General considerations
Pre-operative evaluation
History and physical examination
The importance of proper and thorough preparation of the paediatric patient for anaesthesia and surgery is frequently overlooked in the perioperative period. Given the systemic effects of general anaesthesia and the physiological stress of surgery, an organ system-based approach is optimal for anticipating potential physiological derangements and coexisting disease states that may increase the risk of perioperative complications. Many potential perioperative problems can be preemptively addressed with such an approach. As some children are either pre-verbal or do not fully understand their medical condition, their parents or primary carers should be interviewed carefully to obtain information regarding coexisting medical problems. A thorough review of the patient’s history can reveal conditions that may increase the risk of adverse reactions to anaesthesia and perioperative morbidity, and identify patients who need more extensive evaluation or whose medical condition needs to be optimized before surgery. Tere are also special perioperative concerns regarding children with neurological abnormalities (Table 14.1). A thorough neurological examination should be a routine component of the pre-operative evaluation of children undergoing any neurosurgical procedure. It is critical to be able to document accurately any change in neurological status from baseline. Pre-operative fasting is necessary to minimize aspiration of gastric contents during the operative procedure, and guidelines vary according to local practices. Certain patients presenting for neurosurgical procedures may be at particular risk for aspiration of gastric contents, and caution should be exercised during induction of anaesthesia in these children .
Laboratory evaluation
Pre-operative evaluation and laboratory tests should be tailored to the proposed neurosurgical procedure. Given the risk of significant blood loss associated with surgery, a haematocrit, prothrombin time (PT) and partial thromboplastin time (PTT) should be obtained in order to uncover any insidious haematological disorder. Patients with suprasellar pathology should get serum thyroid hormone levels, as hypothyroidism may delay emergence from anaesthesia and complicate the post-operative neurological evaluation of infants or children. Type and cross-matched blood should be ordered prior to all craniotomies. Electrolyte disturbances may exist in some children with particular intracranial pathologies. Tumours such as craniophar-yngiomas and other lesions affecting the hypothalam-ic-pituitary axis may cause significant perturbations in serum sodium. Some patients with such lesions will present for surgical procedures already on therapeutic agents such as 1-deamino-8-D-arginine vasopressin (DDAVP). Evaluation of serum electrolytes and serum osmolality is necessary for optimal perioperative management of these patients.
Induction of anaesthesia
The perioperative approach to the paediatric patient should take into account the patient’s developmental age and neurological status. Paediatric neurosurgical patients may not fully comprehend the gravity of the proposed surgery and are non-cooperative. Sedatives given before the induction of anaesthesia can ease separation of the child from the parents and the transition from the pre-operative holding area to the operating room. Midazolam given orally is particularly ef ective in relieving anxiety and producing amnesia. Pre-operative sedation should be withheld or administered only with close observation in patients with deteriorating findings on neurological examination or lethargy, because it can induce respiratory depression and interfere with serial neurological examinations. A common problem is an uncooperative toddler who has an intracranial tumour and moderately decreased intracranial compliance, yet is agitated and resistant to separation from parents. Behaviours such as crying can elevate ICP and can present the anaesthesiologist with a dilemma regarding the approach to induction. Patients with elevated ICP may be at increased risk for delayed gastric emptying and thus aspiration. However, a crying, agitated child has demonstrated a tolerance to increased ICP, and the stress of inserting an intravenous catheter for induction of anaesthesia should be inconsequential .
The patient’s neurological status and coexisting medical conditions will dictate the appropriate technique and drugs for induction of anaesthesia. In infants and young children, general anaesthesia can be induced with inhalation of sevoflurane in oxygen. Many practitioners will also use N2O as a component of an inhalational induction. However, there is much debate regarding the use of N2O during neurosurgical procedures. Te use of N2O should probably be considered on an individual case basis. Sevoflurane is not without neurological consequences, as it has been shown to have epileptogenic potential. However, the mechanism of this phenomenon is unclear. Alternatively, if the patient already has an intravenous catheter, anaesthesia can be induced with sedative/hypnotic drugs such thiopental (5-8mg kg-1) or propofol (3-4mg kgf1). Tese drugs rapidly induce unconsciousness and can blunt the haemodynamic effects of tracheal intubation. A non-depolarizing muscle relaxant is then administered after induction of general anaesthesia in order to facilitate intubation of the trachea. Patients with nausea or gastro-oesophageal reflux disorder are at risk for aspiration pneumonitis and should have a rapid-sequence induction of anaesthesia 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, suc-cinylcholine can result in sudden, catastrophic hyper-kalaemia. In children, the routine use ofsuccinylcholine is uncommon. Concerns over occult myopathies, particularly in young males, are the reason. Once again, the use of succinylcholine should be an individualized decision based on the particular risks identified during the pre-operative evaluation.
Maintenance of anaesthesia
There are several classes of drugs used to maintain general anaesthesia. Potent, volatile anaesthetic agents (i.e. sevoflurane, isoflurane and desflurane) are administered by inhalation. Tese drugs are potent cerebral vascular dilators and cerebral metabolic depressants, which can mediate dose-dependent uncoupling of cerebral metabolic supply and demand while increasing cerebral blood volume and ICP. Moreover, the use of these agents can be associated with a significant decrease in cerebral perfusion pressure (CPP), primarily due to a dose-dependent reduction in arterial blood pressure. Tey depress the EEG and may interfere with intraoperative electrocorticography (ECoG). Given these issues, volatile anaesthetics are rarely used as the sole anaesthetic for neurosurgery. Nitrous oxide also acts as a cerebral vascular dilator.
Intravenous anaesthetics include sedative-hypnotics and opioids. Tese agents are also potent cerebral metabolic depressants but do not cause cerebral vaso-dilation. Te sedative-hypnotics propofol, midazolam and thiopental rapidly induce general anaesthesia and attenuate the EEG. Opioid drugs can depress the EEG but not to the same degree as the sedative hypnotics. Fentanyl and other related synthetic opioids including sufentanil have their context-sensitive half-times increase with repeated dosing or prolonged infusions and require hepatic metabolism. As a result, the narcotic effects of these drugs, such as respiratory depression and sedation, may be prolonged. Remifentanil is a unique opioid that is rapidly cleared by plasma este-rases whose context-sensitive half-life remains constant unrelated to the length of infusion. Tis makes it, when administered at a rate of 0.2-1.0 |xg kg-1 min-1, an ideal opioid for rapid emergence from anaesthesia. However, this rapid recovery is frequently accompanied by delirium and inadequate analgesia. Although craniotomies generally do not require aggressive post-operative pain management, the immediate post-operative period generally requires the use of intravenous opioids. Tus, when remifentanil is utilized for maintenance of general anaesthesia, a longer-acting opioid such as morphine or hydromorphone should be part of the plan for the immediate post-operative period. Deep neuromuscular blockade with a non-depolarizing muscle relaxant is maintained to avoid patient movement and minimize the amount of anaesthetic agents needed. Muscle relaxants should be withheld or permitted to wear off when assessment of motor function during neurosurgery is planned. During many neurosurgical procedures, head fixation devices such as the Mayfield head clamp are employed. Te use of muscle relaxation when these devices are employed is extremely important to prevent inadvertent patient movement while fixed in these clamps and pins. Sudden uncontrolled patient movement while in a head fixation device can result in scalp lacerations and even cervical spine injuries .
Positioning
Children with elevated ICP should be transported to the pre-operative holding area and operating room with the head elevated in the midline position to maximize cerebral venous drainage. Once a patient is in the operating room, the neurosurgeons and anaesthesiologists must all have adequate access to the patient. Tis can be particularly challenging in infants and small children, for whom slight displacement of the head or small movements of the endotracheal tube can result in tracheal extubation or endobronchial intubation, so extra care must be taken to ensure the airway is secured after final positioning. Given the fact that many neurosurgical procedures can be quite lengthy, it is important to carefully establish and evaluate adequate positioning prior to prepping and draping of the patient. Additionally, major positional adjustment of the patient during the procedure may be contraindicated if the patient is placed in a head fixation device due to potential for cervical spine injury under anaesthesia.
The prone position is commonly used for posterior fossa and spinal cord surgery in paediatric patients. In addition to the physiological sequelae of this position, a whole spectrum of compression and stretch injuries has been reported. Padding under the chest and pelvis can support the torso. It is important to ensure free abdominal wall motion, because increased intra-abdominal pressure can impair ventilation, cause venocaval compression and increase epidural venous pressure and bleeding. In males, the penis and testicles should be checked to ensure there is no compression. Sof rolls are used to elevate and support the lateral chest wall and hips in order to minimize any increase in abdominal and thoracic pressure. In addition, this allows a Doppler probe to be placed on the chest without pressure. Te head must be carefully flexed to avoid kinking of the endotracheal tube, inadvertently advancing the tube into an endobronchial position or compressing the chin on the chest. Too much flexion for an extended time can cause lower brainstem and upper spinal cord ischaemia, as well as head and tongue swelling from blockage of venous or lymphatic drainage. Tis can lead to post-extubation airway obstruction or croup.
Most neurosurgical procedures are performed with the head slightly elevated to facilitate venous and CSF drainage from the surgical site. However, superior sagittal sinus pressure decreases with greater head elevation, and this increases the likelihood of venous air embolism (VAE). Extreme head flexion can cause brainstem compression in patients with pathological conditions of the posterior fossa, such as mass lesions or Arnold-Chiari malformations. It can also cause endotracheal tube problems, including obstruction from kinking or displacement to the carina or the right mainstem bronchus. Extreme rotation of the head can impede venous return through the jugular veins and lead to impaired cerebral perfusion, increased ICP and cerebral venous bleeding .
Monitoring
Standard monitoring equipment used for all patients undergoing anaesthesia include an ECG, pulse oxim-eter, blood pressure gauge, end-tidal carbon dioxide analyser and thermometer. A precordial or oesopha-geal stethoscope is commonly used in North American practice, but less so in Europe. Given the potential for sudden haemodynamic instability due to VAE, haemorrhage, herniation syndromes, or manipulation of cranial nerves, placement of an intra-arterial cannula for continuous blood pressure monitoring is appropriate for most neurosurgical procedures. An arterial catheter also provides access for sampling serial blood gases, electrolytes, haematocrit and serum osmolality. Central venous pressures may not accurately reflect vascular volume, especially in a child in the prone position. Terefore, the risks of a central venous catheter may outweigh the benefits. Certain patients may arrive in the operating room with ICP monitoring devices. Te intraoperative use of these devices may be useful in particular cases but should not be considered a necessary routine monitor for children undergoing neuro-surgical procedures .
Vascular access
Owing to limited access by the anaesthesiologist to the child during many neurosurgical procedures, optimal intravenous access is mandatory prior to the start of surgery. Central venous catheters are often inserted for intravenous access, central pressure monitoring and infusion of vasoactive drugs in adult neurosurgical procedures. However, paediatric central vein catheters have a small internal diameter (gauge) and are resistant to high flow rates. Two large peripheral venous cannulae are sufficient for most paediatric craniotomies. In the child that has difficult peripheral venous access, central venous cannulation may be necessary. Utilization of the femoral vein avoids the risk of pneumothorax associated with subclavian catheters, and does not interfere with cerebral venous drainage, as may be the case with jugular venous catheters. Furthermore, femoral catheters are more easily accessible to the anaesthesiologist during intracranial operations. As significant blood loss and haemodynamic instability can occur during craniotomies, an intra-arterial cannula provides direct blood pressure monitoring and sampling for blood gas analysis .
Venous air embolism occurs when ambient air enters the vascular system through open venous sinuses in the surgical field during surgical procedures. Te incidence of VAE in children undergoing suboccipital craniotomy in the sitting position is not significantly different from that in adults. However, children appear to have a higher incidence of hypotension and a lower likelihood of successful aspiration of intravascular air via central venous catheters .
Post-operative issues
Close observation in an intensive care unit (ICU) with serial neurological examinations and invasive haemo-dynamic monitoring is helpful for the prevention and early detection of post-operative problems. In certain patients, seizures can occur during the post-operative period and may lead to significantly increased morbidity. When seizures do occur, the response must be prompt: first with basic life support algorithms addressing airway, breathing and circulation, and then with administration of sufficient anticonvulsant drug to stop the seizure. A common approach involves lor-azepam 0.1 mg kg-1 IV (repeated after 10 min if necessary) for immediate control, followed by fosphenytoin 20 mgkg-1, phenobarbitol 20 mg kg-1 or levetiracetam 10 mg kg-1 on a regular schedule for more lasting coverage. Levetiracetam is becoming an increasingly common agent used for seizure prophylaxis in the perioperative period. Te reasons for this change from fosphenytoin and phenobarbitol include the generally more favourable side effect profile of levetiracetam as well as the lack of the need for following drug levels. Phenytoin levels may be altered in sick children in the hospital and free drug levels should be followed rather than total drug levels.
Pain control and sedation present unique challenges in the paediatric ICU. Ideally, post-operative neurosurgical patients are comfortable, awake and cooperative in order to obtain serial neurological examinations. In paediatrics, these goals can be difficult to achieve due to the cognitive level of the patient. Te mainstay of sedation in the paediatric ICU remains a combination of opioid and benzodiazepine administered via continuous infusion. Infants and children receiving sedative infusion for >3-5 days are subject to tolerance and experience symptoms of withdrawal when infusions are discontinued. Propofol is a potent, ultra-short-acting sedative-hypnotic that is extremely useful in adult neurocritical care but has only limited utility in paediatrics because of its association with a fatal syndrome of bradycardia, rhabdomyolysis, metabolic acidosis and multiple organ failure when used over extended periods in small children. While some centres have advocated its use in children under strict controls, propofol is generally limited to operative anaesthesia, procedural sedation and continuous infusions of limited duration (<24 h).
A newer agent, dexmedetomidine, an intravenous a2-agonist, is an ultra-short-acting, single-agent sedative that is often used in the post-operative period. Te purported advantage of this agent is its ability to provide sedation while allowing easy and frequent neurological assessment. Studies involving children are preliminary, but the drug appears to be safe and effective when used for periods of 24 h or less. Opioid cross-tolerance makes it a useful agent for treatment of fentanyl or morphine withdrawal. Transient increases in blood pressure can be seen with boluses followed by hypotension and bradycardia as sedation deepens. Further experience and vetting will be necessary to determine the proper place for this agent in the routine perioperative care of neurosurgical patients.
Anaesthesia for specific neurosurgical procedures
Neonatal emergencies
Myelomeningocele
Myelomeningocele is due to failure of closure of the posterior neural tube, resulting in malformation of the vertebral column and spinal cord and other CNS anomalies. Tis defect occurs during the fourth week of gestation. Tus, there is a problem that arises very early in gestation that results in a wide spectrum of CNS abnormalities. McLone and Knepper hypothesized that the aetiology of this spectrum of abnormalities results from failure of fusion of the neural tube leading to leakage of CSF through this defect during critical CNS development. Tis leakage does not create the conditions necessary for distension of the cranial end of the neural tube, which leads to abnormal development of the CNS and, in particular, the posterior fossa and its contents. Tus, the myelomeningocele itself is essentially the most identifiable external manifestation of a spectrum of potential CNS pathologies whose common aetiology can be traced to very early embryologic development.
The spinal defect can occur anywhere along the vertebral column, although lumbar and low thoracic defects are most common. In the most severe forms (rachischisis), the neural plate appears as a raw, fleshy plaque through a vertebral column defect (spina bif-ida) and the skin. A protruding membranous sac containing meninges, CSF, nerve roots and a dysplastic spinal cord often protrudes through the defect in men-ingocele or myelomeningocele. Tracheal intubation of a neonate with a myelomeningocele can be challenging depending on the size and location of the defect. Te supine patient may be elevated on towel rolls taking care to create a port through which the myelomenin-gocele may protrude. Likewise, the neonate can be positioned in the right lateral decubitus position to allow an unimpeded arc from right to left for laryngoscopy and intubation. Blood and insensible fluid loss is dependent on the size of the myelomeningocele and the amount of tissue dissection required to repair the defect. Hydrocephalus occurs in 80% of neonates with myelomeningocele or encephalocele, and a ventricu-loperitoneal shunt will often need to be inserted immediately after the myelomeningocele is initially repaired or within several days after primary closure.
Intraventricular haemorrhages
Given the fragile nature of their cerebral vascular system, low-birthweight premature neonates frequently develop intraventricular haemorrhage, which can lead to post-haemorrhagic hydrocephalus. Exaggerated fluctuations in blood pressure have been implicated in the development of intraventricular haemorrhage. Evaluation for intraventricular haemorrhage involves a head ultrasound, made easier in infants because of their open fontanelles. Evaluation of the presence and extent of intraventricular haemorrhage should take place prior to administration of any systemic anticoagulants such as heparin or enoxaparin.
Signs and symptoms of post-haemorrhagic hydrocephalus include enlarging occipital-to-frontal circumference, a tense anterior fontanelle and periods of bradycardia. Serial lumbar punctures will temporize the symptoms. However, the definitive surgical approach to this problem is to insert a shunt attached to a subgaleal reservoir. Te major anaesthetic issues in managing these premature neonates are the transport from and to the neonatal ICU and continuation of ventilator and haemodynamic support .
Hydrocephalus
Hydrocephalus is a vexing paediatric neurosurgical condition that has a laundry list of aetiologies including haemorrhage (neonatal intraventricular or sub-arachnoid), congenital problems (aqueductal stenosis), trauma, infection and tumours (especially in the posterior fossa). Unless the aetiology of the hydrocephalus can be definitively treated, treatment entails surgical placement of a ventricular drain or ventriculoperito-neal shunt. Alternatively, the shunt can drain into the right atrium or pleural cavity. Acute obstruction of these shunts should be treated urgently because acute rises in ICP in the relatively small cranial vault of the infant or child can be lethal. Anaesthesia should be established in the obtunded patient with a rapid sequence induction technique followed by tracheal intubation. If intravenous access cannot be established, an inhalation induction with sevoflurane and gentle cricoid pressure may be an alternative in the conscious patient. Te possibility of VAE during placement of the distal end of a ventriculoatrial shunt should always be kept in mind. Post-operatively, patients should be observed carefully because an altered mental status and recent peritoneal incision place them at high risk for pulmonary aspiration once feedings are begun .
Chiari malformations
There are four types of Chiari malformations (Table 14.2). Te Arnold-Chiari malformation (type II) almost always coexists in children with myelodyspla-sia. Tis defect consists of a bony abnormality in the posterior fossa and upper cervical spine with caudal displacement of the cerebellar vermis, fourth ventricle and lower brainstem below the plane of the foramen magnum. Medullary cervical cord compression can occur. Vocal cord paralysis with stridor and respiratory distress, apnoea, abnormal swallowing and pulmonary aspiration, opisthotonos and cranial nerve deficits may be associated with the Arnold-Chiari malformation and usually present during infancy. Patients with vocal cord paralysis or a diminished gag reflex may require tracheostomy and gastrostomy to secure the airway and minimize chronic aspiration. Patients of any age may have abnormal responses to hypoxia and hyper-carbia because of cranial nerve and brainstem dysfunction. Extreme head flexion may cause brainstem compression in otherwise asymptomatic patients .
Type I Chiari malformations can occur in healthy children without myelodysplasia. Tese defects involve caudal displacement of the cerebellar tonsils below the foramen magnum, but patients generally have much milder symptoms, sometimes presenting only with headache or neck pain.
Surgical treatment usually involves a decompres-sive suboccipital craniectomy with cervical laminectomies in the prone position. Given the proximity of the straight and transverse sinus under the occipital bone, massive blood loss and VAE can occur as the bone flap is lifted.
Table 14.2 Chiari malformations
|
Type |
Malformation |
|
Type I |
Caudal displacement of cerebellar tonsils below the plane of the foramen magnum |
|
Type \II |
Arnold-Chiari, associated with myelomeningocele: caudal displacement of the cerebellar vermis, fourth ventricle and lower brainstem below the plane of the foramen magnum. Dysplastic brainstem with characteristic ‘kink, elongation of the fourth ventricle, ‘beaking’ of the quadrigeminal plate, hypoplastic tentorium with small posterior fossa, polymicrogyria and enlargement of the massa intermedia |
|
Type III |
Caudal displacement of the cerebellum and brainstem into a high cervical meningocele |
|
Type IV |
Cerebellar hypoplasia |
Tumours
Posterior fossa tumours
As the majority of intracranial tumours in children occur in the posterior fossa, CSF flow is often obstructed and intracranial hypertension and hydrocephalus are often present. Most neurosurgeons approach this region with children in the prone position. Te patient’s head is generally secured with a Mayfield head frame, although pins used in small children can cause skull fractures, dural tears and intracranial haematomas. Elevation of the bone flap can result in sinus tears, massive blood loss and/or VAE. Surgical resection of tumours in the posterior fossa can also lead to brainstem and/or cranial nerve damage. Table 14.3 lists some of the signs of encroachment on these structures. Damage to the respiratory centres and cranial nerves can lead to apnoea and airway obstruction after extubation of the patient’s trachea .
Suprasellar tumours
Craniopharyngiomas are the most common perisel-lar tumours in children and adolescents and may be associated with hypothalamic and pituitary dysfunction. Steroid replacement (dexamethasone or hydro-cortisone) is generally administered, as the integrity of the hypothalamic-pituitary-adrenal axis may be uncertain. In addition, diabetes insipidus occurs pre-operatively in some patients and is a common postoperative problem. Nocturnal enuresis may result in pre-operative hypovolaemia.
Table 14.3 Effects of surgical brainstem manipulation
|
Brainstem area |
Signs |
Changes in monitor |
|
Cranial nerve V |
Hypertension, bradycardia |
Arterial pressure, ECG |
|
Cranial nerve VII |
Facial muscle movement |
EMG |
|
Cranial nerve X |
Hypotension, bradycardia |
Arterial pressure, ECG |
|
Pons, medulla |
Arrhythmias, hypotension, hypertension, tachy- or bradycardia, irregular breathing pattern |
ECG, arterial pressure, end-tidal carbon dioxide |
Table 14.4 Diagnostic criteria for diabetes insipidus
|
Criterion |
Measurement |
|
Urine output |
 |
|
Serum sodium |
 |
|
Serum osmolality |
 |
|
Urine osmolality |
 |
|
Polyuria persisting |
 |
If diabetes insipidus does not exist pre-operatively, it usually does not develop until the post-operative period. Tis is because there appears to be an adequate reserve of antidiuretic hormone in the posterior pituitary gland capable of functioning for many hours, even when the hypotha-lamic-pituitary stalk is damaged intraoperatively. Te surgical approach to the sella is between the frontal lobes in infants and young children and transnasal in adolescences. Te frontal approach tends to provoke more surgical bleeding and places the infant or child at risk for venous air embolism due the elevated position of the head during the surgery. Terefore, patients undergoing a frontal craniotomy should have adequate intravenous access and monitors such as an arterial catheter and pre-cordial Doppler. Although it occurs primarily af er surgery, diabetes insipidus is associated with suprasellar surgery. Te diagnosis of intraoperative diabetes insipidus is straightforward and is characterized by the criteria described in Table 14.4. Other causes of polyuria must be ruled out (e.g. administration of mannitol, furosemide or osmotic contrast agents, and the presence of hyper-glycaemia). Once the diagnosis of diabetes insipidus is established, a vasopressin infusion is commenced at 1 mU kg-1 h-1 and is increased every 5-10 min to a maximum of 10 mU kg-1 h-1 to decrease the urine output to <2 ml kg-1 h-1. Total maintenance fluids (intravenous and oral fluids) should not exceed the insensible losses plus the obligate urinary losses. It is convenient to calculate the total intravenous fluids as two-thirds of maintenance. Te appropriate intravenous fl uid is 5% dextrose/0.9% saline with 0-40 mEq KCl l-1. Te antidiuretic effect of vasopressin is an ‘all or none’ phenomenon. Once the patient’s urine output is < 2 ml kg-1 h-1, the vasopressin and crystalloid infusion rate should be maintained until the patient is alert enough to take oral fluids and vasopressin derivatives such as DDAVP.
