Neuroplasticity: How the Brain Adapts from Massive Loss
Audrey Hunter
Plymouth State University
The brain is known as the body’s most complex and mysterious organ. While there have been massive advancements in understanding of the brain since the days of phrenology and lobotomies, there are still many aspects of neuroscience that are currently unknown. One topic of relatively recent interest is neuroplasticity, or the brain’s ability to change, reorganize and create new neuronal connections throughout life and following injury. Previously, it was thought that even minor damage to the brain could have devastating consequences, as the brain did not seem to regenerate or heal in ways that other organs could. However, recent research involving patients with massive brain tissue loss shows that this is not necessarily the case.
One mechanism of studying neuroplasticity comes from observing patients who have undergone massive brain tissue loss, particularly from hemispherectomy procedures. Hemispherectomies involve removing half, or one hemisphere, of the brain in order to treat diseases such as Rasmussen encephalitis, intractable seizures, or in response to severe brain injury. Rasmussen encephalitis is a chronic inflammatory neurological disease that typically manifests in one hemisphere and causes symptoms such as encephalitis, hemiparesis, severe seizures and cognitive deterioration. Because it typically affects one hemisphere, a hemispherectomy offers a strong likelihood of permanently ceasing seizure activity in RE patients. The procedure is indicated in a similar way for patients with intractable seizure ahd hemiparesis. In these types of patients, there is often atrophy, poor development and severe seizure activity in one hemisphere, and the procedure is done not only to stop seizure activity but also to prevent it from crossing the corpus callosum and affecting the healthy hemisphere. Furthermore, a hemispherectomy can be used when one hemisphere has become necrotic or severely infected in order to prevent the damage from spreading to the other hemisphere. Unlike a corpus callosotomy (severing of the corpus callosum), a hemispherectomy is frequently more “curative” than palliative, and often patients experience a dramatic improvement in function. Although this procedure is drastic, the results are also dramatic and often life altering and life saving.
There are two main types of hemispherectomy procedures: functional hemispherectomy and anatomical hemispherectomy. An anatomical hemispherectomy involves completely removing all four lobes of the brain and occasionally deeper structures (thalamus, basal ganglia, etc) as well. Following this procedure, the cranial cavity will then fill with cerebrospinal fluid where the hemisphere used to be. A functional hemispherectomy does not actually remove the hemisphere, but instead disables it. There may be partial removal of brain tissue, but the main purpose of this procedure is to cease the functions in one hemisphere.
Initial research on this procedure suggested that the timing of the surgery and the age of the patient were both crucial to preserving brain function. If the procedure is done when the patient is a child, when the brain is still developing, the brain will have an easier time reconfiguring to make up for the loss. A developing brain would be able to evolve with the limitation of one hemisphere, instead of in spite of it, as it would if it had already finished developing. However, further research has shown that the procedure often produces favorable results in adults as well. A study done by the Cleveland Clinic followed 47 patients, all of whom were at least 16 years old at the time of the procedure, and determined that the outcomes of the surgery were overall favorable. At approximately five years post surgery, seizure activity had improved in 77% of patients, and motor, language and cognitive function were typically stable and unchanged (McGovern et al 2019). In a similar manner to pediatric hemispherectomies, there was a low mortality and complication rate. Additionally, a case report involving a 21 year old patient with intractable seizures and hemiparesis had similar results: no worsening of pre-existing deficits and freedom from seizure activities (Krishnan et al 2011). These studies, along with others, showed that this procedure was safe and effective in adult patients, something that many neuroscientists were rightfully uncertain of. With that being said, neuroplasticity seems to be an ongoing, lifelong process as opposed to something that can only occur in development.
With regards to neuroplasticity, there are two main methods: structural neuroplasticity (synaptic connections are strengthened) and functional neuroplasticity (synaptic connections are altered). LTP, or long term potentiation, is the best known example of long term synaptic plasticity and what is thought to be the mechanism underlying learning and memory. LTP involves strengthening synaptic connections through frequent activation, entailing intense stimulation of the presynaptic neuron, which increases the amplitude of the postsynaptic neuron as a result. Therefore, neuroplasticity is often increased by repeating tasks, such as practicing math problems or learning a language, as is often suggested to ward off symptoms of dementia.
At surface level, neuroplasticity can be observed through behavior, especially in patients who have undergone some kind of drastic neurological change. Patients go through both psychological and physiological tests to determine whether or not they have increased, decreased or stable brain function. In addition, imaging is used to actually show neuroplasticity. In a study done by Kliemann et al, functional brain networks in hemispherectomy patients were looked at using an fMRI and compared to scans of control subjects. The study showed that the hemispherectomy patients not only had equally strong interhemispheric connectivity in functional networks as compared to non-hemispherectomy patients, but that the connectivity between functional networks was stronger for hemispherectomy patients post-procedure (Kliemann et al, 2019). The results not only speak to the brain’s ability to reorganize after significant loss, but the success of rehabilitative efforts after surgery. The extra connectivity between networks may reflect a “team” effort that the brain may adaptively use to make up for what was lost- something that is very encouraging for both physicians and patients. This study, among others, proves that a hemispherectomy is effective at reducing seizure activity and allows for the preservation of function, despite how radical it may seem.
One rehabilitative therapy that has been found to be helpful after hemispherectomy procedures is Intensive Mobility Training, or IMT. IMT involves mechanisms such as body-weight supported treadmill training (BWSTT) with the goal of improving mobility, walking and gait in patients using the principles of neural plasticity. A study done by Fritz et al looked into the effectiveness of IMT in patients who presented with hemiplegia post-cerebral hemispherectomy. The study took place over a two week period and involved three hour sessions each day of repetitive exercises, such as proprioceptive activities and static balance activities. As mentioned previously, repeating behaviors is used to increase long term potentiation, which was the justification in mind for such exercises. Overall, IMT was found to be an effective way to improve balance, gait and walking ability, with the most significant improvement in patients who had surgery before the age of five (Fritz et al 2011). This finding could be supportive of the idea of a higher level of plasticity during development, but it could also be a result of damage to the brain as a result of a longer period of incessant seizures or injury as opposed to a result of the hemispherectomy itself. In either case, the results are helpful in guiding post-operative rehabilitation procedures and treatment plans.
While the recent research shows that neuroplasticity exists and that it continues to exist in adulthood, future research is needed to show exactly how and why it works. In doing so, there could be significant improvements in treatments for many kinds of degenerative brain diseases, such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Currently, clinical trials are able to work on early intervention to prevent such diseases in patients with genetic vulnerabilities, but harnessing neuroplasticity in patients who are already symptomatic has not been truly successful yet (Smith 2013). The successful outcomes and bright prognoses of hemispherectomy patients give insight on the power and mechanisms of neuroplasticity, and further research on hemispherectomy patients could be the key to discovering how to help patients in late stages of these degenerative illnesses. Hemispherectomy patients are proof that the brain is resilient, plastic, and most importantly, can overcome massive loss in spite of what seems to be intuitive.
References
A. M. Devlin, J. H. Cross, W. Harkness, W. K. Chong, B. Harding, F. Vargha‐Khadem, B. G. R. Neville, Clinical outcomes of hemispherectomy for epilepsy in childhood and adolescence, Brain, Volume 126, Issue 3, March 2003, Pages 556–566, https://doi.org/10.1093/brain/awg052
Fritz SL, Rivers ED, Merlo AM, et al. Intensive mobility training postcerebral hemispherectomy: early surgery shows best functional improvements. European Journal of Physical and Rehabilitation Medicine. 2011 Dec;47(4):569-577.
Cell Press. (2019, November 20). Brain scans reveal how the human brain compensates when one hemisphere is removed. ScienceDaily. Retrieved May 7, 2020 from www.sciencedaily.com/releases/2019/11/191120070710.htm
Kliemann D, Adolphs R, Tyszka JM, et al. Intrinsic functional connectivity of the brain in adults with a single cerebral hemisphere https://www.sciencedirect.com/science/article/pii/S2211124719313816. Cell Rep 2019;29(8):2398–2407
Krishnan SS, Panigrahi M, Jayalakshmi S, Varma DR. Neuroplasticity in hemispheric syndrome: An interesting case report. Neurol India [serial online] 2011 [cited 2020 May 7];59:601-4. Available from: http://www.neurologyindia.com/text.asp?2011/59/4/601/84346
McGovern, RA, N. V. Moosa, A, Jehi, L, et al. Hemispherectomy in adults and adolescents: Seizure and functional outcomes in 47 patients. Epilepsia. 2019; 60: 2416– 2427. https://doi.org/10.1111/epi.16378
Smith G. S. (2013). Aging and neuroplasticity. Dialogues in clinical neuroscience, 15(1), 3–5.
TED. (2009, April 28). Michael Merzenich: Growing evidence of brain plasticity [Video]. YouTube. https://www.youtube.com/watch?time_continue=164&v=Z41BTeAU7DI&feature=emb_logo