Difference between revisions of "Corpus Callosotomy"

Latest revision as of 07:27, 22 October 2022

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]

Affected Areas of the Brain

Corpus Callosum

The corpus callosum is a thick group of nerve fibers that connects the left and right hemispheres of the brain. Its primary function is to share information between the two hemispheres to coordinate motor movement and other responses to stimuli. Brain imaging suggests it has a topographic structure. It is also hypothesized to have inhibitory effects that prevent uncoordinated motor function. [4]

Right and Left Hemispheres

Each hemisphere of the brain helps to control motor function on the opposite side of the body (i.e. the right hemisphere coordinates movement of one’s left hand). The left hemisphere is typically thought of as the “dominant” hemisphere, as it is generally responsible for speech, language, and dominant hand movement. The right hemisphere processes visual and spatial information, so a connection between the two hemispheres is critical to express in words what one is seeing or experiencing. It also explains why Patient W.J. could process that he was seeing a certain image regardless of which hemisphere it was presented to but could not tell the researcher what he was seeing when the image was shown to his right hemisphere. [5]

Patient Demographics

The target patients for corpus callosotomy are affected by epilsepy but are not eligible for other surgeries, such as focal resections. They generally do not have progressive neurological or medical diseases that would interfere with the procedure. Patients are considered after multiple anti-seizure medications 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. [6]

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. [7]

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.

Efficacy

Research into efficacy of the corpus callosotomy is limited, as a very small number of surgeries have actually been performed. The procedures that have occurred are difficult to study, as patients are often not good candidates for research due to their neurological states. Research over the years has suggested some level of decrease in generalized seizures for patients post-corpus callosotomy.

Recent Study

There are variations of the corpus callosotomy procedure. The partial procedure severs the anterior two-thirds, and the complete procedure is a full division that also severs the posterior one-third. A study conducted by the Children’s Hospital of Michigan examined both types of corpus callosotomy in 20 patients with an age range of 5-19 years. They monitored patients post-operation for 3 years and found that the procedure completely eliminated seizures leading to falls in 35% of the patients. 50% of the patients had between 50% and 75% reduction of those seizures, and 15% had 76-99% reduction. While there were observed deficits in speech and mobility post-operation for some of the patients, those symptoms decreased within 4 weeks. 3 children who received the operation later developed neurological disorders, but those were not clearly linked to the corpus callosotomy.

The study concluded that the corpus callosotomy is effective to treat generalized seizures often resulting in falls. However, it is not effective for treating focal seizures. All patients who previously experienced focal seizures continued to experience them, with some even worsening. One patient who didn’t previously have focal seizures developed them post-operation. [8]

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. Goldstein A, Covington BP, Mahabadi N, et al. Neuroanatomy, Corpus Callosum. (2023, April 3). StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK448209/

5. Hines, T. (2018, April). Brain Anatomy. Mayfield Brain & Spine. https://mayfieldclinic.com/pe-anatbrain.htm

6. Aside-Pooyaa, A. A., Sharanb, A., Neia, M., & Sperlinga, M. R. (2008b). Corpus Callosotomy. Epilepsy & Behavior, 13(2), 271–278. https://doi.org/https://doi.org/10.1016/j.yebeh.2008.04.020

7. 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

8. Luat, A. F., Asano, E., Kumar, A., Chugani, H. T., & Sood, S. (2017). Corpus Callosotomy for Intractable Epilepsy Revisited: The Children's Hospital of Michigan Series. Journal of child neurology, 32(7), 624–629. https://doi.org/10.1177/0883073817697847

9. University of Queensland. (n.d.). Corpus Callosum. Queensland Brain Institute. https://qbi.uq.edu.au/brain/brain-anatomy/corpus-callosum

10. U.S. National Library of Medicine. (n.d.). Understanding Different Kinds of Seizures. NIH MedlinePlus Magazine. https://magazine.medlineplus.gov/article/understanding-different-kinds-of-seizures