Preoperative Evaluation and Preparation

The evaluation of a pediatric neurosurgical patient should include history and physical examination pertaining to the conditions that require special anesthetic considerations. The neurologic status can be assessed from the evidence of raised ICP, altered sensorium, and cranial nerve palsies. Infants with ICH might present with irritability, lethargy, decreased consciousness, failure to feed, bulging fontanale, and cranial enlargement. In children, it may present with early morning headache, vomiting without nausea, diplopia, papilledema, and in very late stage, the Cushing’s triad composed of hypertension, bradycardia, and respiratory changes. The conscious level can be ascertained from age-specific Glasgow Coma Scale (GCS) score. Frequent episodes of vomiting may lead to dehydration and electrolyte imbalances, and increases the risk of aspiration. Hence, serum electrolyte should be determined to identify abnormalities of sodium and potassium following vomiting. Hemoglobin or hematocrit level, typing and cross-matching of blood if the loss is expected to be considerable, should be done and blood should be kept ready before shifting the child to the operating room. Additional studies include electrocardiography (ECG), coagulation profile, as well as renal and hepatic function, as and when required. Children with pituitary tumors should undergo complete endocrine evaluation.

Premedication

In children with suspected increase in ICP, sedative premedication must be avoided as the medications decrease respiratory drive resulting in hypercapnia and further increase in ICP. However, in patients with normal ICP such as those scheduled for repair of vascular lesions like arteriovenous malformations (AVMs) may be sedated so as to allay preoperative anxiety and avoid hypertension thus, preventing rupture of the abnormality. Oral benzodiazepines such as midazolam may be beneficial for small children as they provide sedation without respiratory depression but should be administered under supervision. Some reports suggest that midazolam is not associated with respiratory depression even in children with reduced intracranial compliance. Rather, it (midazolam) reduces anxiety and hence helps separating the child from the parents. It also reduces crying and associated cardiovascular changes, which may further lead to raised ICP. Midazolam also reduces the requirement of analgesics.

Induction of Anesthesia

The goal of anesthetic induction is to avoid increase in ICP owing to associated hypoxia, hypercapnia, and volatile anesthetic–induced increases in CBF. An IV induction with thiopentone or propofol and neuromuscular blockade to facilitate endotracheal intubation is ideal in children with raised ICP. All IV induction agents, except ketamine, would cause a reduction in ICP. However, in children without IV access or with difficult IV access, inhalational induction by facemask with sevoflurane should be preferred as crying or struggling may lead to further increase in ICP. After the IV access is secured, a bolus of thiopentone (1–2 mg/kg) or propofol may be given to prevent the pressure responses of tracheal intubation. Furthermore, the inhalational technique may subsequently be changed to an IV induction. All volatile anesthetics cause an increase in CBF, and hence the ICP. Therefore, ventilation should be controlled as early as possible and mild hyperventilation is to be instituted to prevent rise in ICP. Children at risk for aspiration should undergo rapid-sequence anesthetic induction with thiopentone or propofol followed by rapid-acting muscle relaxant such as succinylcholine or rocuronium.

Maintenance of Anesthesia

Low exhaled concentrations of inhalational agents with mild hyperventilation does not attribute to increased ICP. Hence, anesthesia is maintained either with low end-tidal volatile agents (minimum alveolar concentration, MAC <1) or with total IV anesthesia (TIVA) along with short-acting opioids (fentanyl or remifentanil), with or without nitrous oxide, and controlled ventilation. Halothane is a potent cerebral vasodilator and causes maximum increase in ICP among all the volatile anesthetic agents currently in use. Sevoflurane an agent for induction has almost replaced halothane as far as pediatric neuroanesthesia is concerned. Isoflurane, sevoflurane, or desflurane are all used for maintenance of anesthesia during the neurosurgical procedures. Sevoflurane provides smooth induction followed by a rapid recovery. Recovery is also rapid following desflurane anesthesia. However, nitrous oxide should avoided as it causes increase in ICP and cerebral metabolism. Nitrous oxide also increases the size of the gas-filled space after craniotomy with a propensity to occurrence of significant pneumocephalus and consequent increase in ICP following surgery. Neuromuscular blockade with nondepolarizing muscle relaxants is achieved to prevent patient movement and to minimize the amount of anesthetics required. Children on chronic antiepileptic medications require large doses of muscle relaxants and narcotics due to enzymatic induction in liver. The muscle relaxant should be withheld when assessment of motor function is carried out, e.g., during spinal cord surgery. Fentanyl is the most commonly used opioid, but its half-life increases with repeated dosing. It requires hepatic metabolism, which is immature in premature infants. Hence, the sedative and respiratory depressive effects of fentanyl may be prolonged in these children. Remifentanil is a newer narcotic agent, cleared rapidly by the plasma esterases. The rapid recovery associated with its use may be accompanied by delirium and inadequate analgesia. Hence, it requires supplementation of other analgesics such as morphine during postoperative pain management. Opioids have not been shown to increase ICP in patients undergoing controlled ventilation.

Positioning the Child

The neurosurgical procedures are carried out in different positions each position having a certain challenge. In children, similar positions are made to access the intracranial lesions. The surgeries are usually of prolonged duration, and the small child undergoing the procedure disappears under the surgical drape or a Mayo stand used by the scrub nurse. The anesthesiologist must try to ensure unobstructed view of at least a part of the child. Proper padding of pressure points, protection of the eyes from surgical cleaning solutions, and the access to IV line, airway, and breathing circuits before commencement of surgery are other prerequisites.

Supratentorial surgeries and CSF diversion procedures are carried out with the patient lying supine with the head may be turned to one side. This position allows distribution of the child’s weight over a larger area. The head is placed on a horseshoe or fixed to a three-pin head-holder or Mayfield clamp. Precaution should be taken to prevent overrotation of the neck as it may impede the venous return. The endotracheal tube (ETT) is secured in the nondependent corner of the mouth to prevent oral secretions loosening the adhesive tapes used for fastening.

The prone position is used during suboccipital craniotomy for posterior fossa lesions, spine surgery, and repair of myelomeningoceles (MMCs) and encephaloceles. The weight of the child is supported on bolsters under the chest and pelvis, and the abdomen hangs free between bolsters. Excessive pressure on the abdomen due to an abnormal position impedes ventilation, compresses vena cava, and increases epidural venous pressure and bleeding. Head is placed over a horseshoe rest or field with Mayfield clamps. There may be congestion of face and tongue and pressure sore on malar prominences owing to horseshoe head rest. Extreme flexion of the neck may cause endobronchial intubation because of short trachea and intraoral kinking of ETT; hence, flexometallic tracheal tubes are preferred. Excessive flexion or extension of head may cause brainstem compression in patients with associated Chiari malformation. Excessive flexion of neck may also lead to cervical cord ischemia and quadriplegia.

The lateral position in which cerebellar hemispheric lesions or cerebellopontine angle tumors are surgically approached in adults is rarely required in children. An axillary roll is provided to protect the inferior shoulder and axillary neurovascular structures. The dependent arm may be placed in a low-placed arm board or positioned in hanging.

The sitting position, for neurosurgery in children, is still being practiced in limited centers across the world ( Fig. 36.1 ). In children older than 4 years, it is used for exploration of the posterior fossa with the advantage of reducing intraoperative bleeding and facilitating surgical exposure. The cerebellum falls down under gravity making this position ideal for infratentorial approach surgeries for third and fourth ventricular lesions and cerebellopontine angle tumors. Complications in relation to this position include hemodynamic instability, venous air embolism (VAE), and postoperative tension pneumocephalus. Extreme flexion of neck required in this position may be complicated by midcervical flexion myelopathy and macroglossia noticed during recovery from anesthesia.

Sitting position in a 6-year-old child with posterior fossa tumor.

Mannitol may be administered in doses of 0.25–1 g/kg to attenuate increases in ICP during the intraoperative period.

Monitoring

Intracranial surgery may be associated with hemodynamic changes owing to sudden blood loss, VAE, and manipulation of cranial nerves. Routine monitors for pediatric patients include precordial or esophageal stethoscope, pulse oximeter, ECG, noninvasive blood pressure, and capnography (EtCO ₂ ). Intracranial surgeries are expected to result in significant blood loss, which at times occurs at a rapid pace. Hence, arterial cannulation is done, which permits continuous monitoring of blood pressure and estimation of arterial blood gas, hematocrit, and electrolytes. A central venous catheter provides large-bore IV access and also helps aspiration of air when the sitting position is complicated by VAE. It also helps assessing the volume status of the patients; however, its efficacy is questionable in patients positioned prone. Precordial Doppler and transesophageal echocardiography are sensitive monitors used to detect of VAE, but they are not available everywhere. Hence, end-tidal CO ₂ is relied upon in most centers.

Neurophysiological monitoring is used for early detection of neurologic insult when it is reversible, thereby reducing postoperative morbidity. The surgeries where it can be utilized in pediatric patients include supratentorial lesions involving motor cortex, lesions involving the brainstem and spinal cord (tumors), and spina bifida with tethered cord syndrome. The monitoring modalities include electroencephalography, motor evoked potentials (MEP), sensory evoked potentials (SSEP), and brainstem auditory evoked potentials (BAEP). MEP and SSEP are used to monitor the functional integrity of the motor pathways in the brainstem and sensory pathways leading to sensory cortex, respectively, whereas BAEP is used for the surgery involving brainstem, posterior fossa, and cerebellopontine angle tumors. Inhalational anesthetics and N ₂ O depress the SSEPs during spinal surgery, and hence are avoided. MEPs are most sensitive to the volatile anesthetic agents. TIVA with fentanyl and an infusion of propofol is recommended.

Fluid Management

The goal is to maintain normovolemia and thus hemodynamic stability. Normal saline (NS) is the most commonly used fluid in pediatric neurosurgical procedures. It is mildly hyperosmolar and hence, prevents cerebral edema. However, infusion of large quantities of NS may cause hyperchloremic metabolic acidosis and hypernatremia. Ringer lactate (RL) is slightly hypo-osmolar and hence, may increase cerebral edema. RL also causes increase in intracellular glucose if given in large quantities. Glucose-containing fluids should not be used during neurosurgical procedures as the consequent hyperglcemia may worsen the reperfusion injury. However, in neonates and premature infants, the danger of hypoglycemia should be borne in mind. Blood glucose should be closely monitored in these children along with continuous infusion of glucose at 5–6 mg/kg/min. Children do not need exogenous glucose administration and are able to maintain normal levels along with the associated surgical stress. Blood transfusion should be guided by the degree of blood loss. However, constant oozing into the surgical draping due to craniotomy makes blood loss difficult to be assessed. Initial hematocrit values should be compared before the transfusion.

Temperature Management

Although mild hypothermia has been found to be useful in experimental animal models, it has not been extrapolated to humans undergoing neurosurgical procedures. The complications due to hypothermia such as coagulation abnormalities prevent its routine use in pediatric neurosurgical practice despite encouraging results observed after brain injury. The intraoperative goal is to maintain normothermia thereby ensuring adequate recovery in the postoperative period. It can be achieved intraoperatively, by the use of warm saline and application of heated mattresses and warming devices. Both hypothermia and hyperthermia should be avoided with application of different methods.