Epilepsy Surgery in Children
Since the introduction of surgery for medically intractable epilepsy on Victor Horsley in 1886, the focus of treatment was on adults, rather than on children. In children with epilepsy, consideration of surgery was usually delayed for several years while various drugs were tried in different combinations, in hope that the seizures would disappear with age. However, frequent convulsions and impaired mentation can take a serious toll on a child's development and socialization. With the advent of improvements in operative techniques, and a better understanding of the plasticity present in a child's nervous system, there have been dramatic improvements in the treatment of medically intractable epilepsy. At the Department of Neurosurgery a team approach is taken in the treatment of children with epilepsy. We offer state of the art surgical treatment for intractable seizures, including surgery by experienced practitioners who are innovators in their field.
During the past two decades, there has been a great interest in surgical treatment of epilepsy in childhood. In younger patients, many seizure types appear to be suitable for resective surgery. Furthermore, a growing body of evidence suggests that frequent uncontrolled seizures may have a deleterious effect on the developing brain. Finally, the potential for functional recovery is greatest in the young child. It is wise to operate as soon as the seizures have proved refractory to appropriate medical therapy. Early curative surgery eliminates the adverse effects of seizures on the maturing brain and takes advantage of the plasticity of the immature brain.
To better understand the surgical treatment of epilepsy in children, it is helpful to summarize the basic concepts of brain development and plasticity, and to review the effects of intractable seizures on the developing brain. Taking into account both the knowledge of specific childhood seizure types and the advent of brain plasticity, we review the protocols formulated for childhood epilepsy and appropriate choices of surgical procedures.
Brain Development and Maturation
Central to the renaissance in the treatment of childhood epilepsy is the recognition that the development and the ability of the child's brain to recover from injury justified departure from the traditional dogmas and approaches to adult epilepsy surgery. Neuronal modification and neuroglial proliferation is the process by which a child's brain undergoes maturation. The key to understanding the pathophysiology of brain plasticity is the time dependent maturation of neuronal networks. Nerve cells extend processes (dendrites and axons) that undergo extensive branching and growth to create an intricate network during the first decade of life. This flurry of activity is at its peak at about 8 years of age. There appears to be a period of excess neurons and connections during this time followed by selection of inappropriate or excess connections for eventual cell death. This reduction in the network density plateaus at about the age of 14. Neuronal plasticity is the capacity of the immature brain to modify existing pathways through several different mechanisms, which are dependant on surrounding electrical fields and other appropriate environmental stimuli.
The degree of plasticity seems to be age dependent; the degree of functional recovery corresponds inversely with age at the time of operation.
Impact of Seizures on the Developing Brain
Substantial evidence suggests that frequent seizures interfere with the maturation of the brain. By exposing neurons to repetitive long-term electrical discharges, scientists were able to demonstrate damage to neuronal networks in the developing brain.
Although the controversy continues over the impact of frequent uncontrolled seizures on cerebral development, a large body of evidence suggests an adverse effect. It is therefore incumbent on the pediatric neurologist and neurosurgeon to evaluate medical intractability. If the seizures do not respond to two or three of the most appropriate anticonvulsant medications, then it is unlikely that they will be controlled by any drug regimen. By evaluating these children with intractable epilepsy as early as possible one can minimize the potential damage during critical periods of brain development, and maximize functional recovery resulting from brain plasticity.
Childhood Seizure Disorders
According to the International Classification of Epileptic Seizures, seizures can be classified as either partial or generalized. Partial seizures are those seizures that start in a focal (localized) area of the brain. Generalized seizures do not arise from a discrete part of the brain. The evaluation of children with partial onset seizures requires investigation of lateralization (which side) and localization of the area of seizure onset. It must then be determined whether resection of this area is possible without inflicting a new deficit or aggravating an old one. Some children with generalized seizures have also been helped with surgery. For instance, in some patients with drops attacks a procedure called "corpus callosotomy" has been shown to interrupt the spread of seizures from one hemisphere to the other.
Partial Seizures Associated with Hemiparesis
The Sturge-Weber syndrome is characterized by an abnormality of the blood vessels, frequently in association with a facial nevus. The underlying cerebral abnormalities include progressive brain injury and calcification. Seizures can develop within the first year of life. With the onset of uncontrollable seizures, patients may develop a progressive weakness of their extremities on the side opposite to the brain abnormality and become mentally retarded. If the seizures are medically intractable, surgery is recommended as early as possible, to ensure minimal interference with development and maximal recovery. Those patients with a localized lesion may require only a limited resection. Those with more diffuse involvement accompanied by a hemi paresis are candidates for a more extensive surgery called "hemispherectomy."
Dr. Rasmussen and co-workers initially described and performed a hemispherectomy for this syndrome in 1958. Patients typically present between 4 to 9 years of age with the onset of infrequent focal (partial) seizures. Seizure frequency is usually progressive, with eventual hemiplegia and mental retardation. Magnetic resonance imaging (MRI) reveals abnormalities in the affected hemisphere. Because the condition almost always spreads to the entire hemisphere, limited focal resection is not recommended. Instead it is prudent to reserve resective surgery until the loss of fine movements in the affected hand prior to proceeding to surgery.
Infantile Hemiplegic Epilepsy
This form of infantile hemiplegia incorporates a number of disorders, all of which may lead to intractable partial seizures and hemi paresis. Most of the known underlying causes are congenital. Seizures usually begin within the first few months of life, and the hemi paresis is noticed between the ages of 3 and 6 months. Many of these children will progress to intractable seizures and mental retardation if their seizures are not brought under control.
Complex Partial Epilepsy Not Associated with Hemiparesis
Complex partial seizures of temporal lobe origin are the most common disorder in this category. They are not associated with hemi paresis and rarely result in severe mental retardation. However, the postulated adverse effects on development, accompanying psychosocial problems and the effects of anticonvulsant medications can all be as equally debilitating. The best opportunity for surgical control of seizures, and prevention of psychological and developmental handicaps, is prior to puberty. Whereas in adults with poorly controlled epilepsy there is little urgency to operate, in young children with intractable seizures surgery should be performed as early as possible to promote brain maturation.
Generalized or Secondarily Generalized Seizures
Infantile spasm, also known as West's syndrome, is a characteristic triad of myoclonic seizures, a specific (hypsarrhythmic) pattern of EEG, and, ultimately, profound mental retardation. The seizures are characterized by sudden flexion or extension of the extremities or trunk. They have been described clinically as salaam seizures, jackknife seizures, flexor or extensor spasms, and Blitskrampf-lightning attacks. Although seizure onset is usually between 4-8 months of age, they can start as late as 24 months. Some children respond to medical therapy with a hormone called Adrenocorticotropic hormone (ACTH). For others, the prognosis is extremely poor. Surgical resection can be considered if an epileptogenic focus is found to be localized.
Lennox-Gastaux Syndrome is a severe epileptic disorder of childhood. In the past, children with Lennox-Gastaux and infantile spasms were not considered for surgical treatment because an isolated epileptogenic region could not be identified. Corpus callosotomy had occasionally been performed as a palliative procedure. In some cases positron emission tomography has clearly demonstrated a localized area of interictal hypometabolism suggestive of a cortical abnormality, which has been supported by intraoperative electrocorticography. Histopathological examination of surgical specimens have revealed the presence of structural abnormalities such as cortical dysplasia or cystic gliosis.
Selection Criterion for Pediatric Epilepsy
The surgical approach to pediatric onset epilepsy was traditionally based on principles developed for treatment of adults. Despite most cases of epilepsy beginning in childhood, selection criterion for adult patients were applied unmodified to children. There has been a dramatic evolution in the treatment of epilepsy in children with a departure from the treatment protocols of epilepsy in adults.
Because the maturing brain is adversely affected by frequent seizures and because it retains the capacity to recover from resective surgery, approaches for the surgical management of epilepsy in children are of necessity different from those used for adults. A number of principles used in evaluating adults with intractable epilepsy must be modified when evaluating children.
The traditional definition of medical intractability implies a minimum seizure frequency and a lengthy evaluation period needed to establish lack of responsiveness to all available drugs. In adults, it has been accepted that establishing medical interactability may take several years. In an adult having 1-2 seizures a month, it may take a significant time period, often several years, to establish medical intractability. In a child in whom seizures tend to occur more frequently, with 50-60 seizures per day being not uncommon, intractability usually can be established much more rapidly. In the young, it is not necessary to establish resistance to all classes of antiepileptic drugs. The range of antiepileptic drugs available for children with epilepsy is much smaller than for their adult counterparts. Furthermore, certain pediatric conditions, such as Sturge-Weber, Rasmussen's encephalitis, or congenital brain malformations, rarely respond to antiepileptic medications for an extended period of time. If a child does not respond to two or three appropriate antiepileptic drugs, it is extremely unlikely that the child will respond to any medical regimen.
The toxic effects of anticonvulsants must also be considered when dealing with pediatric patients as candidates for operative intervention. The literature concerning the neuronal toxicity of phenytoin is inconsistent, but a number of studies and case reports do implicate a deleterious effect on parts of the brain.
Adults who develop side effects from anticonvulsant medications will complain or simply modify their dosage. Children tend not to, or are unable to complain and their parents frequently accept the induced apathy or hyperactivity in exchange for improved seizure control.
When considering resective surgery in the very young child, there is no minimum age for surgical candidacy. Although spontaneous remission of seizures is common for some seizures without an associated structural lesion, partial epilepsies with known structural lesions, and not controlled with first and second line drugs, are unlikely to stop as the child grows. In fact, the younger the surgical candidate, the more urgent the need to proceed with surgery. By offering surgical resection at the earliest appropriate time, adverse effects of antiepileptic medications and epileptiform discharges on brain development and behavior can be minimized while at the same time taking maximal advantage of brain plasticity in promoting recovery.
Intractable focal or unilateral seizures arising from a localized area of brain abnormality can adversely affect the functioning of the remainder of the brain. This may lead to a child with a focal brain abnormality that is severely disabled. Because of the widespread effects of a focal seizure focus, a child who is having frequent attacks may appear severely delayed, but once the "abnormal region of the brain" has been removed and the seizures brought under control, developmental progress may be dramatic.
Unlike adults, the production of a new or the aggravation of an existing deficit may be acceptable in a child. In a child with frequent seizures and thus at high risk for severe developmental delay, a loss of strength on one side of the body (hemiparesis) or partial visual loss (hemianopsia) is probably an acceptable price to pay for seizure control and improvement in intellectual development. Also, the deficits will usually improve significantly with time. In patients with Rasmussen's encephalitis or Sturge-Weber syndrome, the underlying disease eventually leads to the same deficit.
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