Introduction
The posterior fossa is a common site for neurosurgical lesions in infants and children. It is one of three cranial fossae and contains the cerebellum, the pons, and the medulla oblongata. Given the limited confines of the posterior fossa, it is a critical location for pathology because space-occupying lesions (tumors, hemorrhages, and structural defects of the cerebellum) will impair cerebrospinal fluid (CSF) flow and compress brainstem structures, which can lead to obstructive hydrocephalus and cardiorespiratory derangements, respectively (Figure 7.1). These aberrations place patients undergoing posterior fossa craniotomies at increased risk of perioperative complications.1, 2
Figure 7.1. The posterior fossa contains vital structures and is a critical space for pathology because enlarging and invasive lesions can lead to obstructive hydrocephalus and cardiorespiratory derangements.
Neurosurgical Lesions in the Posterior Fossa
Tumors
The posterior fossa is the site of approximately 50% of all childhood brain tumors.3 Patients frequently present with the triad of headache, vomiting, and lethargy. Cerebellar involvement manifests as truncal and limb ataxia. While bulbar or facial palsies herald brainstem infiltration. Symptoms in infants and preverbal toddlers include irritability, vomiting, and enlarged cranium. Posterior fossa tumors of childhood in decreasing order of incidence include medulloblastoma, cerebellar astrocytoma, ependymoma, brainstem glioma, and atypical teratoid or rhabdoid tumors. Surgery is most frequently the initial step in the treatment of these tumors for total or near total resection or to obtain a histological diagnosis.
Medulloblastomas frequently occur in the first decade and occur in the inferior medial zone (vermis) of the cerebellum. In the absence of infiltration of the subarachnoid space, gross total resection is the goal of surgery. However, it has a tendency to metastasize. Therefore, magnetic resonance imaging (MRI) of the entire spinal axis is crucial. Radiation therapy, chemotherapy, and surveillance MRI follow the initial surgical resection and frequently require sedation or general anesthesia for these procedures.
Cerebellar astrocytomas typically appear in the end of the first decade. The majority of these tumors arise in the cerebellum and can extend into the brainstem or cerebellar peduncles. These tumors are well circumscribed with both cystic and solid components. Total resection of the tumor is the standard operative course.
Ependymomas arise from the ependymal lining of the fourth ventricle and are situated either in the fourth ventricle or cerebellopontine angle. Half of these tumors present by 3 years of age and under. These tumors are invasive rather than infiltrative and can extend out of the confines of the posterior fossa and can predispose patients to massive blood loss and postoperative cranial nerve dysfunction.
Brainstem gliomas constitute 25% of posterior fossa tumors in children. These tumors produce cranial nerve palsies, cerebellar signs, and motor and sensory dysfunction due to their proximity to cranial nerve nuclei and corticospinal tracts. Given their poor prognosis and high potential for iatrogenic injury to vital brainstem structures, these tumors are typically treated with biopsies or focal resections followed by radiotherapy and chemotherapy.
Chiari Malformations
There are two types of hindbrain herniation or Chiari malformations due to structural defects of the cranium or cerebellum (Figure 7.2). Type I involves only the descent of the cerebellar tonsils and is often an isolated finding. Chiari malformation type II involves descent of the cerebellar vermis, medulla, and fourth ventricle, is strongly associated with hydrocephalus, and includes brainstem dysfunction (see Chapter 8). Structural anomalies of the craniocervical junction may form a CSF-filled cavity (syringomelia) in the spinal canal due to obstruction of CSF flow. Type I Chiari malformations can occur in healthy children without myelodysplasia. These defects also involve caudal displacement of the cerebellar tonsils below the foramen magnum (Figure 7.2). Symptoms such as headaches, neck pain, vertigo, paresthesia, hyperreflexia, paresis, or ataxia typically arise during adolescence. However, the diagnosis can be incidentally made during MRIs for the evaluation of scoliosis. The goal of surgery is to increase the size of the cisterna magna and normalize CSF flow. This involves decompression of the craniocervical junction by a suboccipital decompression, cervical laminectomies, and dural opening with a duraplasty.4 Cauterization of the cerebellar tonsils and lysis of arachnoid adhesions may be considered in some cases.
Figure 7.2. Midsagittal MRI of a child with a Chiari I malformation. Note the descent of the cerebellar tonsils through the foramen magnum.
The Arnold-Chiari malformation type II typically occurs in patients with myelodysplasia. This structural defect consists of caudal displacement of the cerebellar vermis, fourth ventricle, and lower brainstem below the plane of the foramen magnum. Medullary cervical cord compression can lead to vocal cord paralysis with stridor, apnea, dysphagia, pulmonary aspiration, opisthotonos, and cranial nerve deficits. Tracheostomy and gastrostomy are recommended in the most severe cases to secure the airway and to minimize chronic aspiration. Extreme head flexion should be avoided in these patients because it may cause brainstem compression. Early surgical decompression is recommended in order to minimize irreversible medullary dysfunction (cranial nerve palsies and disordered respiratory drive).
Perioperative Concerns
The major perioperative goals in the management of patients with pathological lesions in the posterior fossa are stabilizing intracranial pressure (ICP), preventing damage to the brainstem, and avoiding venous air embolism. In order to optimize the patient prior to surgery, complete preoperative evaluation and physical examination should be performed, as outlined in Chapter 1. Extensive MRI for surgical planning should be reviewed in depth and will require sedation or general anesthesia in uncooperative patients.
Patients with posterior fossa pathology have varying degrees of intracranial hypertension. Posterior fossa tumors obstruct CSF flow, which leads to obstructive hydrocephalus. Vasogenic edema from the tumor will also increase ICP. Treatment ranges from administration of dexamethasone for mild signs to placement of a ventricular catheter or shunt for severe symptoms. Patients should be positioned with the head at a 30-degree angle from horizontal to maximize cerebral venous drainage. Neonates and infants are particularly vulnerable to pharmacologic and physiologic insults. Hypoxia and hypotension are preventable conditions that can cause postoperative neurologic impairment. Therefore, optimizing cerebral perfusion and oxygenation during these intracranial surgical procedures is paramount. While the metabolic requirement of the brain (CMRO2) decreases under anesthesia, the anesthetic agents utilized and positioning the patient may decrease cerebral perfusion pressure, especially since limits of cerebral autoregulation in infants and neonates are unclear (see Chapter 2). Furthermore, intracranial pathology itself can also potentially impair cerebral autoregulation.
Brainstem tumor infiltration and direct compression by adjacent structures can present as cranial nerve dysfunction, long-tract symptoms, and severe hydrocephalus (Figure 7.1), whereas tumors in the medulla oblongata (dorsal and ventral respiratory group) and pons (pneumotaxic and apneustic center) will disrupt generation and maintenance of respiration. Patients with severely impaired brainstem may have vocal cord paralysis, loss of the pharyngeal reflex, and disordered respiratory drive. These severely compromised patients may require tracheostomy and gastrostomy tubes and perioperative mechanical ventilation.
The presence of a congenital heart defect or patent foramen ovale can lead to catastrophic venous air embolism and should negate the use of the sitting position for surgery. Furthermore, prolonged nausea and vomiting and fluid restriction may lead to hypovolemia, which may compromise cerebral perfusion in the setting of intracranial hypertension. The patient’s volume status should be optimized prior to surgery with intravenous fluid therapy.
Intraoperative Management
Induction and Maintenance of Anesthesia
Induction of anesthesia will depend on the infant or child’s symptoms. If the patient’s mental status is stable and he has no intravenous (IV) access, general anesthesia can be induced by mask with sevoflurane and oxygen. Alternatively, anesthesia can be induced with propofol in an existing IV. A non-depolarizing muscle relaxant, vecuronium, or rocuronium should be administered to facilitate intubation of the trachea. Patients who are lethargic or nauseated are at risk for aspiration pneumonitis and require a rapid-sequence induction of anesthesia using cricoid pressure and succinylcholine or high-dose rocuronium. Etomidate and ketamine can be used to induce anesthesia in hemodynamically compromised patients.
Maintenance of General Anesthesia
Drugs for the maintenance of general anesthesia should be tailored to preserve hemodynamic stability and intraoperative neuromonitoring and facilitate rapid emergence. Since specific anesthetic drugs and technique have been shown to have no impact on outcome,5 an opioid (fentanyl, sufentanil, or remifentanil) combined with low-dose isoflurane or sevoflurane or total intravenous anesthesia (TIVA) with propofol and remifentanil infusions are equally effective. However, intraoperative neuromonitoring favors TIVA techniques without the neuromuscular blockade.
Given the potential for massive blood loss and reduced access to the patient during surgery, large peripheral venous cannulae should be secured. Insertion of central venous cannulation may be necessary in patients with poor peripheral veins. The routine use of central venous catheters was not effective in determining volume status in pediatric patients.6, 7 Given the small caliber of infant central venous catheters, its utility as a conduit for aspiration of venous air embolism (VAE) is questionable.8
Related posts:
Chapter 1 – Developmental Approach to the Pediatric Neurosurgical Patient
Chapter 4 – Neuropharmacology
Chapter 10 – Anesthesia for Fetal Neurosurgery
Chapter 12 – Anesthesia for Cerebrovascular Disease in Children
Chapter 21 – Intensive Care Considerations of Pediatric Neurosurgery
Chapter 20 – Perioperative Salt and Water in Pediatric Neurocritical Care
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