Abstract
Surgery is the mainstay of treatment for the child with a posterior fossa tumor. However, posterior fossa tumor surgery does involve greater morbidity and mortality and has a wider variety of complication than surgery in supratentorial compartment. These complications may be avoided by careful perioperative planning with the understanding of patient’s history, neurological findings, imaging studies as well as meticulous microsurgical dissection with full-knowledge of neuroanatomy. The neurosurgeon has a significant role to play in the management of posterior fossa tumors in children and to minimize all possibility of complications.
Keywords
pediatric posterior fossa tumor, surgical complications, surgical Precautions
Highlights
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Cautions for resection of these tumors will vary depending upon the location of the tumor.
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Medulloblastomas most arise from the roof of the fourth ventricle, pushing downward into the ventricle. 35% of medulloblastoma invade the floor of the fourth ventricle and the brainstem. This tumor debulking may lead to a “floor of the fourth syndrome” which includes an ipsilateral nuclear palsy of cranial nerves VI and VII and a contralateral hemiparesis.
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Juvenile pilocytic astrocytomas arise within the cerebellar hemispheres and are usually separated from the ventricle by the ependyma. A variant of the cerebellar astrocytoma actually arises from the brainstem and is dorsally exophytic into the ventricle. Such tumors exit the brainstem, pulling functional tissue up as they grow dorsally. The inexperienced surgeon may shave these tumors off with inadvertently injuring the brainstem in the process.
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Ependymomas take origin from the ependymal lining of the ventricle. Those arising from the floor of the fourth ventricle derive blood supply from multiple small perforating vessels arising from the brainstem. These vessels must be meticulously coagulated and cut, as avulsing them may cause them to retract and bleed into the brainstem. A variant of the ependymoma arises from ependymal rests at the lateral margin of the foramen of Luschka and grows out the foramen into the cerebellopontine angle. These tumors often encase the lower cranial nerves as well as the vertebra-basilar complex, and may invade the side of the pons.
Introduction
The incidence of brain tumors in children is 2.6 to 4 per 100,000. Half of the brain tumors found in childhood occur in the posterior fossa, with medulloblastoma, juvenile pilocytic astrocytoma (JPA), and ependymoma being the big three. Tumors of the fourth ventricle offer a unique challenge to the neurosurgeon because they lie deep in the brain in close proximity to a number of vital structures. Posterior fossa surgery involves greater morbidity and mortality and has a wider variety of complications than surgery in the supratentorial region. In addition, multiple reviews have shown that the single most important determinant of survival in children with posterior fossa tumors is the extent of surgical resection. The pediatric neurosurgeon must achieve a maximal safe resection.
Thus surgery for pediatric posterior fossa tumors is important not only to the survival but also to the quality of the survival. To date there is no study to report the true complication rate of pediatric posterior fossa surgery. Sawaya et al. classified the complications associated with craniotomy into three major categories: neurologic, regional, and systemic complications. In this article, we focus mainly on neurologic and regional complications associated with posterior fossa surgery for children with ependymoma, medulloblastoma, and pilocytic astrocytoma. We also review the causes of delayed diagnosis and the precautions that can be taken in the preoperative and intraoperative period to minimize complications ( Fig. 28.1 ).
Delayed Diagnosis or Misdiagnosis of Posterior Fossa Tumor
Previous studies have indicated that delays in the diagnosis of childhood brain tumors may be much longer than those associated with other pediatric neoplasms. Dobrovoljac et al. found the median prediagnostic symptomatic interval (PSI) was 60 days, with a parental delay of 14 days and a doctor’s delay of 30 days. Only 33% of brain tumors were diagnosed within the first month after the onset of signs and symptoms. In children older than 2 years, most common initial complaints were headache, nausea/vomiting, seizures, squint/diplopia, ataxia, and behavioral changes. In children younger than 2 years, the most common initial presenting symptoms were seizures, vomiting, head tilt, and behavior changes. These signs and symptoms are nonspecific, thus making the diagnosis early in the course often difficult.
The mean age at presentation of children with medulloblastoma or ependymoma is 5 years or less, whereas the mean age at presentation of the child with a JPA is 9 years. JPAs often exhibit a long history of ataxia and elevated intracranial pressure (ICP) resulting from gradual obstructive hydrocephalus. Neck pain may be a presenting symptom in cases of chronic tonsillar herniation. Medulloblastomas may produce symptoms and signs similar to those for pilocytic astrocytoma, but because they are malignant tumors and grow more rapidly, the time course of progression is usually over weeks rather than months. Ependymomas typically arise from the floor of the fourth ventricle. Nausea and vomiting as a result of irritation of the “vomiting center” near the obex is often an initial symptom. Progressive headaches usually herald the evidence of a brain tumor in children; however, headaches may be experienced by 5% to 30% of elementary school children. Most children with headache as an initial symptom of a brain tumor will show additional signs and symptoms within a relatively short period. In the study of the Childhood Brain Tumor Consortium with 3276 patients, less than 3% of children with headache caused by a brain tumor had no other abnormality on neurologic examination.
An earlier diagnosis of pediatric posterior fossa brain tumor has not been seen since the popularity of computed tomography and magnetic resonance imaging (MRI). Only a high degree of suspicion, a detailed clinical history, and a targeted neurologic examination lead to more accurate and timely diagnosis.
Presurgical Precautions
Although most children present with ventricular obstruction and symptoms of increased ICP, the vast majority can be observed in an intensive care setting and do not require placement of an emergent shunt, third ventriculostomy, or external ventricular drain (EVD). Within 12 hours of initiating intravenous steroids, most children will recognize improvement in nausea/vomiting or headache, allowing surgery to be performed on an elective basis. There are rare cases in which a child declined neurologically and had to be operated on emergently, but such cases are uncommon.
Most children with posterior fossa tumors do not require permanent shunts. The need for external ventricular drainage is determined at the time of surgery based on the turgor of the dura after the craniectomy. Again, the majority of children do not require external ventricular drainage, even though the ventricles may appear quite enlarged on preoperative imaging.
Intraoperative Precautions
Several age-dependent factors enter into the decision-making of positioning, anesthesia, and postoperative care. They are discussed in the following subsections.
Blood Transfusion
The major challenge of tumor surgery in young children is that of blood loss. The circulating blood volume of a young child is estimated at 70 cc per kilogram body weight. Loss of more than 1.5 blood volumes runs the risk of a coagulopathy. The anesthesiologist should begin replacement early when it becomes apparent that a transfusion will be necessary. Washed red blood cells are less likely to cause intraoperative problems with hyperkalemia in the child requiring large volumes of blood. Irradiated red blood cells may be given to reduce the likelihood of viral transmission to a compromised host, especially if the child is likely to require chemotherapy after surgery.
Positioning and Fixation
There are three possibilities for positioning: prone, lateral decubitus, or sitting. Each of the positions requires the head to be pinned as long as the patient is more than 2 years old. Use of pins in infants can lead to skull penetration producing depressed skull fracture, pneumocephalus, dural laceration, hematoma, or postoperative abscess. Posterior fossa surgery in children younger than 2 is probably safer if the child is in the prone position with the face down on a padded horseshoe headrest. It is important to adjust the width of the horseshoe to ensure that there is no pressure on the eyes. In this position, malar eminences or the forehead area is at risk to get pressure sores. Placing rest-on foam over the face, with the adhesive side to the skin, may help avoid pressure injury. For children over 3 years of age, the pediatric pins are utilized but tightened to only 40 pounds of pressure per inch until the pins penetrate the outer table of the skull. It is important for the pin placement to avoid the thin squamous temporal bone and shunt tubing, if present.
Prone position or concorde position (prone with neck flexed) affords many ergonometric advantages such as better visualization, better exposure, greater surgeon comfort, and minimized risk of air embolus. The most significant disadvantage of the prone position is venous congestion that can lead to more significant blood loss and soft tissue swelling of the face. This congestion can be improved by elevating the head above the level of the heart. In the prone position, care must be taken to protect the pressure points—such as the ulnar nerve at the elbow, the common peroneal nerve across fibular head, and the lateral femoral cutaneous nerve at the iliac crest—to avoid skin breakdown and compression neuropathy. Two longitudinal padded rolls are placed under the patient, and the knees and ankles are padded.
In the lateral decubitus position, the patient is lying on his side. This allows superior visualization in the lateral recesses or cerebello-pontine angle. The disadvantage of the lateral position is that the anatomy is not centered, so the surgeon must visualize all anatomic structures as rotated. The patient is placed on his side with a soft roll placed in the axilla of the dependent arm to prevent brachial plexus injury or vascular compression, and the dependent leg is padded with special attention paid to the fibular head of the upper leg to avoid peroneal palsy.
The sitting position with the patient positioned sitting upright offers a clear operative field because blood and cerebrospinal fluid (CSF) drain out of the operative site. Some studies also showed better lower cranial nerve preservation in the sitting position. However, there are many risks to the sitting position. The most significant dangers are cardiovascular instability and hypotension, venous air embolism (VAE), and subdural hematoma. Precordial Doppler ultrasonic flow and end-tidal CO 2 should be monitored throughout the case for detecting VAE. In adult studies, the incidence of end-tidal CO 2 –detected VAE is up to 15.2% compared with only 1.4% in the prone position. If the child has a shunt, it should be occluded before surgery in a sitting position to decrease the risk of subdural hematoma. For the same reason, mannitol should be used with caution in the sitting position because it has been implicated in the development of subdural hematomas. Other risks of the sitting position include tension pneumocephalus, cervical myelopathy, thermal loss (especially in children), surgeon discomfort, and the rapid escape of CSF from the ventricular system. When applying the head holder, the pin sites must be covered with Vaseline gauze to minimize entry of air. If air embolism occurs, the wound should be packed with a saline-soaked sponge, the head lowered immediately, and the atrial catheter aspirated by anesthesia to attempt to remove the embolus from the left atrium. If the embolus is severe, the patient should be placed in the left decubitus position.
Selection of Surgical Approach
The safest and most direct approach to the fourth ventricle is the midline suboccipital approach. In children, the dura is not firmly adherent to the skull, so it is safe to drill close to or even on top of the major venous sinuses. The superior and lateral limits of the craniotomy are the transverse and sigmoid sinuses. Inferiorly, the craniotomy should always include the posterior edge of the foramen magnum to prevent laceration of the brain against the closed bony rim when cerebellar elements are retracted downward or if hematoma or swelling should occur postoperatively. To expose the posterior arch of C1, monopolar cautery should be used with caution when dissecting the soft tissue over C1 (especially at the superolateral surface) to prevent vertebral artery injury. In infants or young children, C1 is often cartilaginous, and the dorsal arch does not fuse until age 3 years. C1 laminectomy is helpful for lesions that herniate through the foramen magnum. It is important to remember that extending a laminectomy below C2 in young children increases the risk of swan neck deformity.
All techniques for dural incision require crossing the occipital and annular sinuses, which may be very large in infants under age 2 years. If there is significant bleeding from the midline occipital sinus, it should be controlled with obliquely placed hemostatic clips or suture ligatures. The arachnoid is opened next over the cisterna magna to allow drainage of CSF. Gentle separation of the cerebellar tonsils will expose the cerebello-medullary fissure through the opened vallecula, giving an unimpeded view of the inferior roof of the fourth ventricle. The locations of the vermian branches of posterior inferior cerebellar artery (PICA) should be carefully dissected out because they are often tethered to the tonsils and the walls of the cerebello-medullary fissure. If there is a limitation of this exposure when approaching the rostral portion of the fourth ventricle, incising the inferior vermis of the cerebellum and retracting the two halves of the vermis may provide a greater working angle in this area and a better visualization of the midline inferior portion of the superior medullary velum and fastigium. If there is extension of the tumor through one of the foramina of Luschka into the cerebello-pontine cistern, the ipsilateral tonsil and cerebellar hemisphere can be retracted dorsally to expose it. Sometimes it is necessary to do a secondary retromastoid approach to completely resect the tumor.
Techniques for intradural exposure and resection of the tumor will vary depending upon the location and size of the tumor. Medulloblastomas can be found within the cerebellar hemisphere, but most arise from the roof of the fourth ventricle, pushing downward into the ventricle; 35% invade the brainstem, often at the obex or the floor of the fourth ventricle. If the ventricular floor is invaded by tumor, caution must be taken in manipulation of the tumor as it is debulked to avoid “floor of the fourth syndrome,” which includes an ipsilateral nuclear palsy of cranial nerves VI and VII and a contralateral hemiparesis. Irritation of the obex can induce persistent postoperative vomiting and present an aspiration risk.
JPAs arise within the cerebellar hemispheres and are usually separated from the ventricle by the ependyma. On some occasions, however, they also invade the floor of the fourth ventricle. A variant of the cerebellar astrocytoma actually arises from the brainstem and is dorsally exophytic into the ventricle or laterally into the cerebello-pontine angle. Such tumors exit the brainstem, pulling functional tissue up with them as they grow dorsally, much like the sides of a volcano. The inexperienced surgeon may be inclined to shave off these tumors flush with the floor of the ventricle or with the side of the stem, inadvertently injuring the brainstem in the process.
Ependymomas, by definition, take origin from the ependymal lining of the ventricle. They must be debulked carefully as the capsule is dissected away from neural tissue. Those arising from the floor of the fourth ventricle derive their blood supply from multiple small perforating vessels arising from the brainstem. These vessels must be meticulously coagulated and cut, because avulsion may cause them to retract and bleed into the brainstem. If they do, they should not be pursued. Gentle, regulated suction, gentle irrigation, and time will allow them to stop oozing without injuring the brainstem. A variant of the ependymoma arises from ependymal rests at the lateral margin of the foramen of Luschka and grows out the foramen into the cerebello-pontine angle. These tumors often encase the lower cranial nerves as well as the vertebro-basilar complex, and may invade the side of the pons. This tumor is one of the most formidable posterior fossa tumors for the surgeon. The skin incision begins midline but curves up behind the ear on the involved side. This allows bony removal across the midline, up to the torcular herophili, and around to the sigmoid sinus of the involved side. By gently elevating the involved cerebellar hemisphere and opening the telovelar space (cerebello-medullary fissure), one can dissect out the entire tumor and accomplish a gross total resection. One-third of these children may require temporary tracheostomies and gastrostomies, but most of them can be decannulated by 6 months postoperatively.
Intraoperative Monitoring
Intraoperative monitoring during posterior fossa surgery may be helpful if there is danger of violating the brainstem or cranial nerves. In addition, children are as much at risk of neurologic deterioration during various neurosurgical procedures as adults, and benefit as much from intraoperative monitoring. Neurophysiologic monitoring consists of two main categories of techniques: monitoring techniques and mapping techniques. Monitoring is the identification of the source of surgically induced neurophysiologic changes, allowing prompt correction of the cause before permanent neurologic impairment occurs. Mapping includes those techniques that allow the functional identification and preservation of anatomically important nervous tissue.
Monitoring refers to the continuous assessment of the functional integrity of neural pathways. The common option for direct monitoring of brainstem function is brainstem auditory-evoked potentials (BAEP). This technique produces five waves that correspond to the proximal cochlear nerve, the distal cochlear nerve, the cochlear nucleus, the superior olivary complex, and the lateral lemniscus/inferior colliculus in response to auditory stimulation. Evidence of pontomesencephalic transmission of the impulses implies that the brainstem has not been compromised. Another monitoring technique, somatosensory evoked potentials (SSEP), follows sensory signals through the medial lemniscus, tracing the pathway more laterally. Due to its distance from the floor of the fourth ventricle, SSEP is less sensitive than BAEP. Mapping with direct stimulation of the facial nerve or facial nucleus can be used to identify the integrity of cranial nerve fibers or the safe entry zones for entering the brainstem.
Finally, it is important to mention that intraoperative monitoring tends to cause the surgeon to leave more tumor behind. Recognizing that the most important predictor of survival in pediatric posterior fossa tumors is currently a gross or near-total resection, the neurosurgeon relying on intraoperative monitoring must still accomplish this goal, or the tumor will likely progress and the child will die.
Complication During the Postoperative Period
Resectable Residual Tumor
Although resectable residual tumor is not a real surgical complication, we have to remember that the single most important determinant of outcome for the majority of cases of pediatric medulloblastoma/ependymoma/pilocytic astrocytoma is the extent of surgical resection. The pediatric neurosurgeon must achieve a maximal safe resection.
Multiple reviews of outcomes of children with posterior fossa tumors have demonstrated that, regardless of histology, the extent of resection is the most important predictor of outcome. In trials of ependymoma and medulloblastoma, a gross total resection or near-total resection has been shown to double the 5-year survival of the child compared with a lesser resection. This concept is currently being questioned for certain molecular subtypes of medulloblastoma and may not remain as important as new targeted molecular therapies come into play.
It has been our practice to inform the parents preoperatively that the child will undergo a postoperative MRI within 48 hours of surgery, and that if this scan demonstrates any resectable residual tumor, the child will be returned to the operating room to remove that remnant. This is true unless the surgeon stopped the initial resection because of excessive vascularity or invasion of critical structures.
Hydrocephalus
Eighty percent of children with posterior fossa tumors have hydrocephalus at the time of presentation due to obstruction of the fourth ventricle. As such, the management of hydrocephalus is usually the first intervention. In the past, many patients with tumors and hydrocephalus underwent temporizing preoperative shunting at presentation to treat hydrocephalus and make tumor resection elective. Sainte-Rose et al. have advocated endoscopic third ventriculostomy at presentation rather than shunting. However, more recently it has been observed that preoperative dexamethasone results in significant alleviation of the symptoms and a reduction in vomiting within 24 to 48 hours. Given this, an appropriate alternative to permanent shunting is perioperative external ventricular drainage, especially if a patient presents with lethargy or obtundation. When an EVD is placed in the face of a large posterior fossa mass, consideration must be given to the possibility of upward herniation, and the rate and quantity of CSF drainage must be carefully monitored. Most of the time, CSF diversion, either temporary or permanent, is not required, and the hydrocephalus resolves once the tumor is removed. The height of an EVD can be gradually increased in the postoperative period, and in most cases the EVD can be successfully removed within a week to 10 days postoperatively.
Today, only about 10% to 20% of patients with cerebellar and posterior fossa tumors require permanent shunting. Risk factors for shunt dependence include younger age, larger preoperative ventricle size, more extensive tumors, and the presence of metastatic disease. CSF diversion is rarely required in children older than age 10. When persistent hydrocephalus is present, either ventriculo-peritoneal shunting or endoscopic third ventriculostomy can be considered. If a shunt is required for a malignant tumor, there may be an increased risk of extraneural metastasis through the shunt tubing (especially to the peritoneum).
Pneumocephalus
Pneumocephalus in the ventricles and subdural space is not uncommon after fourth ventricular surgery, especially when patients are operated upon in the sitting position. It frequently results from overdrainage of CSF through an EVD during surgery. If tension pneumocephalus is recognized intraoperatively, the patient should be placed in Trendelenburg position and the operative bed irrigated to replace air with the irrigating fluid. Symptomatic postoperative tension pneumocephalus can be treated with a small frontal burr hole to relieve the pressure caused by the trapped air. Intraventricular air may cause ventriculo-peritoneal shunt malfunction due to an air lock within the valve.
Vascular Injury
Injury to major vessels is rare with fourth ventricular surgery. The most likely artery to be injured is PICA. Most patients with PICA injury present with postoperative flocculonodular lobe dysfunction causing nausea, vomiting, nystagmus, vertigo, and inability to stand or walk without appendicular dysmetria ( Fig. 28.2 ).
