Neurosurgery in children

Neurosurgery in children

Intracranial Physiology and Pathophysiology

There are a number of anatomic differences between children and adults that affect central nervous system physiology, especially intracranial pressure (ICP).

 At birth, the dura mater is covered by the calvaria, which consist of ossified plates connected by fibrous sutures and open fontanelles. The fontanelles close by approximately 10 to18 months of age but do not fully ossify until the teenage years. Thus, the infant skull is more compliant and may slowly expand in response to increasing ICP. These same structures offer a great deal of resistance to acute elevations in ICP. Infants and young children may not exhibit clinical signs of intracranial hypertension until the process is significantly advanced and the cranium can no longer accommodate a further increase in ICP.

 By the time an infant demonstrates the classic clinical signs of elevated ICP such as bradycardia, hypertension, papilledema, and pupillary changes, the disease process is likely very advanced. In contrast to adults, infants and young children may present with vague signs and symptoms such as increased head circumference, expanding sutures, bulging fontanelles, “sundowning” of eyes, lethargy, poor feeding, irritability, and possibly lower motor deficit.

 After ossification of the fontanelles, for a time, children may be more vulnerable to brain injury from increased ICP because of a relatively higher brain tissue to blood and cerebrospinal fluid intracranial volume than in the adult.

The limits of autoregulation are also different in infants and children. In adults, normal ICP ranges between 8 and 15 mmHg, whereas in infants, it may be as low as 2 to 4 mmHg. The cerebral autoregulation limit is shifted to a significantly lower value of mean arterial blood pressure (20 to 60 mmHg). The “margin of safety” may be narrower because infants are less able to compensate for the changes in blood pressure. Global cerebral blood flow (CBF, measured as ml/min/100 g of brain tissue) in children is greater than in adults, but in infants and premature babies, it is lower. The lower limit of CBF needed to sustain neuronal integrity is unknown in these patients. In infants with pathologic conditions resulting in a shift of the intracranial compliance curve to the right, cerebrospinal fluid production alone may be a significant contributor of increased ICP. Infants are at risk for ischemia when mean arterial pressure is low, whereas systemic hypertension may result in intraventricular hemorrhage; therefore, large fluctuations in systemic blood pressure may be deleterious. The response to hyperventilation may, also, be exaggerated and ischemia may ensue with very low PCO2 levels (less than 20 mmHg).

Preoperative Considerations

Managing the pediatric neurosurgical patient requires a thorough understanding of the neuropathologic lesion and its potential impact on ICP and respiratory and cardiovascular function.

 Equally important is the assessment of co-existing diseases, medications, intravascular volume status, and anesthetic history.

Most neurosurgical patients present to the anesthesiologist after extensive diagnostic imaging. Nevertheless, a thorough history and physical examination are useful in evaluating for signs of intracranial hypertension. The severity of the symptoms of intracranial hypertension depends on the duration of the elevated ICP. Acute ICP elevation may result in changed sensorium or coma, whereas a less acute rise in ICP may result in headaches, irritability, poor feeding, or increased head circumference in infants. Increases in ICP may be caused by a number of different central nervous system pathologic conditions, including hydrocephalus, brain tumor, or trauma.

Laboratory studies are dictated by the underlying pathology. Patients who are to undergo surgery for brain tumors require at least acquiring a hemoglobin level and type and screen. Serum electrolytes should be checked if there is a possibility of disturbance of sodium homeostasis, because of hormonal alterations or vomiting and intravascular volume contraction.

The pediatric airway may present challenges to the anesthesiologist. This challenge is even greater in patients with other craniofacial deformities presenting for surgical correction. Children may have congenital heart disease. Renal function should be assessed particularly if radiographic dyes are to be used because these can be nephrotoxic.

Patients who are on anticonvulsant therapy may have altered drug metabolism.

Measurement of anticonvulsant levels is only necessary if there have been recent dosing changes or seizures have worsened. Preoperative arterial blood gas analysis may be indicated in patients with altered mental status and those with underlying pulmonary pathology.

Premedication:

The majority of children who present for surgery may benefit from sedation for anxiolysis, because anxiety may cause further increases in ICP. However, caution is advised to prevent hypoventilation and associated increases in PaCO2 and ICP.

Sedation can be achieved with intravenous midazolam if an intravenous line is in place.

Midazolam may also be given orally and may be preferable because it is unlikely to produce

respiratory depression in doses of up to 0.7 mg/kg to a maximum of 20 mg. Opioids and other medications that depress respirations should be used cautiously, if at all, in patients with elevated ICP. Ketamine, which increases both CBF and cerebral metabolic rate of O2, increases ICP, and may lower seizure threshold, is usually avoided. Children exhibiting signs of acute ICP elevation will likely be obtunded and do not require any sedation.

Intraoperative Considerations

Induction of Anesthesia

The choice of anesthetic technique for the neurosurgical patient may present a particular challenge to the pediatric anesthesiologist.

 Induction of anesthesia is guided by the patient’s medical condition and age-dependent normal physiology in a patient population ranging from the premature infant to adolescents and young adults.

Typically, young children without acute ICP issues who have no indwelling intravenous line in place will undergo an inhalation induction through a face mask. Because all volatile anesthetics cause an increase in CBF, ventilation should be controlled as soon as possible to achieve mild hyperventilation and decrease PaCO2 to offset any anesthetic-induced increase in CBF. This age group is also more prone to airway irritability. Laryngospasm and bronchospasm during induction can increase PaCO2 and result in elevated CBF and ICP.

 In situations in which an intravenous catheter is in place, induction of anesthesia can be accomplished with agents such as thiopental or propofol, which reduce ICP. If the patient is at risk for aspiration, rapid sequence or modified rapid sequence induction can be performed.

For major procedures such as for a brain tumor resection, craniofacial reconstruction, and spine surgery, many clinicians will insist on at least two peripheral intravenous lines and an arterial line. For patients with a high risk for venous air embolism (VAE) such as sitting craniotomies, a right atrial catheter may be indicated. VAE, when detected, must be treated promptly by “flooding” the surgical field, aspiration of the

right atrium (if an atrial catheter is in place), repositioning the patient, and volume and vasopressor resuscitation, as indicated.

Positioning

Correct positioning of the patient for neurosurgery is of utmost importance, and attention to detail will ensure patient safety and comfort. The position of the patient varies with the surgical procedure, but the basic principles are the same as in adults.

The eyes must be protected from drying and injury in all positions. In most neurosurgery cases, the patient will be supine with the head either midline or turned to the side. Neck flexion may result in migration of the endotracheal tube to the mainstem bronchus or may occlude the jugular vein impeding venous drainage and increasing intracranial volume and pressure. Extra care should be exercised with securing the endotracheal tube in this position. Use of chest rolls or an appropriate table is necessary to allow free abdominal movement and prevent respiratory compromise.

 The prone position increases the risk of eye injury from direct ocular pressure and hypoperfusion.

For lateral positions, appropriate padding and stabilization is critical to prevent stretch, ischemia, and pressure injury to the axilla, extremities, and dependent eyes or ears (unless the patient is in head pins).

The sitting position, still used for some posterior fossa procedures, requires careful padding of all pressure points and securing the patient to the bed to ensure safety and surgical field stability. VAE is a risk in this position or any position where the head is significantly elevated. Therefore, in addition to the monitors noted previously, precordial Doppler monitoring is indicated to detect and treat VAE.

Maintenance of Anesthesia

The maintenance of anesthesia, generally, is accomplished with a balanced technique of opioids, volatile anesthetics, and muscle relaxants. As noted, all volatile anesthetics can cause cerebral vasodilation and increase ICP. Isoflurane and sevoflurane appear to have minimal effects on CBF and cerebrovascular reactivity to CO2 in concentrations of 0.5 to 1.5 minimum alveolar concentration.

 A number of neurophysiological parameters may be used to detect cerebral or spinal cord ischemia. These

include somatosensory evoked potentials, motor evoked potentials, and, less commonly, electrocorticography or electroencephalography. Other monitors that may be used in infants include transcranial cerebral oximetry and transcranial Doppler.

Inhalation anesthetics may alter evoked potentials and are generally used in low doses of less than 0.5 minimum alveolar concentration, or avoided altogether. When intraoperative monitoring of motor evoked potentials is used, the use of muscle relaxants is also contraindicated. In these instances, a total intravenous anesthetic may be used. Infusion of short-acting opioids such as fentanyl, sufentanil, or remifentanil can provide adequate intraoperative analgesia with predictable and rapid emergence permitting timely postoperative neurologic assessment.

Temperature control is an important consideration in the management of pediatric neurosurgical patients. Mild to moderate hypothermia may be neuroprotective and may be therapeutic in the presence of ischemia or hypoxia. In infants, reduced body temperature may result in several undesirable scenarios. Premature and term infants who become hypothermic will have markedly increased oxygen consumption. In infants, hypothermia can result in decreased drug metabolism, increased lactate production and metabolic acidosis, peripheral vasoconstriction, and shift of the O2–hemoglobin dissociation curve to the left. Severe hypothermia may result in cardiac arrhythmias.

Other complications of hypothermia include prolonged emergence from anesthesia, coagulopathy, immunodeficiency, and derangement in serum glucose metabolism.

Fluid Management

Fluid management is a very important part of neurosurgical patient care. The goal of fluid management is maintenance of cerebral perfusion, which usually translates into maintenance of isovolemia, iso-osmolarity, and iso-oncotic blood volume.

 Normal saline (0.9% NaCl) is the most common crystalloid used in neurologic patients. It is slightly

hyperosmolar (308 mosm) and is thought to attenuate cerebral edema. Hyperglycemia is associated with worse brain injury after ischemia; therefore, dextrose administration is not used routinely. Infants, particularly those who are preterm, are at higher risk for hypoglycemia. This group should have blood glucose measurements taken during long procedures and dextrose administered if indicated. In patients with existing intracranial hypertension, drugs may be used to reduce ICP. Furosemide, a loop diuretic, is often used to induce diuresis and decrease cerebrospinal fluid production. Hyperosmolar therapy with mannitol or hypertonic saline (3% or higher usually in intracranial trauma) is often used. These agents should be given after good communication between the surgeon and anesthesiologist. Blood and blood component therapy use is guided by the degree of blood loss, starting hematocrit level, and blood coagulation studies

Postoperative Considerations

Postoperative care is dictated by the complexity of the surgical procedure and the co-existing disease. Generally, intracranial and other major neurosurgical procedures will require postoperative care in the pediatric intensive care unit. These patients usually require frequent monitoring for changes in neurologic status, evidence of coagulopathy and bleeding, seizure activity, ICP fluctuations, and, in some cases, respiratory support. Patients who have undergone extracranial procedures and who have no significant comorbidities may be cared for routinely in an inpatient unit.

 Effects of Hypothermia in Infants

Increased oxygen consumption: 36% in premature babies and 23% in full-term infants

Decreased drug metabolism

Increased lactate production and metabolic acidosis

Peripheral vasoconstriction

Left shift of O2–hemoglobin dissociation curve

Cardiac dysrhythmias

 Formulas for Estimated Blood Volume and Allowable Blood Loss in Children

Allowable blood loss:

((Starting hematocrit − lowest tolerated hematocrit) × estimated blood volume)/starting hematocrit

Total estimated blood volume:

5-kg infant at 80 ml/kg = 400 ml

10-kg child at 70 ml/kg = 700 ml

40-kg child at 60 ml/kg = 2400 ml

70-kg adult at 60 ml/kg = 4200 ml