Epilepsy Surgery in Children
Since the introduction of surgery for medically intractable epilepsy in 1886, the focus of treatment was on adults, rather than on children. In hope that the childhood seizures would disappear with age, surgery was usually delayed for several years while various drugs were tried in different combinations. However, frequent convulsions and impaired development may take a serious toll on a child's mental growth and socialization. With the recent improvements in operative techniques, and a better understanding of the potential of a child's nervous system, there have been dramatic improvements in the treatment of epilepsy that cannot be treated with medicine (intractable). At the Department of Neurosurgery, a team approach is taken for the surgical treatment of intractable seizures.
During the past two decades, there has been a great interest in surgical treatment of epilepsy in childhood. In younger patients, many seizure types respond to surgery. Furthermore, a growing evidence suggests that frequent uncontrolled seizures may have a harmful effect on the developing brain. Finally, the potential for functional recovery is greatest in a young child. It is wise to operate as soon as the seizures have proved resistant to appropriate medical therapy. Early surgery helps to eliminate the adverse effects of seizures on the maturing brain. In addition, surgery helps to trigger the brain's neurons to rearrange their connections which may improve overall memory formation and learning processes (plasticity).
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 specific childhood seizure types and the advent of brain plasticity, the team at the Department of Neurosurgery reviews the protocols formulated for childhood epilepsy and appropriate surgical procedures.
The ability of the child's brain to recover from injury justified departure from the traditional views and approaches considered with adult epilepsy surgery. Neuronal modification and neurological proliferation are the processes 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 dependent on surrounding electrical fields and other appropriate environmental interactions.
The degree of plasticity seems to be age dependent; the degree of functional recovery corresponds inversely with age at the time of operation.
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, there is a body of evidence which suggests an adverse effect. 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, potential damage during critical periods of brain development may be minimized and brain plasticity may be maximized.
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 separate 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 removal of this area is possible without causing 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 drop attacks, a procedure called "corpus callosotomy" has been shown to interrupt the spread of seizures from one hemisphere to the other.
The Sturge-Weber syndrome is characterized by abnormal blood vessels, and are frequently associated with facial nerve bundles. The underlying cerebral abnormalities include progressive brain injury and calcification (abnormal deposits of calcium in areas of the brain). 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 developmentally delayed. If the seizures cannot be treated medically, surgery is recommended as early as possible, to ensure minimal developmental interference and maximal recovery. Patients with a localized lesion may require only a limited resection. Those with more diffuse involvement accompanied by a hemiparesis (weakness on one side of the body) 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 between 4 to 9 years of age typically have infrequent focal (partial) seizures. Seizure frequency is usually progressive, with eventual hemiplegia and mental impairment. 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 usually recommended to delay surgery until there is loss of fine movements in the affected hand.
This form of infantile hemiplegia incorporates a number of disorders, all of which may lead to intractable partial seizures and hemiparesis. Most of the known underlying causes are congenital. Seizures usually begin within the first few months of life, and the hemiparesis is noticed between the ages of 3 and 6 months. Many of these children will progress to intractable seizures and mental impairment if their seizures are not brought under control.
Complex partial seizures of temporal lobe origin are the most common disorder. They are not associated with hemiparesis and rarely result in severe mental impairment. However, the likely adverse effects on development, accompanying psychosocial problems and the effects of anticonvulsant medications can all be as equally debilitating. Having surgery prior to puberty is usually best for controlling seizures, and preventing psychological and developmental handicaps. In adults with poorly controlled epilepsy there is little urgency to operate. However in young children with intractable seizures, surgery should be performed as early as possible to help promote brain maturation.
Infantile spasm, also known as West's syndrome, is diagnosed by involuntary twitching-like seizures, a specific (hypsarrhythmic) pattern of EEG, and profound mental impairment. The seizures are characterized by sudden 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 to 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 (cause of epilepsy) region could not be identified. Corpus callosotomy had occasionally been performed as a palliative (eliminate symptoms, not condition) procedure. In some cases, positron emission tomography (PET) 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 (faulty neurons) or cystic gliosis. Vagus Nerve Stimulation (VNS) can also help.
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 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 to establish a lack of responsiveness to all available drugs. In adults, it has been accepted that establishing medical intractability may take several years. In an adult having 1-2 seizures a month, it may take a significant amount of time, often several years, to establish medical intractability. In a child whose 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 adults. 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 harmful 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 indifference 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, these seizures 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 likely an acceptable option 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.