Porth's Essentials of Pathophysiology, 4e

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Nervous System

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rarely cures. Most histologically benign tumors infil- trate the normal brain tissue, preventing total resection and allowing for tumor recurrence. Furthermore, brain tumors seldom metastasize, except within the CNS itself. Brain tumors can be divided into three basic types: primary intracranial tumors of neuroepithelial tissue (e.g., neuroglia, neurons), primary intracranial tumors that originate in the cranial cavity but are not derived from the brain tissue itself (e.g., meninges, primary CNS lymphoma, pituitary gland tumors [discussed in Chapter 32]), and metastatic tumors. 1,4,50 Collectively, neurogliomas of astrocytic origin are the most common type of primary brain tumor in adults, and medulloblas- tomas the most common type in children. Whatever the type, brain tumors commonly present with symptoms related to a disruption in neuronal activity (seizures), signs of increased ICP (progressive headache, nausea and vomiting, drowsiness, visual abnormalities), focal motor or sensory deficits, and cognitive dysfunction. 7,50 Neuroglial Tumors Neuroglial tumors, which are the most common form of primary brain tumors, derive from astrocytes, oligoden- drocytes, and ependymal cells. 1,4,51 Gliomas are divided into four grades: grades I and II are low-grade tumors, whereas grade III and IV are high-grade tumors. Astrocytomas. There are two types of astrocyte tumors: infiltrating and noninfiltrating astrocytomas. Infiltrating astrocytomas account for 80% of adult primary brain tumors. 1 They occur most commonly in the fourth through the sixth decade. Although they usually are found in the cerebral hemispheres, they also can occur in the cerebellum, brain stem, or spinal cord. The most common presenting signs and symptoms are seizures, headaches, and focal neurologic deficits related to the location of the tumor. Infiltrating astrocytomas are char- acterized by a spectrum of histologic differentiation that includes diffuse astrocytomas (grade II); anaplastic astro- cytomas (grade III); and the least differentiated and most aggressive, glioblastomas (grade IV). 1 The grade I designation is reserved for the noninfil- trating pilocytic astrocytomas, which are distinguished from other astrocytomas by their cellular appearance and their benign behavior. Typically, they occur in chil- dren and young adults and usually are located in the cerebellum, but they also can be found in the floor and walls of the third ventricle, in the optic chiasm and nerves, and occasionally in the cerebral hemispheres. These tumors grow slowly and, in the cerebellum par- ticularly, may be treated by resection. Oligodendrogliomas. Oligodendrogliomas are tumors of the oligodendrocytes or their precursors. 1 They rep- resent approximately 5% to 15% of neuroglial tumors and are most common in the fourth and fifth decades of life. The tumors are found mostly in the cerebral hemispheres, with a predilection for white matter. The prognosis of persons with oligodendrogliomas is less pre- dictable than for persons with infiltrating astrocytomas.

SUMMARY CONCEPTS

BrainTumors Primary brain tumors account for 2% of all cancers in the United States. 4,47 Metastasis to the brain from other sites is more common. Central nervous system tumors are the second most common malignancy in children and adolescents, with the incidence rate being highest in infants and children 5 years of age or younger. 48 The etiology of brain tumors is largely unknown. Several CNS tumors are associated with rare genetic conditions, most commonly the autosomal dominant disorder neurofibromatosis 1 (see Chapter 6). Although a large number of studies have examined the relation- ship between environmental and occupational factors, only high-dose radiation is a proven risk factor. 49,50 Irradiation given to treat intracranial and extracranial cancers, including prophylactic irradiation for leuke- mia, increases the incidence of brain tumors. Types ofTumors The term brain tumor refers to a collection of intracra- nial neoplasms, each with its own histology, site of ori- gin, treatment, and prognosis. 1,4 For most neoplasms, the term malignant is used to describe the tumor’s lack of cell differentiation, its invasive nature, and its ability to metastasize. However, the terms benign and malignant do not apply to brain tumors in the same sense as they do to tumors in other parts of the body. In the brain, even a well-differentiated and histologically benign tumor may grow and cause death because of its location. Also, tumors in the brain are rarely benign because surgery ■■ Infections of the CNS may be classified according to the structures involved (the meninges [meningitis] or brain parenchyma [encephalitis]) and the type of organism causing the infection (bacteria or virus).The damage caused by infection may predispose to hydrocephalus, seizures, or other neurologic defects. ■■ Except in neonates, most cases of bacterial meningitis are pneumococcal or meningococcal. The most common symptoms are sudden onset of headache, fever, and stiffness of the neck (nuchal rigidity). ■■ Viral meningitis manifests in much the same way as bacterial meningitis, but the course is less severe and symptoms resolve spontaneously. ■■ Encephalitis is usually a viral infection of the brain. In addition to fever, headache, and nuchal rigidity, patients often experience neurologic disturbances.