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What is agenesis of the corpus callosum (ACC)?

Agenesis of the corpus callosum is a congenital (lifelong) brain abnormality that occurs when the corpus callosum does not develop as it should during the early prenatal period. It can occur as an isolated condition or in association with other brain abnormalities or physical or medical conditions.

What is the corpus callosum?

The corpus callosum is the largest midline structure of the brain. It

• Begins to develop around the 10th to 11th week of pregnancy
• Consists of over 200 million nerve fibers that connect the two hemispheres of the brain
• Transfers and integrates motor, sensory, and cognitive information between the cerebral hemispheres
• Continues to mature throughout pregnancy and into childhood and adolescence

MRI showing brain with corpus collasum. Location of corpus callosum, a tipped "c" shape in the center of the brain, is indicated by white arrow.	MRI showing brain without corpus callosum (ACC).

What types of callosal disorders can occur?

The types of callosal abnormality that occurs depends on the cause and timing of the disruption to prenatal brain development. If the corpus callosum does not form during the prenatal period, it will not develop later. Currently, there is some variation in how callosal conditions may be described in medical reports and by medical providers. Below is a list of the most common terms used to describe these conditions.

• (ACC) Agenesis of the corpus callosum: All or a portion of the corpus callosum is absent; this includes both complete and partial ACC
• (AgCC) Agenesis of the corpus callosum: All or a portion of the corpus callosum is absent. This acronym has appeared more recently in some research literature.
• (c-ACC) Complete agenesis of the corpus callosum: The corpus callosum is completely absent
• (p-ACC) Partial agenesis of the corpus callosum: A portion of the corpus callosum is absent; most often it is the posterior (back) portion that is missing
• Hypogenesis of the corpus callosum: Another term sometimes used to describe partial ACC
• Hypoplasia of the corpus callosum: The corpus callosum is present, but is abnormally thin
• Dysgenesis of the corpus callosum: The corpus callosum is present but is malformed in some way; this includes p-ACC and Hypoplasia

How often do callosal disorders occur?

It is difficult to determine the precise incidence of ACC and other callosal disorders since a brain scan is required to diagnose the condition, and no one knows how many individuals may have ACC but not be aware of it.

The development and increased use of new and sophisticated imaging technologies (including prenatal technologies) are helping researchers and medical professionals identify this condition earlier in life as well as across the lifespan. Most sources estimate callosal disorders can occur in up to 4 individuals per 1,000 in the general population and occurs more frequently among persons with developmental disabilities (22-24 per 1,000). Recently, a group of researchers in the U.S. reported an incidence rate of 1 in 4,000 live births, based on a review of prenatal and neonatal imaging studies, but these researchers suggested this may be an underestimate of the true occurrence rate.

What causes ACC and other callosal conditions?

It is often difficult to determine the precise cause of the callosal abnormality, although medical testing, including genetic testing, can sometimes provide valuable information. ACC researchers, including geneticists, are working to gain a better understanding of the causes of callosal conditions.

Possible causes include:

• Chromosome errors
• Inherited genetic factors
• Prenatal infections or injuries
• Prenatal toxic exposures
• Structural blockages, such as cysts
• Metabolic disorders
• Other unknown factors

How are callosal conditions diagnosed?

A brain scan is necessary to diagnose callosal conditions. Brain scans include:

• Prenatal ultrasound
• Prenatal magnetic resonance imaging (MRI)
• Computerized tomography (CT scan)
• MRI (provides the most detailed information)

What other conditions may be associated with callosal abnormalities?

Although a callosal abnormality can occur as an isolated condition, it may also occur in association with other brain abnormalities or physical or medical conditions. It can occur as part of a syndrome (a specific grouping of congenital abnormalities) or in association with a chromosome abnormality (e.g. Trisomy 13 or 18). For this reason, some persons with callosal disorders are quite healthy, while others may have additional medical or physical conditions that impact health and development to varying degrees. The lists below identify the more common brain anomalies and syndromes associated with ACC.

Brain anomalies that sometimes occur with ACC:

• Hydrocephalus
• Arnold- Chiari malformation
• Schizencephaly (deep clefts in brain tissue)
• Holoprosencephaly (failure of the forebrain to divide into lobes)
• Migration disorders

Syndromes associated with ACC:

• Aicardi
• Andermann
• Shapiro
• Acrocallosal
• Menkes
• Mowat-Wilson
• FG
  

Individuals and families often find it helpful to consult genetic specialists after the initial diagnosis of a callosal disorder for diagnostic purposes and to aid in future family planning.

Common characteristics associated with ACC:

Research on the impact of callosal disorders is in the early stages and new information continues to emerge. Yet, current research suggests that persons with callosal disorders share some common physical, communication, social, behavioral and cognitive features. However, individual differences exist among persons with callosal conditions, ranging from subtle developmental and cognitive challenges to more severe disability. These differences are due in part to whether an individual has additional physical, medical, or behavioral conditions that also affect health, development, and intellectual abilities.

The following lists identify characteristics commonly associated with ACC. Individual differences are common and not all persons with ACC will experience these characteristics.

Health-related/Physical:

• Vision impairments (near/farsightedness, nystagmus, strabismus, problems with depth perception)
• Low muscle tone (hypotonia)
• Early feeding difficulties/gastric reflux/chewing and swallowing difficulties
• Abnormal head and facial features
• High tolerance to pain
• Sleep difficulties (e.g. getting to sleep, nighttime waking, bed-wetting)
• Seizures or spasticity
• Hearing impairments
• Elimination problems including chronic constipation
• Less common: Genito-urinary defects, cardiac abnormalities, skeletal defects, metabolic disorders, genetic conditions

Developmental

• Delays in attaining motor milestones such as sitting, walking, riding a bike
• Early speech and language delays (particularly in expressive communication)
• Clumsiness/poor motor coordination
• Delayed toilet training

Social/behavioral

• Generally happy disposition, enjoys being with others
• Socially immature
• Lack of self awareness
• Inability to take the perspective of others
• Difficulties understanding and acting on the social cues of others
• Difficulty maintaining attention
• Restlessness or hyperactivity
• Fearfulness
• Obsessive/compulsive behaviors
• Challenges with peer interactions and relationships that seem to increase with age and the complexity of the social situations

Cognitive/Communication

Although mental retardation can occur in association with callosal conditions, individuals with ACC may also have normal intelligence. Yet, research suggests that even those with isolated ACC and normal intelligence may experience subtle neuropsychological and cognitive challenges.

• Early speech and language delays (particularly with expressive communication)
• Misinterpretation of the nonverbal communication of others (i.e. their facial expressions or tone of voice)
• Difficulties with complex tasks, abstract reasoning, problem solving (such as managing money and schedules, responding to novel  situations)
• Limited insight into their own behavior and limitations
• Difficulty imagining the consequences of their own behavior
• Difficulty understanding abstract language/concepts 
• Difficulty understanding slang, sarcasm, and sophisticated humor
• Difficulty understanding others’ perspectives and emotions
• Confabulation (communicating information that is untrue while perceiving that it is true)
  

Are treatments available for persons with callosal disorders?

Since the corpus callosum forms early in fetal development, if it does not develop then, it will not develop later. At this time, there are no medical treatments specifically for callosal conditions. However, treatment is available for other physical and medical conditions sometimes associated with callosal disorders, such as seizures, hydrocephaly, and sensory impairments.

In addition, children and adults with callosal conditions can benefit from individualized developmental therapies, educational supports, and adult services. It is important to obtain evaluations, consultations, and interventions from a wide variety of medical, allied health, educational, and social work professionals, beginning early in life and continuing throughout childhood and into adulthood. Below is a list of professionals who are potential resources for persons with ACC and other callosal disorders.

• Adult service providers
• Behavioral psychologists
• Early intervention specialists
• Early childhood special educators
• Endocrinologists
• Geneticists
• Neurologists
• Neuropsychologists
• Occupational therapists
• Ophthalmologists
• Pediatricians/Pediatric nurse practitioners
• Physical therapists
• Special educators
• Speech-language pathologists
  

As researchers continue to learn more about the causes and impact of callosal conditions, families and professionals may gain additional information and knowledge to better support individuals with disorders of the corpus callosum.

ACC Information Sheet in .pdf
ACC Information Sheet in MS Word

This information is intended for educational purposes only. Please consult your healthcare professionals for diagnosis and treatment. This information reflects the current status of research and knowledge at the time of production (3/2/08) and is subject to change as new research findings emerge.