largest white matter structure in our body. Formation of corpus callosum begins from day 74 of gestation with it gaining its basic shape by 18-20 weeks of gestational age. The rostrum of corpus callosum is the last part to form and hence its presence on scan indicates normal structure.

Corpus callosum serves the function of integrating information from both hemispheres and thus processing sensory, motor and cognitive signals.

DEFINITION

ACC is a congenital cerebral malformation characterised by a complete or partial morphological absence of corpus callosum, and not its functional or behavioural abnormalities.

It can be classified into complete agenesis or partial agenesis depending on the part of corpus callosum affected. Partial agenesis, also known as hypogenesis, usually shows absence of splenium.

It may occur as an isolated problem, along with other brain abnormalities (ventriculomegaly) or with problems of other organs (congenital heart defects).

Another morphological classification is as follows:
Type 1- Large Probst bundles formed due to inability of axons to cross the midline

Type 2- Probst bundles not seen due to failure of formation of axons

Causes for ACC are varied and may include the following:

1. Use of alcohol by mother during pregnancy
2. Infection or injury during 12th to 22nd week of IUL
3. Phenylketonuria
4. Genetic factors

   • Aneuploidy
   • Chromosomal rearrangements
   • Mutation in DISC1 gene

It can also be present with other congenital conditions, including:

• Apert syndrome

• Joubert syndrome

• Arnold-Chiari malformation

• Dandy-Walker syndrome

• Aicardi syndrome

• schizencephaly

• hydrocephalus

• Trisomy 18 and 13

SYMPTOMS AND SIGNS

Patients may present with a wide range of symptoms with some being greatly impaired and others being largely asymptomatic.

Common symptoms include:

• Autism like features - difficulty in abstract reasoning, social skills and communication

• Delayed developmental milestones

• Mental retardation

• Seizures

• Impaired hand-eye coordination

• Feeding issues

• Visual or auditory memory deficits

DIAGNOSIS AND EVALUATION

1. MRI

   It is the investigation of choice for ACC. The following signs can be seen:
   

   • Racing car sign on axial imaging of lateral ventricles
   • Gyri radiating outward forming a radial pattern, called the sunray appearance
   • Colpocephaly
   • Probst bundles may be seen
   • Antenatal ultrasound
2. It may show some signs suggestive of ACC:
   
   • Teardrop configuration due to colpocephaly
   • Dilation and elevation of third ventricle
   • Absent septum pellucidum

   By fetal ultrasound, agenesis can be diagnosed as early as 16th week of gestational time, but with a false positive rate of 0 to 20%.

3. Neuropsychological evaluation

   Imaging can have two differential diagnoses - holoprosencephaly (failure of separation of forebrain into right and left hemispheres) and septo-optic dysplasia.