Hemispherectomy

Hemispherectomy is a surgery that is performed by a neurosurgeon where an unhealthy hemisphere of the brain is disconnected or removed. There are two types of hemispherectomy. Functional hemispherectomy refers to when the diseased brain is simply disconnected so that it can no longer send signals to the rest of the brain and body. Anatomical hemispherectomy refers to when not only is there disconnection, but also the diseased brain is physically removed from the skull. This surgery is mostly used as a treatment for medically intractable epilepsy, which is the term used when anti-seizure medications are unable to control seizures.

The first anatomical hemispherectomy was performed and described in 1928 by Walter Dandy. This was done as an attempt to treat glioma, a brain tumor.^[1] The first known anatomical hemispherectomy performed as a treatment for intractable epilepsy was in 1938 by Kenneth McKenzie, a Canadian neurosurgeon.^[2] Krynaw, a neurosurgeon from South Africa, was one of the first to perform and report a case series on hemispherectomies in 1950. He performed the surgery on pediatric patients with infantile hemiplegia, specifically as a treatment for their seizures and cognitive impairment.^[3] His hemispherectomy technique removed the damaged hemisphere except the thalamus and caudate structures. Krynaw reported good outcomes overall, although there was one post-operative death. Specifically, there was an overall theme of improvement in weakness, spasticity and cognition.^[3] Amazingly, ten out of the twelve patients had seizures prior to the operation and none of the patients had seizures afterwards.^[3] Other neurosurgeons began performing hemispherectomies as well, primarily for the treatment of seizures. For the most part, the surgeries would go well initially, but there was a general theme of subsequent deterioration and even death years after the surgery. As a result of the complication risk and the introduction of new anti-seizure medications, the popularity of the procedure began to decline in the 1950s.^[4] Oppenheimer and Griffith were one of the first to describe the potential complications, and they reported their findings in 1966, describing superficial hemosiderosis, granular ependymitis and obstructive hydrocephalus.^[5] They posited a theoretical solution to this problem, a surgery that is now known as a functional hemispherectomy.^[5] Rasmussen was one of the first neurosurgeons to develop and apply a functional hemispherectomy in practice. He initially made modifications to the original hemispherectomy by preserving the least epileptogenic quarter or third of the hemisphere, hoping this would ameliorate the known complications of the original anatomic hemispherectomy. Although this modification seemed to solve this issue, patients undergoing the modified hemispherectomy continued to have seizures, which was problematic. Therefore, he further modified his surgery to functionally sever residual portions of the frontal and parieto-occipital lobes.^[4] This surgery, the functional hemispherectomy, has been further modified over the years by several different neurosurgeons, and to this day there is not a consensus as to which exact technique should be used. Hemispherotomy refers to some of the more recently developed approaches to disconnect the epileptic hemisphere while minimizing brain removal and the risk for complications.^[6]

There are two main types of hemispherectomy: Anatomical and Functional.

Anatomical hemispherectomy refers to the resection and removal of an entire hemisphere of the brain, which includes all four lobes, with or without the removal of basal ganglia and thalamus.^[6]

Functional hemispherectomy refers to surgeries that disable the function of one hemisphere, while maintaining its blood supply and without physically removing the entire hemisphere from the skull.^[6] Functional hemispherectomies are performed more frequently than anatomical hemispherectomies due to their lower complication rates.^[7] However, they do carry a risk of incomplete disconnection, which refers to when the surgeon inadvertently leaves remnants of fibers that continue to connect the hemisphere to the brain and body. These remaining fibers can be problematic, as they may lead to seizure recurrence.

Another term that falls under the hemispherectomy umbrella includes hemidecortication, which is the removal of the cortex from one half of the cerebrum, while attempting to preserve the ventricular system by maintaining the surrounding white matter. Hemidecortication was originally developed as a possible strategy to mitigate some of the complications seen with complete anatomical hemispherectomy.^[6]

The term hemispherotomy refers to a surgery that is akin to a functional hemispherectomy in that it functionally severs the damaged hemisphere from the other and leaves some of the severed hemisphere within the skull, but the difference is that it removes even less tissue from the skull.^[6] The term hemispherotomy is now used as an umbrella term to describe the group of modern techniques and procedures that predominate at most contemporary epilepsy centers.^[7]

There is no statistically significant difference in seizure-free rates between the four different types of surgeries:  Hemispherotomy, functional hemispherectomy, anatomical hemispherectomy and hemidecortication. The overall rate of seizure freedom is estimated to be 73.4%.^[7]  However, hemispherotomy procedures may be associated with a more favorable complication profile.^[7]

The typical candidates for hemispherectomy are pediatric patients who have intractable epilepsy due to extensive cerebral unilateral hemispheric injuries.^[8][6] In addition, the seizures should ideally be emanating from that same hemisphere. In some situations, a hemispherectomy may still be performed if there are seizures from both hemispheres, as long as the majority come from one side. In order to assess the patient’s epilepsy completely, patients undergo extensive testing, including EEG and MRI. Most patients also undergo other studies including functional MRI (fMRI), positron emission tomography (PET) or magnetoencephalography (MEG).

Today, hemispherectomy is performed as a treatment for severe and intractable epilepsy, including for young children whose epilepsy has been found to be drug-resistant.^[9] The most common underlying etiologies include malformations of cortical development (MCD), perinatal stroke and Rasmussen’s encephalitis.^[6] MCD is an umbrella term for a wide variety of developmental brain anomalies, including hemimegalencephaly and cortical dysplasia. Other less common underlying etiologies include hemiconvulsion-hemiplegia epilepsy syndrome and Sturge-Weber syndrome.^[7]

Patients often shave the area of the scalp that will be involved with the surgery. Patients undergo general anesthesia and are unconscious for the procedure. The surgical site is sterilized, after which the skin is incised. A substantial portion of the bone is removed, followed by incision of the dura, which is the outer covering of the brain. There are several blood vessels that have connections with both sides of the brain, and these are carefully identified and clipped in such a way that spares the healthy hemisphere. Ultimately, a bundle of fibers that connect both of the cerebral hemispheres, the corpus callosum, is removed which results in the functional separation of one hemisphere from the other. Portions of the cerebral lobes from the damaged side of the brain are removed, depending on the specific procedure being performed. The surgeon may leave some brain tissue, such as the thalamus or choroid plexus. After completing the resection, the surgical site is irrigated with saline, the brain covering called the dura is sutured back together, the bone that was removed is replaced and the skin is sutured. This surgery often takes four to five hours.^[8] Patients often spend a few nights in the hospital post-operatively, and they undergo physical and occupational therapy soon after the surgery.^[8]

The most common complication from surgery is hydrocephalus, a condition in which fluid accumulates within the brain, and this is often treated with a shunt to divert the fluid away. The rate of shunts following surgery ranges from 14–23%.^[6] Other complications include wound complications, epidural hemorrhages, subdural hemorrhages, intraparenchymal hemorrhages, intracranial abscesses, meningitis, ventriculitis and venous thrombosis.^[7] Additional epilepsy surgery following hemispherectomy is rare (4.5%),^[7] but may be recommended if there is a residual connection between the two hemispheres that is causing frequent seizures. Mortality rates are low and estimated to be <1% to 2.2%.^[6][7] Most patients do not experience changes in cognition, but some individuals may be at risk.^[7] A visual deficit called contralateral homonymous hemianopsia is expected to occur in most patients, where the entire visual field contralateral to the removed hemisphere is lost.^[6] There is a risk of motor deficits, and this is variable.^[6] Other possible complications include infection, aseptic meningitis, hearing loss, endocrine problems and transient neurologic deficits such as limb weakness.^[6]

Since seizures are the most common indication for hemispherectomy surgery, most research on hemispherectomy analyzes how the surgery affects seizures. Many patients undergoing surgery obtain good surgical outcomes, some obtaining complete seizure freedom (54–90%) and others having some degree of improvement in seizure burden.^{[6][7][10][11]} A recently developed scoring system has been proposed to help predict the probability of seizure freedom with more accuracy:  HOPS (Hemispherectomy Outcome Prediction Scale).^[11] Although it cannot definitively predict surgical outcome with exact precision, some physicians may use it as a guide. The scoring system takes certain variables into consideration including age at seizure onset, history of prior brain surgery, seizure semiology and imaging findings.

There is also data pertaining to how hemispherectomy affects the body in other ways. After surgery, the remaining cerebral hemisphere is often able to take over some cognitive, sensory and motor functions. The degree to which the remaining hemisphere takes on this additional workload often depends on several factors, including the underlying etiology, which hemisphere is removed and the age at which the surgery occurs.^[12]

In terms of postoperative motor function, some patients may have improvement or no change of their weaker extremity,^[10] and many can walk independently.^[12] Most patients postoperatively have minimal to no behavioral problems, satisfactory language skills, good reading capability,^[12] and only a minority of patients have a decline in IQ.^[13] Predictors of poor outcome may include seizure recurrence and structural abnormalities in the intact hemisphere.^[12]

Ultimately, risks and benefits should be weighed on an individual basis and discussed in detail with the neurosurgeon. Many patients have excellent outcomes, and the International League Against Epilepsy (ILAE) reports that “about one-fifth of hemispherectomy patients are gainfully employed and even fewer live independently.”^[14]

The Brain Recovery Project is a non-profit corporation which funds new research and is based in the United States. This corporation hosts an annual two-day conference for patients who have had hemispherectomies and their families. There are several purposes to this reunion. The main goal is to educate patients and their families on the surgery and its necessary subsequent rehabilitation. It also serves as a way for patients and families to connect with one another, learn from specialists in the field and often offers research enrollment.^[14]

• Corpus callosotomy
• Hemispherectomy Foundation

• Battro, Antonio M. (2001). Half a Brain is Enough: The Story of Nico. Cambridge University Press. ISBN 978-0-521-78307-1.
• Kenneally, Christine (July 3, 2006). "The Deepest Cut". The New Yorker. pp. 36–42.
• Choi, Charles Q. (March 2008). "Do You Need Only Half Your Brain?". Scientific American. 298 (3): 104. Bibcode:2008SciAm.298c.104C. doi:10.1038/scientificamerican0308-104.

• The Deepest Cut by Christine Kenneally, The New Yorker