Epilepsy
Surgical treatment has become a viable option for many patients with medically intractable epilepsy. Two major considerations should be kept in mind. Chronic administration of anticonvulsant drugs, such as phenytoin and carbamazepine induces rapid metabolism and clearance of several classes of anaesthetic agents including neuromuscular blockers and opioids. Terefore, the anaesthetic requirements for these drugs are increased and require close monitoring of their effect and frequent redosing. Intraoperative neurophysiological monitors can be used to guide the actual resection of the epileptogenic focus and general anaesthetics can compromise the sensitivity of these devices. Furthermore, if cortical stimulation is utilized to mimic the seizure pattern or identify areas on the motor strip, neuromuscular blockade should be antagonized.
Occasionally, a repeat craniotomy must be performed in a child shortly af er the primary procedure. An elective repeat craniotomy may be necessary for removal of grids and strips used for invasive EEG monitoring and subsequent resection of the seizure focus. It is important to avoid administration of N2O until the dura is opened, as intracranial air can persist up to 3 weeks following a craniotomy, and N2O in these situation can cause rapid expansion of air cavities and result in tension pneumocephalus .
Post-operative seizures are an uncommon but devastating complication. Prophylaxis in the peri-operative period and aggressive treatment of new convulsions are well-recognized mainstays of care. While phenytoin is the agent used most commonly for prophylaxis, maintaining therapeutic serum levels can be challenging. Levetiracetam is becoming increasingly comm on and in many instances supplanting phenytoin as the choice for prophylaxis. B oth drugs can be administered intravenously, but, unlike phenytoin, administration of levetiracetam does not require following serum drug levels to monitor for toxicity. Alternative agents frequently used in paediatrics include pheno-barbital, carbamazepine and valproic acid. Status epi-lepticus can be treated with lorezepam 0.1 mg kg-1 IV push over 2 min or diazepam 0.5 mg kg-1 PR as effective agents. Lorezapam may be repeated after 10 min and accompanied by fosphenytoin 20 mg kg-1 IV or IM if initial doses are ineffective. Although potentially compounding respiratory depression, phenobarbital 20 mg kg-1 is also an effective first-line anti-epileptic drug.
The vagal nerve stimulator is another advance in the surgical treatment of epilepsy. Although its exact mechanism of action is not understood, it appears to inhibit seizure activity at brainstem or cortical levels. Its placement has shown benefit with minimal side effects in many patients who are disabled by intractable seizures. At this time, there are few published series of vagal nerve stimulation studies in children, but it is estimated that there is a 60-70% improvement in seizure control in children receiving vagal nerve stimulation, with the best results in those with drop attacks. Te anaesthetic management of patients for implantation of vagal nerve stimulators should focus on the coexisting diseases. Te procedure itself involves creation of a left subpectoral muscle pocket for the impulse generator and isolation of the left vagal nerve for positioning of the electrode.
Neurovascular disease
The perioperative management of paediatric patients with vascular anomalies should focus on optimizing cerebral perfusion. Operative management is commonly associated with massive blood loss and these patients require several IV access sites and invasive haemodynamic monitoring. Haemodynamic stability during intracranial surgery requires careful maintenance of intravascular volume. Massive blood loss should be anticipated and treated with blood replacement therapy. Hypotension can transiently be treated with vasopressor infusion (dopamine) during fluid resuscitation.
Moyamoya syndrome
Moyamoya syndrome is a rare, chronic vaso-occlusive disorder of the internal carotid arteries that presents as transient ischaemic attacks or recurrent strokes in childhood. Te cause is unknown, but the syndrome can be associated with previous intracranial radiation, neurofibromatosis, Down’s syndrome and a variety of haematological disorders. Te anaesthetic management of these patients is directed at optimizing cerebral per-fusion. Tis includes ensuring generous pre-operative hydration and maintaining the blood pressure within the patient’s pre-operative range. Moyamoya syndrome is a vasculopathy characterized by chronic progressive stenosis to occlusion at the apices of the intracranial internal carotid arteries including the proximal anterior cerebral arteries and middle cerebral arteries, and is associated with: prior radiotherapy to the head or neck for optic gliomas, craniopharyngiomas and pituitary tumours; genetic disorders such as Down’s syndrome, neurofibromatosis type I (NF1) (with or without hypothalamic-optic pathway tumours), large facial haemangiomas, sickle-cell anaemia and other haemo-globinopathies; and autoimmune disorders such as Graves’ disease, congenital cardiac disease, renal artery stenosis and others. Moyamoya disease is the idio-pathic form of moyamoya, while moyamoya syndrome is defined as the vasculopathy found in association with another condition, such as neurofibromatosis, sickle-cell disease or Down’s syndrome. Maintenance of normocapnia is essential in patients with Moyamoya syndrome because both hyper- and hypocapnia can lead to steal phenomenon from the ischaemic region and further aggravate cerebral ischaemia. An opioid-based anaesthetic provides a stable level of anaesthesia for these patients and is compatible with intraopera-tive EEG monitoring. Once the patient emerges from anaesthesia, the same manoeuvres that optimize cerebral perfusion should be extended into the post-operative period. Tese patients should receive intravenous fluids to maintain adequate cerebral perfusion and adequate narcotics to avoid hyperventilation induced by pain and crying.
Arteriovenous malformations
Arteriovenous malformations (AVMs) result from improper formation of the arteriolar-capillary network that provides a connection between arteries and veins in the brain. Te embryonic origins of these malformations are unclear. Malformations not large enough to produce congestive heart failure usually remain clinically silent unless they cause seizures or a stroke or until the acute rupture of a communicating vessel results in subarachnoid or intracerebral haemorrhage. Overall, 80-85% of all paediatric AVMs present with haemorrhage. It may present with seizures, headache or focal neurological deficits. Haemorrhagic AVMs have been associated with a 25% mortality rate. Rebleeding rates are approximately 6% for the first 6 months, then 3% per year afterwards. Tis lesion produces neurological deficits through mass effect or from cerebral ischaemia that is due to diversion of blood to the AVM from the normal cerebral circulation (‘steal’).
Vein of Galen malformations
Vein of Galen malformations (VOGMs) are direct connections between cerebral arteries and existing veins. Unlike AVMs, they do not usually have a nidus and, in some cases of arteriovenous fistula, may exist as a single pathological connection between an artery and vein. In VOGMs, single or multiple small arterial vessels directly drain into the vein of Galen. Te result of this type of direct connection is markedly increased cerebral venous pressure, leading to increased ICP and potential haemorrhage. In some VOGMs, the connections have such rapid flow rates that children develop high-output cardiac failure and hydrocephalus. Te management of the neonate with VOGM is primarily supportive with vasopressors and mechanical ventilation for progressive congestive heart failure. Aggressive embolizations in the interventional radiology suite are first-line therapies for neonates with VOGM. Infants and children with residual lesions may require additional embolizations and surgery for the placement of ventriculoperitoneal shunts for hydrocephalus .
Ventriculoscopy
Technological advances in minimally invasive endo-scopic surgery have entered the neurosurgical arena. Te anaesthetic considerations for these evolving techniques are the same as for any other neurosurgical procedures, as discussed in this topic. Endoscopic third ventriculoscopy has become an accepted procedure for the treatment of obstructive hydrocephalus in infants and children. Despite the relative safety of this procedure, bradycardia and other arrhythmias have been reported in conjunction with irrigation fluids and/or manipulation of the floor of the third ventricle.
Traumatic brain injury
Paediatric head traum a requires a multiorgan approach to minimizing morbidity and mortality. A small child’s head is often the point of impact in injuries, but other organs can also be damaged. As secondary insults can progressively worsen outcome, basic life-support algorithms should be applied immediately to assure a patent airway, spontaneous respiration and adequate circulation. Immobilization of the cervical spine is essential to avoid secondary spinal cord injury with manipulation of the patient’s airway until radiological clearance is confirmed. Blunt abdominal trauma and long-bone fractures frequently occur with head injury and can be major sources of blood loss. In order to assure tissue perfusion during the operative period, the patient’s blood volume should be restored with crystalloid solutions and/or blood products. Ongoing blood loss can lead to coagulopathies and should be treated with specific blood components.
Infants with shaken baby syndrome often present with a myriad of chronic and acute subdural haema-tomas. As with all traumatic events, the presence of other coexisting injuries, fractures and abdominal trauma should be identified. Craniotomies for the evacuation of either epidural or subdural haematomas are at high risk for massive blood loss and VAE. Postoperative management of these victims is marked by the management of intracranial hypertension and, in the most severe cases, determination of brain death.
Resuscitation
Traumatic brain injury (TBI) is an evolving process that extends beyond the initial insult. Progression of the primary neuronal injury can be attenuated by preventing hypotension and hypoxia. Rapid implementation of advanced life-support algorithms is an essential first step in the management of head-injured patients.
Securing a patent airway and restoring oxygen-ation and ventilation are paramount in advanced life-support algorithms. Secondary injury results from the pathological sequelae of the primary injury: oedema and ischaemia due to cortical compression, hypotension, hypoxia and so on. Inappropriate m anipulation of a patient with an unstable fracture can exacerbate both primary and secondary injuries. Anaesthesiologists caring for a child with a potential cervical spine injury should know that spinal cord injuries in children commonly occur without actual evidence of spinal bone fractures on plain cervical radiographs. Tese injuries are known as SCIWORA (spinal cord injuries without radiologic abnormality). Injuries to the cervical spine in particular are often difficult to recognize but may sometimes be identified by odontoid displacement or pre-vertebral swelling on a radiograph. As a result, CT is frequently indicated when a spinal injury is initially suspected in a child with trauma. Sometimes, as with brain injury, there can be a delay in the onset of neurological deficits with SCIWORA injuries. However, infants and young children are predisposed to C1-2 cervical disruptions, which may be difficult to diagnose in cervical spine X-rays. Tere should be minimal cervical spine manipulation during tracheal intubation of the head-injured infant.
Age-appropriate normograms of blood pressure in paediatric patients should dictate the end points of resuscitation. Hypotension is associated with greater mortality in children than in adults. Vavilala and colleagues confirmed this notion by correlating hypotension to increased morbidity and mortality. Terefore, hypotension secondary to trauma should be treated aggressively by securing large-bore intravenous catheters and restoration of fluid deficits. After normoten-sion has been achieved, CPP management should be instituted according to the haemodynamic parameters listed below. Te choice of resuscitation fluids remains controversial, whether it is crystalloid, colloid or hypertonic saline. However, the administration of hypertonic saline has been shown to improve various clinical parameters, although not survival rate.
Intracranial pressure monitoring
The use of ICP monitoring in infants and children with severe head injury with a Glasgow Coma Scale score <8 is strongly supported by several clinical studies. However, as noted above, age-specific differences in cerebral haemodynamics necessitate modifying these variables. All retrospective studies on the effects of ICP have demonstrated that pronounced morbidity and mortality occur after the ICP is persistently elevated above 20 mmHg. Although there is an age-related decrease in normal ICP in infants and young children, these retrospective studies demonstrate that the ‘safe’ upper limit for ICP is similar in adults. However, the value of ‘<20 mmHg’ was artificially set, and lower increments were not fully tested in paediatric patients. Intracranial pressure measurements from ventricular catheters and fibre-optic intraparenchymal transducers have a good correlation. However, ventricular catheters also provide a conduit to withdraw CSF.
Management of cerebral perfusion pressure
One of the most controversial topics in the management of TBI patients is the goal of cerebral perfu-sion. Te goals of therapy should be based on the data obtained from cerebral vascular monitors. Several paediatric studies have demonstrated that lower CPP is associated with better outcome when compared with adult values. Downard and colleagues retrospectively reviewed the course of 188 children with TBI and concluded that there were no survivors with CPP <40 mmHg. Furthermore, 10 mmHg increases in CPP up to 70 mmHg did not improve the Glasgow Outcome Scale score. Chambers and colleagues determined the critical thresholds for CPP in relation to the patient’s age. Tey reported that CPP in head-injured patients should be 48 mmHg at 2-6 years, 54 mmHg at 7-10 years and 58 mmHg at 11-15 years. Terefore, maintaining CPP above 40 mmHg appears to be associated with the best outcome in paediatric patients. However, studies in infants need to be performed in order to assess the efficacy of even lower CPP.
Drugs
Endotracheal intubation and mechanical ventilation mandates prolonged sedation and, in severe cases, neuromuscular blockade. Sedation is carried out with intermittent doses ofmidazolam, fentanyl or morphine as first-line drugs to minimize discomfort and agitation. Brain swelling can initially be managed by hyper-ventilation and elevating the head above the heart. Should these manoeuvres fail, mannitol can be given at a dose of 0.25-1.0 g kg-1 IV. Tis will transiently alter cerebral haemodynamics and raise serum osmolality by 10-20 mOsm kg-1. Furosemide is a useful adjunct to mannitol in decreasing acute cerebral oedema and has been shown in vitro to prevent rebound swelling due to mannitol. All diuretics will interfere with the ability to utilize urine output as a guide to intravascular volume status. The use of hypertonic saline (3% NaCl) has been shown to decrease ICP and improve CPP in paediatric patients.
High-dose barbiturate therapy produces a burst suppression pattern on the EEG and results in a reduction in cerebral metabolic rate. Refractory intracranial hypertension can be treated by thiopen-tal infusions. However, myocardial depression and hypotension are unacceptable side effects of this therapy and need to be countered by volume loading and ionotropic support .
Decompressive craniotomy
Reduction of severe intracranial hypertension that is refractory of less invasive manoeuvres may warrant surgical decompression of haemorrhagic and oedema-tous brain tissue. Polin and colleagues reported the effect of bifrontal decompressive craniectomy in a mixed population ofhead-injured patients. Favourable outcome correlated with pre-operative ICP <40 mmHg, treatment within 48 h of presentation and age <18 years. Taylor and colleagues prospectively evaluated the utility of early decompressive craniectomy within 24 h after the time of injury. Tey reported a significant decrease in ICP and improved 6-month outcome in the children undergoing the decompressive craniectomy. Cho and colleagues reported their experience with bifrontal or frontotemporal craniotomy for management of refractory intracranial hypertension due to shaken baby syndrome. Tey found a significant decrease in mortality in the craniotomy group when compared with the medically managed children. Jagannathan and colleagues reported the outcome of 23 craniotomies performed on children (mean age of 11.9 years). Sixteen patients survived and 81% of the survivors returned to school and had a reasonable quality of life. Despite the severity of this surgical approach, studies in children and adults demonstrate the utility in reducing refractory intracranial hypertension and an improvement in outcome. Terefore, decompressive craniotomy has a role as a last attempt at managing moribund head-injured infants and children.
Induced hypothermia
Induced hypothermia has been shown unequivocally to reduce neurological injury in pre-clinical studies. Its efficacy in the clinical setting has been controversial. Head cooling and mild hypothermia have been demonstrated to be protective in asphyxiated neonates. However, induced hypothermia in adult TBI has mixed results. Adelson and colleagues demonstrated in a Phase II trial that induced hypothermia can be a safe therapeutic option in children with TBI. A National Institutes of Health-sponsored Phase III trial is underway. Recently, an international multicentre trial of induced hypothermia in paediatric patients reported that hypothermia did not improve the neurological outcome and may increase mortality.
Conclusion
The management of paediatric patients for neurosur-gery is currently based on a few randomized trials and draws heavily from data derived from adult series. Evidence-based management of paediatric patients is still evolving. Terefore, fundamental knowledge of age-related differences in cerebral vascular physiology and anatomy is essential for the perioperative management of paediatric neurosurgical patients.
