|Year : 2008 | Volume
| Issue : 1 | Page : 82-87
Anesthesia for pediatric epilepsy surgery
Rebecca Jacob1, Sanjib Das Adhikary1, Roy Thomas Daniel2
1 Department of Anaesthesia, Christian Medical College, Vellore, India
2 Department of Neurological Sciences, Christian Medical College, Vellore, India
Department of Anaesthesia, Christian Medical College, Vellore - 632 004, Tamil Nadu
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Epilepsy surgery for the child is increasingly being offered as a management option even in infants, due to advances in neurosurgery and pediatric neuro-anesthesia, coupled with a better understanding of neurophysiological evaluation/monitoring. Anesthesia for children in the setting of a major surgery like epilepsy surgery presents a variety of challenges. This article deals with the physiology of electroencephalogram and the effects of anesthetic agents on neurophysiology, sedation and anesthesia for investigative procedures and definitive surgery with major blood loss and fluid shifts, with special emphasis on the small child and awake craniotomy in the older child.
Keywords: Anesthesia, electrocorticogram, epilepsy surgery
|How to cite this article:|
Jacob R, Adhikary SD, Daniel RT. Anesthesia for pediatric epilepsy surgery. J Pediatr Neurosci 2008;3:82-7
Pediatric epilepsy is more common than is generally acknowledged or perceived. The current treatment strategies include use of conventional antiepileptic drugs (AEDs), new AEDs, ketogenic diets, vagus nerve stimulation and epilepsy surgery. The last is usually considered only if the epilepsy is intractable. The commonest definition for intractable epilepsy is when seizures continue despite maximally tolerated doses of more than two AEDs with an occurrence of an average of one seizure per month for approximately 18 months with no more than a 3-month seizure-free period in these 18 months.  At the stage when the epilepsy is intractable and surgery is considered, the child is examined to determine whether he has a surgically remediable epilepsy syndrome with good electro-clinical-radiological concordance. To this end, he is subjected to neurological and neuropsychological evaluation, multiple electroencephalograms (EEGs), video telemetry and magnetic resonance imaging (MRI). If the lesion that is producing epilepsy may be safely removed without causing major functional deficits, he is considered for surgery. If, however, there is no concordance in the above investigations, he goes into Phase II studies, where he is subjected to prolonged invasive EEG monitoring, nuclear medicine and WADA (sodium amylobarbital) studies. Surgery is offered if concordance is obtained and the risk of incurring functional deterioration is considered minimal or acceptable.  Surgically remediable epilepsy can be treated by either resective or disconnective surgery. The surgical techniques are guided either by image guidance or by intraoperative neurophysiological monitoring. With the advent of newer surgical techniques and anesthetic agents and a greater understanding of neurophysiology in the intraoperative setting, surgery for epilepsy is now being undertaken even in very small children.
In this article, we will deal with the physiology of EEG and the effects of anesthetic agents on neurophysiology, sedation and anesthesia for investigative procedures, anesthesia for definitive surgery, along with management of major blood loss and fluid shifts, with special emphasis on the small child and awake craniotomy in the older child.
| Physiology of Electroencephalogram|| |
The electroencephalogram (EEG) is used to help identify conditions like epilepsy, infarction of the brain, consciousness and unconsciousness. However, the correlation of EEG with anesthetic agents is less clear. The EEG signals contain three parameters: amplitude, frequency and time. Amplitude is the height of the wave, frequency is the number of cycles per second the wave crosses the zero voltage line and time is the duration of epileptic activity. In cases of deeply placed epileptic foci, the spread of epileptiform activity is also of significance. The usual base frequency in a normal conscious patient is in the beta wave range (>13 Hz), and this comes to alpha range (8-13 Hz) with closure of eyes or mild sedation. The events that lead to the production of higher frequency is termed activation, and the events that lead to slower frequencies like theta or delta (<4-7 Hz) are referred to as depression of EEG. Epilepsy is recognized by high-voltage spike waves, whereas conditions like cerebral infarction appear as low voltage in EEG. These high-voltage spike waves have certain recognizable patterns and occur in areas specific to a particular type of epileptiform activity. However, these activities are not seen or recorded where the cortical activity is depressed by use of agents like antiepileptic medications or anesthetics. Therefore, to localize the epileptic focus, spike activity may be evoked or produced during surgery by using different maneuvers or pharmacologic agents.
| An Overview of the Anesthetic Agents and their Impact on Electroencephalogram|| |
Anesthesia tends to suppress normal EEG and suppress spike activity. However, it appears that spiking activity with volatile anesthetics may be expressed differently according to the type of epilepsy or location of the epileptiform focus.  Volatile anesthetics have both pro- and anticonvulsive properties. Increased spiking activity is seen with high concentrations of isoflurane. And sevoflurane, but very high concentration of isoflurane, sevoflurane and propofol, on the other hand, cause burst suppression and electrical silence. Propofol reduces spike activity in a dose-dependent manner.  Opioids are used intraoperatively for analgesia and for reduction of the dose requirements of other agents like inhalational and intravenous agents. , Remifentanil, alfentanil and fentanyl all increase spiking in abnormal brain - this is considered an advantage for monitoring. ,, The patients on prolonged antiepileptic drug treatment require a higher dose of opioids for effective analgesia. Hyperventilation also increases spike count.  It seems preferable to use low concentrations of anesthetic agents so as to facilitate neurophysiologic monitoring. However, this may lead to awareness. To avoid this, a depth-of-anesthesia monitor like a BIS monitor may be used where possible, with enough opioids so as to provide adequate analgesia. Agents like droperidol have been used in adults and pediatrics during awake craniotomy, but its sedative effects and postoperative drowsiness have been the main disadvantages. It can also induce seizure when given along with inhalational agents.  Agents like dexmedetomidine, a selective alpha 2 adrenoreceptor agonist, also have been used recently as an adjuvant for awake craniotomy in pediatric patients, but doses used by different authors are varied and no standard dosing schedule has been recommended yet.
Local anesthetics have both excitatory and depressive effects based on their plasma concentrations. At low concentrations, they are depressive and at higher concentrations, they are excitatory. They are used in patients where the surgery is planned awake or under conscious sedation and in anesthetized patients. It often requires a very large volume, and the maximum allowable dose of local anesthesia to block the scalp circumferentially must be carefully calculated. If used in higher concentration, it often can exceed the toxic dose; so, it has to be used in more diluted concentration, with addition of vasoconstrictors like adrenaline or phenylephrine to prevent rapid absorption. In spite of these precautions, there are reports of toxic adverse reactions with these agents. ,
This surgery is major and elective. It is therefore important to do a careful and comprehensive preoperative history and evaluation of the patient. Have a frank discussion with the parents regarding all the procedures to be undertaken, including blood transfusion, before obtaining informed consent. The anesthesiologist should always try to build a good rapport with the child. This is very important if the child requires the procedure to be done under conscious sedation.
Many of these children are developmentally delayed and uncooperative. They may have carious or loose teeth, gum hypertrophy, enlarged adenoids or tonsils, cardiac involvement; and all of them will be on one or more seizure medications. Chronic carbamazepine and phenytoin usage is associated with resistance to muscle relaxants and narcotics. ,, Premedication may be indicated, but the effects of the drugs, especially benzodiazepines, on EEG should be taken into account. Ideally, routine premedication should be avoided. If it is planned to administer any agents as premedication, it should be discussed with the neurologist and the dose adjusted accordingly.
Investigations requiring sedation/anesthesia
These children are often subjected to multiple investigations. With a little care and cooperation between departments, EEG and MRI may be done during the same session of sedation/anesthesia, or MRI and surgery may be done at the same sitting.
EEG and MRI are done at sites in the hospital other than the operation suite. Often sedation is given for EEGs by technicians using drugs like triclofos. If that 'does not work,' they 'add on' other sedatives. This may lead to dangerous respiratory and cardiovascular depression. We advocate that these areas be well equipped for resuscitation and the staff trained in pediatric airway management and resuscitation in case of over-sedation or seizures.
MRI may be done under sedation using a drug like triclofos or an inhalational agent like sevoflurane with a laryngeal mask airway (LMA). In this case, an anesthetic machine which is compatible with MRI is required. Another option is an intravenous infusion of propofol. However, the infusion pump will have to be kept outside the magnetic range. Ketamine should be avoided due to its epileptogenic properties. Due to the effect of the magnetic field on ferromagnetic objects, there are limitations on monitoring and the use of infusion pumps and ventilators in the MRI suite.
Minor procedures like MRI may be carried out under sedation with pulse oximetry alone. If general anesthesia is required, then end tidal carbon dioxide (EtCO 2 ) and noninvasive blood pressure (NIBP), along with pulse oximetry, should be included in monitoring.
A child undergoing major, complex, 'long-duration' procedures involving massive blood loss and fluid shifts requires meticulous monitoring of oximetry, invasive arterial pressure, central venous pressure, temperature, expired carbon dioxide and anesthetic gases, urine output (indwelling catheter), serial hematocrit, arterial blood gases, electrolytes and coagulation parameters.
The skull of a child is larger in proportion, compared to the proportion in an adult. The scalp and skull being very vascular, massive bleeding may be expected. Therefore, continuous intra-arterial blood pressure and central venous pressure monitoring becomes mandatory. These monitors help in optimizing volume replacement. It is better to avoid using the neck veins to gain access to central veins as there is a potential for obstruction of veins draining the brain. A depth-of-anesthesia monitor is helpful to measure the level of anesthesia, prevent awareness and prevent overdose of inhalational or narcotic agents.
It is very important to secure the endo-tracheal tube, all lines, adding extensions to the venous access lines where necessary and monitors meticulously as once the patient is positioned and draped it is very difficult to access the patient. Pressure points should be carefully padded.
Special neurological monitoring
Intraoperative electrocorticogram (ECoG) is the most common intraoperative neurophysiological monitoring done under anesthesia during the procedure. It is used mainly to delineate the extent of the epileptogenic foci or to confirm the success of the predetermined resection. The most important prerequisite for intraoperative ECoG is sufficient brain exposure. Usually 12-16 electrodes are used. They are placed in such a way that post-resection, ECoG is possible with the use of the same electrodes. In most cases, they also serve to detect the post-electric stimulation discharges caused during mapping. Intraoperative cortical mapping is preferred in those children where cooperation and orientation are doubtful intraoperatively, such as those with generalized epilepsies, lesion-related and cryptogenic epilepsies such as Lennox-Gastaut syndrome, infantile spasms, all disorders causing intractable and disabling seizures, early cognitive disturbances, impaired intelligence and behavioral deficits. This makes them unsuitable for awake craniotomy or surgery under conscious sedation.
Another essential requirement of cortical resections is identification of the sensory and motor strip. It is usually done by using sensory evoked potential (phase reversal) and/or direct cortical stimulation (looking for movement on stimulation) under anesthesia. When surgery is planned under local anesthesia, that is, as awake craniotomy, then, only direct cortical stimulation is done. However, due to the possibility of seizure induction during electrical stimulation, a dose of fast-acting barbiturates is always kept ready to treat the seizure.
Keep the ambient temperature up, as once the scalp is retracted temperature loss from the raw surfaces is rapid. An 'underbody' hot air warmer, intravenous (IV) fluid warmers and airway heat-moisture exchangers are useful in maintaining the child's body temperature. Hypothermia leads to increased wound infection  and coagulopathy. , It may also suppress EEG waves. Hyperthermia should also be avoided as it leads to neuronal death. Now, air blowers are available for the underbody blankets, which can be made to cool the baby by blowing cool air through the blankets in case of hyperthermia.
An ideal anesthetic regime would rapidly induce sleep without interfering with the EEG, provide analgesia when required, provide cerebral protection with a stable ICP, provide hemodynamic stability and end with a rapid awakening to a safe state when the procedure is finished. The ideal anesthetic does not quite exist, and the choice of anesthetic depends on the experience of the anesthetist with each agent, the procedure contemplated and the patient himself. The smaller the patient and the more complex the surgery, the more difficult it is to manage the blood loss, hemodynamics and temperature. Intracranial pressure should be maintained normal or low to help surgical access. Though a raised ICP is not usually a problem in these cases, increased ICP can result from positive end expiratory pressure, poor positioning obstructing venous drainage, lack of muscle relaxation and elevated CO 2 .  Isoflurane and sevoflurane with mild hyperventilation may be used.  Propofol will produce a consistent reduction in cerebral blood volume and ICP. Neuroprotection is done best by ensuring adequate cerebral perfusion. Mild hypothermia and anesthetic drugs have also been reported to be neuroprotective. 
Blood and fluid management
Some epilepsy surgeries, such as hemispherectomy, are associated with very large blood loss. This should be anticipated, adequate monitoring in place and large-bore IV lines available for transfusion. The target for fluid therapy is normovolemia. Normal saline is the commonly used crystalloid. Large volumes of normal saline will cause acidosis. Ringer lactate decreases osmotic pressure and can cause brain swelling. Colloids such as 'voluven' are an option. The child may tolerate low hematocrit; but if transfusion is done after filling him with colloids, then there is the risk of 'overfilling' the intravascular volume. If on the other hand, transfusion is left till too late on a relatively hypovolemic child, then the rapid transfusion could lead to hyperkalemia, acidosis and hypocalcemia, which may result in cardiac arrest. Transfusion should therefore be started early and volume maintained.  There is also a need to check coagulation profile and get specific blood products like FFP and cryoprecipitate, if required, as in cases of massive transfusion.
| Awake Craniotomy|| |
Intracranial procedures requiring a conscious patient are challenging to the anesthesiologist, especially when the patient is a child. The challenges associated with this technique are - to provide adequate sedation, analgesia, with respiratory and hemodynamic control while keeping the patient conscious and cooperative for neurological testing. Resection of lesions near eloquent areas of brain, such as speech and language, mandates the use of this technique for excision of the lesion. Different types of anesthetic techniques for awake craniotomy have been described in the literature. ,,,, However, most of these are for adult patients. There have been few case reports recently where these techniques have been modified for use in children down to 11 years of age. Compared to the adult, where the patient is fully awake and conscious throughout the surgery, in the pediatric patient a sleep-awake-sleep technique is usually preferred. All the reports of awake craniotomy in children have emphasized one single point - that of proper patient selection for the procedure. ,,
Complications during these procedures are often related to airway obstruction, agitation, drowsiness and respiratory depression. These problems are very difficult to treat once the patient is positioned for a craniotomy, the skull opened or the procedure started. Even though different reports suggest different agents during the management, not a single anesthetic agent has proven ideal by randomized trial; and every drug has its advantages and disadvantages. There are different agents that have been tried as anesthetics in these cases, such as sevoflurane, propofol, remifentanil, dexmedetomidine, droperidol and fentanyl - alone or in combination. The most commonly used for sleep-awake-sleep technique, as reported in the literature, are propofol with fentanyl. The advantage of propofol is that if used properly, the patient can breathe spontaneously and no active airway management is necessary. However, in higher doses, respiratory depression and hypotension are seen. Sevoflurane, on the other hand, has to be administered through an LMA or ETT, which requires manipulation of the airway during the procedure. Dexmedetomidine has recently been introduced. It is an effective sedative, but it has a propensity for bradycardia and hypotension even though it does not have depressive effects on the respiratory system. There are no large clinical trials to date comparing different anesthetic agents in children during awake craniotomy; so it is left to the neuroanesthesiologists to decide on the technique and agents with which they are most familiar and which will suit the specific patient. We describe here, broadly, the steps of an awake (conscious) craniotomy with advantages and disadvantages of the particular agent or technique.
Awake craniotomy with intraoperative cortical stimulation is most commonly done in patients where the lesions affect the eloquent areas of the brain. This technique allows the excision of the lesion without significant neurological deficits such as aphasia. The prerequisite on the part of the patient is his/her psychological stability. The patient should be old enough to understand that it is very important to cooperate during the procedure. If there is any doubt regarding his cooperation, he should not be considered as a candidate for awake craniotomy. The preoperative visits and consultations with the patient and his parents are very important. Various means of description and explanations of the procedure, like videos, slides or photographs of previous surgeries, may have a role in helping him imagine, understand and accept his role intraoperatively. The task which is expected to be evaluated during the procedure has to be rehearsed preoperatively. It is very important to obtain the confidence of the patient irrespective of the number of visits required preoperatively.
On the day of the surgery, the patient is usually not given any premedications except for steroids and H 2 blockers and is kept nil per oral. The doses of anticonvulsants are decided after discussion with the attending neurologist. If the child is apprehensive, a small bolus of short-acting benzodiazepine like midazolam is administered after securing an intravenous line. All the personnel in the operating room should be aware that the patient is conscious and so unnecessary noise or verbal exchanges should be kept to a minimum. The temperature of the operating room should be made comfortable for the patient. All emergency drugs and equipment should be available and checked. An appropriate-size laryngeal mask airway (LMA) is kept ready. The bladder is not usually catheterized as it is uncomfortable and can aggravate the patient's anxiety. After positioning the patient on a well-padded table, a nasal cannula is placed to administer oxygen and to monitor end-tidal carbon dioxide during the procedure. A depth-of-anesthesia monitor can be placed to monitor the depth of anesthesia during the procedure. We now describe the most commonly used technique described in the literature with drugs that are available in India. Fentanyl 1 µg.kg -1 along with propofol in increments from 0.5 to 2 mg.kg -1 is administered slowly until the patient goes to sleep.  An infusion of propofol 150-200 µg.kg -1 .min -1 is then started, and an arterial cannula is inserted under local anesthesia to measure continuous arterial pressure. The maximum amount of local anesthetic is decided on the basis of the body weight and divided into three volumes for blockade of scalp nerves, the pin sites and for infiltration of the surgical incision site along with dural infiltration. The concentration of bupivicaine used varies from 0.1 to 0.25% depending on the requirement and body weight of the patient. Sometimes, if the requirement is more than the allowable dose of bupivicaine, an additional amount of lignocaine in varying concentration (1-2%) is added to bupivicaine to make up the adequate volume required for the procedure. After application of the three-pin Mayfield head holder and positioning of the patient, the surgeons are allowed to drape the patient in such a way that the anesthesiologists have a clear view of the patient's face, have access to the patient and monitoring and have enough space for any airway management if required. The planned incision site is then infiltrated with the local anesthetic mixture. During infiltration if the patient responds, up to 0.5 µg.kg -1 of fentanyl is given incrementally. Mannitol (0.5 g.kg -1 ) is administered slowly during the flap removal. After raising the skin and bone flap, the outer layer of dura is usually infiltrated with local anesthetic mixture and an incision is made to expose the cortex. Once the dura is incised, the anesthetic agent is usually stopped and over a period of 10 to 15 minutes, the patient usually wakes up. This correlates well with the depth-of-anesthesia monitoring. Direct electrical stimulation is done on the cortex to localize motor and speech areas. The patient is usually kept awake and continuously evaluated by the neurophysiologist during the time of resection, ensuring that no disturbances to patient language and other higher functions occur. There is a chance of seizure occurring at this time. It usually gets resolved on stoppage of stimulation. However, if clinical seizure occurs, then it has to be treated with boluses of propofol or thiopentone. The patient can lose his ability to maintain his airway in these conditions; so an LMA may be used to intervene if required. An intubating laryngeal mask (ILMA) or proseal LMA is easier to place than an LMA classic. Once the mapping is complete, the anesthetic agent (propofol) infusion is restarted as per requirement. Ondansetron 0.15 mg.kg -1 is administered prophylactically. Propofol or anesthetic agents are stopped once the scalp wound is closed. The patient is allowed to regain consciousness slowly. The patient is shifted to the recovery room once he becomes oriented and obeys verbal commands.
| Conclusion|| |
It is now increasingly being recognized that the treatment of epilepsy in the child needs to be prioritized in order to not lose the crucial phase of brain development when the plasticity of the brain is maximal. Allowing a child to grow to adulthood with normal cognition and capability to earn a livelihood contributes immensely to the family and society. Thus younger children are now presenting for major surgery on the brain in the hope that early treatment will ameliorate their epilepsy without increasing morbidity and enhance brain development. The anesthetic management of these cases is widely different and challenging in its own right. The anesthetist is thus called upon to play a major role in the management of the very young child who presents for epilepsy surgery or the older child who undergoes an awake craniotomy. With marked improvement in the understanding of neurophysiology, newer techniques of surgery and anesthesia, more children with epilepsy can now be offered the benefit of a curative surgery.
| Acknowledgement|| |
Dr. K. Srinivasa Babu, Neurophysiologist, Christian Medical College, Vellore, for his invaluable help in understanding neurophysiology.
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