Nervous System

Published on 19/03/2015 by admin

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Last modified 19/03/2015

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Nervous System

The nervous system is an exceedingly complex entity responsible for the sensory, motor, and cognitive activities of the human body. The nervous system contains groups of neurons organized anatomically and specialized functionally for specific activities. The neurons are supported by the glia—the astrocytes, oligodendroglia, microglia, and ependyma in the central nervous system and the Schwann cells in the peripheral nervous system. This chapter examines a spectrum of diseases of the nervous system and their rule in the differential diagnosis of common neurologic clinical presentations, including headache, vertigo, seizures (epilepsy), hydrocephalus, stroke, and coma.

Cerebrovascular Disease

Cerebrovascular disease presents as a transient ischemic attack or the more severe and persistent neurologic deficit of stroke. It stems from underlying pathology of the extracranial or intracranial cerebral vasculature. The major categories are ischemic strokes due to thrombosis, embolism or hypoxia, and hemorrhagic strokes due to rupture of a cerebral vessel. Global cerebral ischemia is caused by hypotension, hypoperfusion, and low flow states and results in multifocal infarcts in the border zones (watershed areas) at the interface between the perfusion zones of 2 major arteries or more diffuse encephalopathy.

Significant obstruction of a component of the carotid or vertebrobasilar arterial trunks leads to focal cerebral ischemia or infarction. In situ thrombosis of a cerebral artery is usually secondary to atherosclerosis or, less commonly, arteritis associated with infections or collagen-vascular diseases. Other cases of cerebral infarction are due to emboli to the cerebral vasculature from thrombi formed in a diseased heart, the aorta, or a major extracranial cerebral artery. The effects of arterial occlusion can be mitigated to a variable extent by the collateral circulation, particularly through the circle of Willis at the base of the brain. Pale, nonhemorrhagic infarcts are produced by in situ thrombosis, whereas hemorrhagic infarcts due to influx of blood from collateral vessels are produced with cerebral emboli. The distinction between infarction due to in situ thrombosis versus embolization is important for optimal clinical treatment, which does not call for the use of anticoagulants in cases of hemorrhagic infarcts due to cerebral emboli.

Hypertension is the most common and important cause of primary intracerebral (intraparenchymal) hemorrhage. Other causes include vascular malformations and hematologic disorders. Hypertension produces cerebral arteriolosclerosis and Charcot-Bouchard microaneurysms. Rupture of the microaneurysm leads to hemorrhage into the brain parenchyma, with frequent extension into the ventricles and subarachnoid space. Hypertensive hemorrhages originate in the basal ganglia in approximately 75% of cases and other sites in the remainder. The most common cause of a major primary subarachnoid hemorrhage is the rupture of a saccular (or berry) aneurysm, located at bifurcation sites of the arteries of the circle of Willis.

Brain Tumors

Tumors of the central nervous system are either primary or metastatic. The more common metastatic brain tumors may take origin from virtually any primary neoplasm, but the most frequent are lung, breast, melanoma, kidney, and colon. The primary tumors of the central nervous system are classified as gliomas and nonglial neoplasms, including neuronal tumors and meningiomas. The gliomas are the most common primary tumors of the brain and include astrocytomas, oligodendrogliomas, and ependymomas. In children, most brain tumors arise in the posterior fossa and include astrocytomas and medulloblastomas of the cerebellum and gliomas of the brainstem, whereas in adults, most brain tumors arise in the cerebral hemispheres. The distinction between benign and malignant lesions is blurred because of the infiltrative growth pattern, frequent involvement of vital structures, and the tendency for lower-grade lesions to transform over time to higher-grade lesions, including the glioblastoma multiforme. Meningiomas are typically benign tumors of adults that arise from the meningoepithelial cells of the arachnoid, become attached to the dura, and produce symptoms by compression of adjacent structures. Most tumors of peripheral nerves are derived from Schwann cells. Acoustic neuroma is a single lesion that produces a mass effect in the cerebellopontine angle. Neurofibromatosis, or von Recklinghausen disease, is the prototype of a group of inherited disorders known as phacomatoses, in which defects of the neural crest lead to multifocal lesions of the nervous system and the skin.

Degenerative Diseases

Degenerative diseases are characterized by loss of neurons in various regions of the gray matter in selective patterns. These patterns characterize the various clinicopathologic conditions that have obscure etiologies. Dementia, or progressive loss of cognitive function, is a major manifestation of the degenerative diseases. Alzheimer disease is characterized by cerebral atrophy, most pronounced in the frontal, temporal, and parietal lobes and associated with the microscopic findings of neurofibrillary tangles, senile (neuritic) plaques, and amyloid angiopathy. Huntington disease is inherited with an autosomal dominant pattern and is characterized by dementia plus uncoordinated movements (chorea) and by atrophy of the frontal lobes and the caudate nucleus. Creutzfeldt-Jakob disease is characterized by spongiform degeneration of the cerebral cortex, with the pathogenesis involving mutated proteins called prions. Parkinsonism, as seen in idiopathic Parkinson disease and related conditions, is a clinical syndrome with impaired facial and voluntary muscle movements, intention tremor, rigidity, and stuttering gait. The underlying mechanism is impairment of the nigrostriatal dopaminergic system, with prominent neuronal degeneration in the substantia nigra and the locus ceruleus.

Infectious Diseases

Infections of the central nervous system may develop as a result of seeding of microorganisms via the hematogenous route, direct implantation from trauma or medical intervention, local spread from a contiguous site such as the paranasal sinuses, or retrograde spread along a peripheral nerve, as is the case with certain viral infections such as herpes simplex and rabies. Infectious meningitis of the leptomeninges and the cerebrospinal fluid (CSF) presents with fever, somnolence, and stiff neck. Examination of the CSF is important to differentiate acute pyogenic bacterial meningitis (numerous white blood cells with neutrophil predominance, high protein, low glucose) from aseptic (viral) meningitis (lymphocytic pleocytosis, moderate protein increase, normal glucose) and chronic forms of meningitis, including tuberculous meningitis (pleocytosis with mononuclear cells or mixed mononuclear cells and neutrophils, markedly increased protein level, and moderately reduced or normal glucose level). Parameningeal infections consist of brain abscess, subdural empyema, and spinal epidural abscess. Neurosyphilis occurs late in the course of approximately 10% of untreated patients and may be manifest as meningeal-meningovascular disease, dementia paralytica (general paresis), or tabes dorsalis. A number of viruses can produce encephalitis or encephalomyelitis, characterized by meningeal and parenchymal, particularly perivascular, inflammation. The viruses include arthropod-borne viruses (e.g., eastern and western equine encephalitis), herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus, cytomegalovirus, poliomyelitis, rabies, human immunodeficiency virus types 1 and 2, and viruses responsible for so-called slow virus infections, including progressive multifocal leukoencephalopathy. Immunosuppressed patients are particularly susceptible to fungal infections such as Candida albicans, Mucor species, Aspergillus fumigatus, and Cryptococcus neoformans and protozoal infections such as Toxoplasma gondii.

Disorders of the Motor Neuron, Peripheral Nerve, Neuromuscular Junction, and Skeletal Muscles

Peripheral neuropathies manifest as subacute or chronic sensory and motor dysfunction resulting from metabolic, toxic, or nutritional disorders or vasculitis, particularly polyarteritis nodosa (PAN). Guillain-Barré syndrome is an acute, rapidly progressive, ascending paralysis due to inflammatory demyelination of peripheral nerves, with potential for reversal. Myasthenia gravis is manifest as muscle weakness due to autoimmune attack on acetylcholine receptors at the neuromuscular junction. Muscular dystrophies are a heterogeneous group of inherited diseases manifest as progressive muscle weakness and degeneration of skeletal muscle, usually with onset in childhood (e.g., Duchenne muscular dystrophy). Polymyositis and dermatomyositis are autoimmune, inflammatory disorders producing proximal muscle weakness with or without skin rash.