14 Anesthetic Considerations and Postoperative ICU Care
The surgical management of intractable epilepsy has evolved owing to advances in intraoperative neuroimaging and electroencephalography (EEG). Recent advances in pediatric neurosurgery have exploited these technologies and dramatically improved the outcome in infants and children. The chapters in this section highlight the age-dependent aspects of the perioperative management of the pediatric neurosurgical patient.
Physiological Differences in Pediatrics
Age-dependent differences in cerebrovascular physiology have a significant impact on the perioperative management of neurosurgical patients. Cerebral blood flow (CBF) is coupled tightly to metabolic demand, and both increase proportionally immediately after birth. Wintermark and colleagues determined the effect of age on CBF.1 Using computed tomography (CT) perfusion techniques, they reported that CBF peaked between 2 and 4 years of age and settled at 7 to 8 years of age. These changes mirror changes in neu-roanatomical development. The autoregulatory range of blood pressure in a normal newborn lies between 20 and 60 mm Hg,2 reflecting the relatively low cerebral metabolic requirements and blood pressure of the perinatal period. Although children younger than 2 years have lower baseline mean arterial pressures, they have lower autoregulatory reserve and can theoretically be at greater risk of cerebral ischemia.3 These factors place the infant at risk for significant hemodynamic instability during neurosurgical procedures compared with adults.
Preoperative Evaluation and Preparation
A thorough preoperative, organ system—based evaluation of the pediatric patient is essential to minimize perioperative morbidity because infants are at higher risk for perioperative morbidity and mortality than any other age group.4 Respiratory- and cardiac-related events account for a majority of these complications and necessitate a thorough history and physical examination. First, a complete airway examination is essential, because some craniofacial anomalies may require specialized techniques to secure the airway.5 Next, a thorough cardiovascular evaluation should be completed with an eye toward congenital heart disease that may not be apparent immediately after birth. A pediatric cardiologist should be consulted to evaluate all patients with suspected problems to identify any lesions and assess cardiac function before surgery. Optimal cardiac function is crucial intraoperatively because massive blood loss, swings in blood pressure, electrolyte shifts, and aggressive fluid administration may lead to depression of myocardial contractility and acute myocardial failure. Finally, a variety of medical conditions often accompany pediatric patients with epilepsy, and these need to be addressed when formulating the anesthetic plan.
Tuberous sclerosis (TS) is a hamartomatous disease that usually presents with cutaneous and intracranial lesions, the latter leading to medically intractable epilepsy.6 Hamarto-matous lesions frequently infiltrate and disturb the cardiac, renal, and pulmonary systems as well. Cardiac rhabdomyomas can be detected in a majority of these patients and can lead to dysrhythmias, obstruction of intracardiac blood flow, and abnormal conduction through the bundle of His. Therefore, all patients with TS should have a preoperative echocardiogram and electrocardiograph to detect any functional defects. Renal lesions often result in hypertension and azotemia, both of which may complicate the conduct of anesthesia.
Sturge-Weber syndrome, or encephalotrigeminal angio-matosis, is another of the phakomatoses and is characterized by port-wine facial stains and ipsilateral leptomeningeal angiomas. Intracranial angiomata with calcification (“railroad sign”) produce cerebral atrophy, mental retardation, and seizures that are often refractory to medical management. Extracranial angiomata, including lesions involving the airway, have been reported, and congenital glaucoma is present in a third of patients. Thus, airway management, intraocular pressure, and intraoperative hemorrhage are important considerations.
Preoperative laboratory tests should be tailored to the proposed neurosurgical procedure. Hypercoagulation develops early after resection of brain tissue in pediatric neurosurgical patients, as assessed by thromboelastography.7 Given the risk of significant blood loss associated with craniotomies, a hematocrit, prothrombin time, and partial thromboplastin time should be obtained to uncover any insidious hemato-logical or coagulation disorders. Type- and cross-matched blood should be available before all craniotomies.
All patients presenting for epilepsy surgery have undergone drug treatment of their seizures. Each drug class has side effects that can affect the conduct of anesthesia. The traditional anticonvulsant drugs—phenobarbital, phenytoin, and carbamazepine—are potent inducers of hepatic microsomal p450 enzymes. Cerebyx (fosphenytoin) and Carbatrol (oxcarbazepine) are recent reformulations of the latter two drugs. The hepatic p450 enzymes mediate bio-transformation and enhanced elimination of many drugs. Long-term administration of these specific anticonvulsant drugs results in drug resistance and increases requirements for both nondepolarizing muscle relaxants and opioids administered during general anesthesia.8 Patients on chronic anticonvulsant drug therapy with phenobarbital, phenytoin, and carbamazepine need to be closely monitored for drug effect, and dosage should be increased accordingly. In general, the newer classes of anticonvulsant drugs do not appear to alter the metabolism of anesthetic drugs. However, other side effects have been reported with chronic administration of these new drugs. Topiramate has been shown to cause an asymptomatic anion gap metabolic acidosis because of inhibition of carbonic anhydrase.9 This can exaggerate the metabolic acidosis that often occurs as a result of hypoperfusion because of massive blood loss. Sodium valproate is associated with platelet abnormalities and can cause bleeding disorders. Sodium valproate and felbamate can induce liver failure, and patients receiving these drugs should have the appropriate laboratory tests to determine the baseline line platelet and liver function before surgery.
The ketogenic diet is a high fat, low carbohydrate, low protein regime that promotes a chronic metabolic state of ketosis and acidosis. For reasons that remain unclear, this diet has proven to be a very useful adjunct in the treatment of many children with intractable epilepsy.10 Although adequate calories are provided with fat, carbohydrate intake is limited to 5 to 15 g/day and hypoalbuminemia is common. Because the target metabolic state can be disrupted by administration of carbohydrate-containing intravenous (IV) solutions or by ingestion of the sweetened syrups contained in some premedications, these should be avoided by the anesthesiologist. Although both normal saline and lactated Ringer’s are acceptable fluid choices, there is limited margin for additional acidosis, and the acid—base status must be monitored closely during surgery. Bicarbonate, plasma glucose, and serum ketone levels should be measured pre-operatively, then sampled regularly to avoid hypoglycemia or excessive ketosis (serum or urine ketones >160 mg/dL).
Anesthetic Management
Premedication
Separation from parents and perioperative anxiety play a significant role in the care of the pediatric patient and are related to the cognitive development and age of the child. Preoperative sedatives given before the induction of anesthesia can ease the transition from the preoperative holding area to the operating room.11 Midazolam administered orally is particularly effective in relieving anxiety and producing amnesia. If an indwelling IV catheter is in place, midazolam can be slowly titrated to achieve sedation. If intraoperative electrocorticography (ECoG) is planned, the dosage of midazolam and other benzodiazepines should be reduced to minimize the depressant effects on the electroencephalogram (EEG).
Induction of Anesthesia
The patient’s neurological status and co-existing medical conditions will dictate the appropriate technique and drugs for induction of anesthesia. In infants and young children, general anesthesia can be induced with inhalation of sevo-flurane and nitrous oxide in oxygen. Sevoflurane has been shown to have epileptogenic potential.12 However, the mechanism of this phenomenon is unclear. Alternatively, if the patient already has an IV catheter, anesthesia can be induced with sedative/hypnotic drugs such thiopental (5–8 mg/kg) or propofol (3–4 mg/kg). These drugs rapidly induce unconsciousness and can blunt the hemodynamic effects of tracheal intubation. A nondepolarizing muscle relaxant is then administered after induction of general anesthesia to facilitate intubation of the trachea. Patients with nausea or gastroesophageal reflux disorder are at risk for aspiration pneumonitis and should have a rapid-sequence induction of anesthesia performed with thiopental or propofol immediately followed by a rapid-acting muscle relaxant and cricoid pressure. Rocuronium can be used when succinylcholine is contraindicated, such as for patients with spinal cord injuries or paretic extremities. In these instances, succinylcholine can result in sudden, catastrophic hyperkalemia.
Airway Management
Given the high incidence of respiratory morbidity and mortality in pediatric patients, a thorough examination of the airway and the use of appropriate equipment and techniques are mandatory. Because the trachea is relatively short, an endotracheal tube can easily migrate into a mainstem bronchus if an infant’s head is flexed or turned. Therefore, great care should be devoted to assuring proper position of the endotracheal tube during tracheal intubation. Patients undergoing awake craniotomies are always at risk for airway compromise caused by sedation, seizure, or obstruction from positioning. Therefore, the patient’s face should be accessible to the anesthesiologist for manipulation of the airway and ventilation of the lungs.
Positioning
Patient positioning for surgery requires careful preoperative planning to allow adequate access to the patient for both the neurosurgeon and anesthesiologist. This issue is especially important in patients undergoing awake craniotomies. In this case, the patient has to be in a comfortable position throughout the surgical procedure. A clear channel should be created in front of the patient’s face to facilitate communication and facial observation during the neuropsychological assessment. If cortical stimulation or induction of the seizure is planned, the patient’s limbs must be easily visualized.
Frequently, neurosurgical procedures are performed with the head slightly elevated to facilitate venous and cerebro-spinal fluid (CSF) drainage from the surgical site. However, superior sagittal sinus pressures decrease with increasing head elevation, and this situation increases the likelihood of venous air embolus.13 Extreme rotation of the head can impede venous return through the jugular veins and lead to impaired cerebral perfusion and increased intracranial pressure (ICP) and venous bleeding.
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