A. Central nervous system (CNS) tumors are the most common solid cancers in children. They occur almost as often as the leukemias. Although the overall 5-year survival over the past 3 decades has improved significantly, CNS tumors are still the leading cause of childhood cancer deaths [1].

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B. The majority of pediatric brain tumors are located in the posterior fossa. The opposite is true in adults, in whom two-thirds of brain tumors are supratentorial with only 15% to 20% of brain tumors in adults occurring in the posterior fossa. Approximately half of these tumors in children are malignant. Children may also have a more favorable outcome than do adults with histologically similar tumors.

CLINICAL PEARL

Most childhood brain tumors are in the posterior fossa, a critical location

II. Neuroanatomy of the posterior fossa

A. The posterior fossa is the largest and deepest of the three cranial fossae. It comprises the cerebellum, the pons, and the medulla oblongata, and lies between the foramen magnum and the tentorium cerebellum. The tentorium cerebellum, an extension of the dura mater, forms a roof or tent over the posterior cranial fossa, separating it from the cerebral hemispheres.

B. The posterior fossa is a critical location for pathology because of the limited space and the potential for involvement of brainstem nuclei. The cerebellum comprises three functionally distinct regions: The vestibulocerebellum, the spinocerebellum, and the cerebrocerebellum which, respectively, control the vestibular system, body and limb movements, and internal feedback, planning, and cognition. The cerebellum has two hemispheres; it also has a narrow, unpaired, midline zone called the vermis.

C. The cerebellum is configured with a large number of tight folds arranged in the style of an accordion, and has numerous, small granule cells. It has a myriad of functions and circuitry loops with the rest of the brain. Its role is not only in motor control and muscle tone, but in cognition, attention, emotion, and speech articulation. Hence, pathology to this region is critical [2]. The cerebellum regulates and coordinates axial and girdle musculature and receives spinal, vestibular and auditory input relayed by the brainstem nuclei. It integrates these various inputs in order to calibrate and fine tune motor activity. The cerebellum also receives and modulates dopaminergic, serotonergic, noradrenergic, and cholinergic inputs. The pons, superior to the medulla, is separated from it by a groove through which the sixth, seventh, and eighth cranial nerves emerge. It is a communication center and relays signals from other parts of the brain to the cerebellum. The pons also has nuclei dealing with a wide range of functions including sleep, respiration, swallowing, bladder control, hearing, equilibrium, and facial expressions. The medulla is the inferior portion of the brainstem and continues on to the spinal cord. The medulla is a highly complex neural structure crowded with cranial nerve nuclei. It contains cardiac, respiratory, vomiting, and vasomotor centers and deals with autonomic functions such as breathing, heart rate, and blood pressure. Therefore, it is not surprising that with all of this neural circuitry, tumors in the posterior fossa present a challenge for the anesthesiologist in the care of these highly vulnerable pediatric patients with such tumors [3].

III. Types of posterior fossa tumors and associated pathophysiology

A. Medulloblastomas are the most common pediatric brain tumors [1,2]. They arise from the cerebellar vermis, in the roof of the fourth ventricle and may grow to fill the fourth ventricle. They occur mostly in the first decade of life. These tumors are usually confined to the posterior fossa, but disseminated disease can occur, spreading along the craniospinal axis via the cerebrospinal fluid (CSF) pathways, and may also present with back pain or lower limb weakness. Disseminated disease is a predictor of poor outcome [1,2]. Occasionally, metastatic disease is seen outside of the CNS sometimes with bone or bone marrow disease especially in infants. Typically, medulloblastomas present with signs of increased intracranial pressure (ICP) related to obstructive hydrocephalus due to the growth of the tumor, and obstruction of CSF pathways at the fourth ventricle as well as at the Aqueduct of Sylvius. Since medulloblastomas arise in the cerebellar midline at the vermis, truncal or axial instability is often seen, a common finding in midline tumors. The predominant symptoms are the triad of early morning headache (60%), vomiting (67%), and ataxia or unsteady gait (40%). Nausea (39%) is also significant [4]. The lone presenting symptom in young children may be repeated vomiting which results in failure to thrive. Increased head circumference is seen in children whose cranial sutures are not yet fused; however, in these children, virtually no detectable symptoms develop early [4]. Not until spinal fluid flow becomes markedly compromised and causes rapidly enlarging head circumference, will the intracranial tumor be diagnosed. School-aged children may demonstrate a decline in school performance due to vision changes that may accompany hydrocephalus.

B. Astrocytomas comprise 40% of tumors in the posterior fossa in children. They are cerebellar astrocytomas [2]. They usually occur in the first decade and arise from glial cells, specifically astrocytes, named for their star-like appearance. They have a propensity for midline axial structures such as the brainstem and cerebellar vermis. The low-grade pilocytic astrocytomas, referred to as grade I, are slow-growing tumors that rarely spread and present the best chance for survival [2]. They form inside cysts, hence the term pilocytic. For these tumors, the degree of surgical resection is the single most important factor in determining prognosis [1,2]. After complete resection, the 10-year survival is 70% to 100%. Fibrillary astrocytomas, grade II, usually spread within the cerebellum and tend to recur. Presenting symptoms are produced by raised ICP, including headache made worse with exertion, recumbency, and early morning nausea and lethargy. The patient may also have difficulties with speech, may be confused, disoriented and have ataxic gait. As with medulloblastomas, most astrocytomas develop sporadically; however, in some patients there is a genetic tendency [2,4]. Patients with genetic syndromes including neurofibromatosis type I and tuberous sclerosis are at a higher risk of developing tumors of glial origin.

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C. Gliomas are characterized based on their histologic similarity to mature glial cells, which are cells that surround and support the proper functioning of nerve cells. Glial cells include astrocytes and oligodendrocytes. Most gliomas originate from astrocytes; so astrocytomas are also gliomas. Glial cancers, mostly infratentorial in children, consist of many heterogeneous tumors that include pilocytic astrocytomas, fibrillary astrocytomas, ependymomas, and the diffuse, intrinsic pontine gliomas [1]. In the posterior fossa, they can arise from the midbrain, pons, medulla and upper cervical cord. Pontine gliomas are the most common. Ten to twenty percent of posterior fossa neoplasms in children are brainstem gliomas and are seen in the first decade of life [2]. As magnetic resonance imaging (MRI) technology has advanced, several types have been described based on their presenting morphology. They are characterized based on their origin, tendency to remain localized, direction and extent of tumor growth, degree of brainstem enlargement, degree of growth outside of their site of origin, presence or absence of cysts, hemorrhage, and hydrocephalus [1,2]. In 2000, Choux [5] described four types of these tumors using computed tomography (CT) and MRI technology to define them. Intrinsic focal, exophytic focal, and cervicomedullary tumors, which are noninfiltrative, sharply demarcated from the surrounding tissues and are low histologic grade tumors. They also cause less brainstem edema. The intrinsic type lacks visible extension into other areas, while the exophytic type may extend posteriorly in the direction of least resistance. These three are also more amenable to surgical resection followed by radiation treatment and they have a good prognosis. The fourth type is described by Choux [5] as being diffuse and does not have well-demarcated boundaries. Their infiltrative, poorly marginated borders are less amenable to complete surgical resection. They cause diffuse infiltration and swelling in the brainstem and are refractory to treat [2]. Tissue diagnosis may alter treatment regimens.

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D. Ependymomas are tumors that arise from cells lining the ventricular system and the central spinal canal [6]. Ependymomas in the posterior fossa begin in the floor of the fourth ventricle and present as a fourth ventricular mass with cerebellopontine angle predilection. They are classified as benign or low-grade ependymoma versus anaplastic ependymoma, malignant or high grade. Half of the cases occur before the age of 5 years. Due to their location in the CSF pathways, they are likely to disseminate via the CSF. Extraneural metastases have been reported in the liver, lungs, peritoneum, pleura, and lymph node with dissemination being more frequent in children younger than 3 years old [2]. This is due to the delay in diagnosis in this age group in whom the subtle symptom presentations of lethargy and irritability are nonspecific and the tumors are, therefore, larger at the time of diagnosis; and there is reluctance to perform radiation therapy in young children due to the potential for radiation-induced neurotoxicity [1]. Since ependymomas arise in the fourth ventricle, they cause symptomatic compromise of the CSF pathway and lead to obstructive hydrocephalus and generalized intracranial hypertension [2]. Their location near the area postrema of the fourth ventricle or near vagal nuclei in the medulla is significant in that they may present with recurrent vomiting [1]. The current standard of therapy for infratentorial ependymomas includes surgical excision, the completeness of surgical excision being the most predictive variable on outcome [2,6]. The majority of complete responders have total tumor removal. However, tumor removal may be incomplete due to location near the brainstem, the worst location. Survival rates for ependymoma therefore depend on age, location, grading, as well as their histologic characteristics [2,6].

IV. Diagnostic tests and neuroimaging

A. CT is usually the first-line neuroimaging modality for patients with posterior fossa tumors [4].

B. Patients can also be diagnosed and followed up with nonenhanced and contrast-enhanced MRI of the brain and spine, an integral part of management. MRI scans have become the preferred diagnostic study for pediatric brain tumors as they avoid radiation and provide better definition of the tumor than CT. With MRI, the tumor can also be studied in multiple planes. Magnetic resonance angiography can aid in showing a better image of tumor vessels if they cannot be adequately seen on a routine MRI. Magnetic resonance spectroscopy is useful in evaluating the biochemical composition of CNS tumors, specifically choline, a marker of biomembranes, n-acetyl aspartate, a neuronal cell marker, taurine and other mobile lipids [4].

C. Single photon emission computed tomography (SPECT) is a clinical aid using radio tracer uptake to differentiate postradiation changes and gliosis from tumor recurrence [4].

V. Surgical interventions and treatment

A. The goal of therapy for all of the posterior fossa tumors is to eradicate the tumor while causing the least morbidity. Circumscribed and well-encapsulated tumors, as demonstrated by neuroimaging, can be surgically resected more completely.

B. Radical resections are especially important for children under 2 years old because of the risks of radiation to the developing brain [2,4]. Maximal tumor resection of pediatric gliomas, though controversial in adults, tend to be more valuable for children [1].

C. Many brainstem gliomas have margins that blend imperceptively with normal tissue, preventing complete resection of the tumor, so stereotactic biopsies may be the only feasible option.

D. The mainstay of treatment for pilocytic astrocytomas and ependymomas is maximal surgical resection [2]. As many of these tumors produce hydrocephalus by obstructing the flow of CSF, this is occasionally treated by performing an endoscopic third ventriculostomy or an external ventricular drain (EVD) at the time of craniotomy. If the surgical procedure has not reestablished the CSF pathways, a ventriculoperitoneal (VP) shunt may be inserted.

VI. Anesthetic management

A. Preoperative evaluation. A baseline neurologic evaluation is essential in order to be able to recognize changes from this baseline when the patient emerges from anesthesia, and to be able to differentiate from possible new neurologic impairments. Postural headaches, or irritability in a young child which worsens in the recumbent position, are suggestive of raised ICP. Full fontanel, widely separated cranial sutures, cranial enlargement, papilledema, and diplopia are also suggestive of raised ICP. If there has been protracted vomiting, the patient may be at risk for pulmonary aspiration as well as electrolyte imbalance. If the patient has any right to left intracardiac shunting due to a patent foramen ovale or ductus arteriosus, the risk of paradoxical air embolization from venous air embolism is higher in the sitting surgical position.

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B. Airway. Airway evaluation will determine whether special intubating techniques are needed, particularly in the case of distorted airway anatomy.

C. Neuromuscular blockade. Although a small and transient increase in ICP has been described with succinylcholine, the benefits of using succinylcholine for rapidly establishing intubation conditions may greatly outweigh the risks of pulmonary aspiration. High-dose rocuronium at 0.8 mg/kg may also be used to rapidly establish intubating conditions, aided by an opioid to blunt the hemodynamic response to laryngoscopy and endotracheal intubation.

D. Surgical position. Positioning requires care to pad vulnerable areas including the ulnar nerve at the elbow and peroneal nerve at the knee. The preferred surgical approach to posterior fossa tumor is the prone position, with neck flexion for optimal exposure, and the head of the patient fixed in pins, with the abdomen made free for movement during respiration. Neck flexion can change the location of the tip of the endotracheal tube, or even cause kinking of the small endotracheal tube in the posterior pharynx, so breath sounds should be reassessed after positioning. Lateral or park bench position may also be used. The lateral position may result in decreased compliance of the down-side lung [7]. In this position, a pad should be placed in the axilla to minimize the patient’s weight on the lower arm and shoulder, and to avoid brachial plexus injury. In either case, head up tilt is often employed to decrease hemorrhage, but this increases the risk of air embolism. The relatively larger head size in children places them at increased risk for air emboli. If the sitting position is adopted, a precordial Doppler monitor, the most sensitive noninvasive monitoring technique, will be essential to detect air embolism, a potentially catastrophic event. It should be placed in the middle third of the sternum or second to fourth intercostal space, to the right of the sternum, or between the right scapula and the spine in prone infants less than 6 kg [8]. Although a multiorifice, central venous catheter for aspirating any venous air is used in adults, this may not be feasible in infants and small children due to the bore of the catheter and the difficulty of aspirating any embolized air. To test for proper functioning of the Doppler probe, agitated saline may be injected through a right atrial catheter or peripheral line while listening for characteristic Doppler sounds. Doppler combined with end-tidal gas monitoring and arterial blood pressure monitoring is essential in the sitting position. A decrease in end-tidal carbon dioxide, however, is not specific for venous air embolism as a decrease in cardiac output by any cause will have the same effect.

CLINICAL PEARL

Positioning plays a critical role in posterior fossa surgery.

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