Corpus Callosotomy

By Laurel Hamilton

A corpus callosotomy is a surgical procedure to alleviate symptoms of epilepsy, particularly in cases where epilepsy is resistant to drug treatment. [1] Epilepsy stems from unusual electrical activity in the brain. Severing the corpus callosum, a thick bundle of nerve fibers connecting the two hemispheres of the brain, reduces epilepsy’s impact by preventing the abnormal signals from traveling across the whole brain and confining seizures to one hemisphere. This procedure is primarily helpful for atonic (drop attack) and other generalized seizures as they spread throughout the brain, as opposed to focal seizures which affect a specific part of the brain. Severing connectivity between the brain hemispheres led to further research into how the two halves function together and how they continue to operate, if unable to communicate with one another.

History

The corpus callosotomy (“split-brain”) was first performed in the 1940s to treat severe epilepsy for 26 different patients. Initial research indicated that the procedure had no significant effects on seizures, thought, or behavior; however, these early attempts likely did not sever the corpus callosum completely. [2]

In the 1950s, neuroscientist Roger Sperry found evidence of major changes to brain function in animals that had a severed corpus callosum. He continued exploring this with Patient W.J., a World War II veteran who began having seizures after receiving a blow to the head with a rifle. Sperry ran an experiment with W.J. after his corpus callosotomy in which information was given to his right or left hemispheres by showing images in his left or right fields of vision. When W.J. saw the image, he was asked to press the button and explain what he saw. W.J. had no problems doing so with his left hemisphere, but when images were presented to his right hemisphere, he would press the button yet say that he saw nothing. This indicated that severing the corpus callosum did in fact lead to gaps in communication between the two hemispheres of the brain.

Research into the corpus callosotomy is limited, as it is a rare procedure to begin with, and patients who undergo the operation are rarely neurologically healthy enough to effectively participate in experiments. Early research found results similar to those of W.J. and suggested that post-operation the two hemispheres are completely unable to communicate. Later research indicates some levels of coordination, as patients are generally still able to execute previously learned motor tasks, even if they struggle to learn some new ones. Overall, split-brain research is inconclusive on whether consciousness is still unified after the brain is divided. Neuroscientists have come up with different results, and there is not yet a widely accepted theory. [3]

Patient Demographics

Individuals of all racial, gender, and age demographics are susceptible to developing epilepsy. Epilepsy is linked to genetic variations in ion channels that regulate ion flow in neurons and signaling. It is also associated with disturbances to typical nerve activity such as traumatic brain injuries, brain abnormalities, and conditions that deprive the brain of oxygen. When neurons send the incorrect signal through the brain, seizures may occur. There are two main types of seizures. [4]

Focal Seizures

Focal seizures originate in a singular area of the brain. Symptoms vary greatly depending on which part of the brain with which the seizure is associated. These symptoms range from sudden emotional responses (e.g. intense feelings of joy) to abnormal motor sensations (e.g. one hand involuntarily moving) to performing unconscious, repetitive acts, known as automatisms. Corpus callosotomy is not an effective treatment for focal seizures as they already occur in only one part of the brain and are therefore limited to one hemisphere as is.

Generalized Seizures

Generalized seizures are caused by unusual neuron activity occurring in both hemispheres of the brain. There are various types of generalized seizures, typically characterized by symptoms of muscle contractions or a loss of consciousness. Corpus callosotomy limits these seizures to one half of the brain in severe cases, primarily for instances of atonic and tonic seizures. Atonic seizures, also referred to as ‘drop attacks,’ are associated with a sudden loss of muscle strength, while tonic seizures cause muscle stiffening. Both types may lead to falls and potential injuries due to the loss of muscle control.

The target patients for corpus callosotomy are not eligible for other surgeries, such as focal resections, and they generally do not have progressive neurological or medical diseases that would interfere with the procedure. Patients are considered after multiple medication treatments have been unsuccessful. Corpus callosotomy is also reserved for individuals experiencing bodily harm due to falls caused by seizures. There are a number of steps taken before confirming the procedure, including extensive brain MRI testing, EEG monitoring, and physical examinations. [5]

References

1. Cleveland Clinic medical. (2024, February 5). Corpus Callosotomy. Cleveland Clinic. https://my.clevelandclinic.org/health/treatments/11546-corpus-callosotomy

2. Wolman, D. (2012, March 14). The Split Brain: A Tale of Two Halves. Nature News. https://www.nature.com/articles/483260a

3. de Haan, E. H. F., Corballis, P. M., Hillyard, S. A., Marzi, C. A., Seth, A., Lamme, V. A. F., Volz, L., Fabri, M., Schechter, E., Bayne, T., Corballis, M., & Pinto, Y. (2020). Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness. Neuropsychology review, 30(2), 224–233. https://doi.org/10.1007/s11065-020-09439-3

4. U.S. Department of Health and Human Services. (n.d.). Epilepsy and Seizures. National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/health-information/disorders/epilepsy-and-seizures