Nervous System
Disorders of the Motor Neuron, Peripheral Nerve, Neuromuscular Junction, and Skeletal Muscles
In the newborn, certain forms of intracranial hemorrhage are usually related to birth trauma, and these include subdural hemorrhage, subarachnoid hemorrhage, and posterior fossa hemorrhage. However, other factors, particularly prematurity and asphyxia, are involved in periventricular and intraventricular hemorrhage. Periventricular-intraventricular hemorrhage originates in the germinal matrix and occurs with increasing frequency in relation to the degree of prematurity of the infant. Such bleeding causes a high mortality rate. Surviving infants often develop cerebral palsy.
The time of onset of prenatal injury predicts the type of maldevelopment and resultant prenatal encephalopathy characterized by defects in the formation of the neural tube (first trimester), neural proliferation and migration (second trimester), and neural organization and myelination (third trimester). Defects in neural tube formation in the first trimester result in anencephaly, encephalocele, or holoprosencephaly (arrhinencephalia), the latter characterized by a single ventricle with defective olfactory and optic systems, and impairment of caudal closure results in meningomyelocele. During the phase of neuronal proliferation, a decrease in number of neurons leads to microcephaly, whereas an increase results in megalencephaly. With defective neuronal migration, gyral formation does not occur, resulting in lissencephalia (smooth brain) or other lesions, such as agenesis of the corpus callosum. Abnormalities in intrauterine cerebral blood flow, if severe, can result in the rare disorder of hydranencephaly and, if less severe, porencephaly characterized by cystic spaces in the brain parenchyma.
Spinal dysraphism includes several conditions characterized by congenital failure of fusion of the midline structures of the spinal column. The resultant clinical spectrum ranges from an asymptomatic bony abnormality (spina bifida occulta) to severe and disabling malformation of the spinal column and spinal cord (meningomyelocele). Lesions in the lumbosacral region and higher may produce paraplegia and loss of bowel and bladder control; hydrocephalus develops in approximately 90% of cases. The hydrocephalus is related to a congenital deformity of the hindbrain, known as the Arnold-Chiari malformation, in which the posterior fossa structures are downwardly displaced into the spinal canal and interfere with the circulation and absorption of CSF.
Hydrocephalus, characterized by enlargement of the ventricles of the brain, results from increased formation or decreased absorption of CSF (communicating hydrocephalus) or from blockage of one of the normal outflow paths of the ventricular system (obstructive hydrocephalus). Obstructive hydrocephalus often results from a congenital stenosis of the cerebral aqueduct of Sylvius, but a brainstem tumor or a posterior fossa tumor encroaching on the fourth ventricle that obstructs one of the medial or lateral apertures can produce the same effect. In adults, brain tumors are the usual cause of obstructive hydrocephalus. Communicating hydrocephalus may occur in premature infants after intraventricular hemorrhage. In children and adults, communicating hydrocephalus with increased intracranial pressure may follow an intracranial hemorrhage or infection. Adults also may have normal-pressure hydrocephalus, which must be differentiated from ventricular dilatation secondary to brain atrophy (hydrocephalus ex vacuo).
Hypotonia is an important clinical sign of neurologic problems in infants and young children. Classically, the infant hangs like a rag doll when lifted under the abdomen and exhibits weakness and flaccidity of all muscles. Muscle weakness coexisting with hypotonia indicates involvement of the peripheral nervous system, whereas the presence of brisk reflexes and a positive Babinski sign indicate involvement of the central nervous system. Friedreich ataxia, which is inherited in an autosomal recessive fashion, is the most common of the spinocerebellar degenerations in older children. The differential diagnosis includes various congenital myopathies and connective tissue diseases. Neurologic disease in infants and young children can also result from several inherited single gene defects of lipid metabolism (Niemann-Pick disease, Gaucher disease, and metachromatic leukodystrophy).
Brain tumors in children are found most commonly in the posterior fossa. The more common astrocytomas and medulloblastomas develop from the parenchyma of the cerebellum. Symptoms include evidence of cerebellar dysfunction (ataxia of the trunk and extremities) and obstruction of CSF flow, leading to headache, nausea, and vomiting. Other tumors include ependymomas, which originate from the ependymal cells lining the ventricular system, and brainstem gliomas. Treatment of posterior fossa tumors involving a combination of surgery, radiation therapy, and chemotherapy, can yield a favorable prognosis, whereas the prognosis for brainstem gliomas is generally poor.
Tuberous sclerosis is a neurocutaneous syndrome caused by a genetic mutation that occurs spontaneously or is inherited as an autosomal dominant trait. Tubers, foci of abnormal neural tissue growth, form in the nervous system and the retina. The clinical features in childhood are dominated by epilepsy and mental retardation, although some patients may have neither manifestation. Cutaneous manifestations include adenoma sebaceum, depigmented nevi, a shagreen patch in the lumbar area, and subungual fibromas. Some patients have cardiac tumors, known as rhabdomyomas, or angiomyolipomas in the kidneys or both. Sturge-Weber disease, which occurs sporadically, presents with a characteristic port-wine nevus that is apparent at birth. Brain lesions consist of hypervascularity and calcification in the leptomeninges and gray matter. The course is progressive, with increasing seizures and hemiparesis.
The headache syndromes include migraine (vascular headache), cluster headache (a migraine variant), muscle contraction headache (often stress related), and headache due to temporal (giant cell) arteritis. Temporal (giant cell) arteritis is an inflammatory disease that occurs in older individuals and affects the temporal branches of the external carotid artery. A steady, aching pain in the temporal and occipital regions, often accompanied by malaise and fever, is symptomatic. The temporal and occipital arteries are firm, tender, and pulseless. Histologically, the arteries are infiltrated by lymphocytes, plasma cells, and giant cells, and the lumen is occluded by organized thrombus. Intracranial vessels are affected occasionally, and blindness can result when ophthalmic arteries are involved. The generalized malaise, muscle pain and stiffness, fever, and other symptoms constitute an associated syndrome of polymyalgia rheumatica.
Dizziness is a general term used to describe a variety of feelings related to a disturbed sense of relation to space, including unsteadiness and giddiness. Vertigo refers more specifically to a sensation of exterior motion, often described as spinning, turning, or rotating of the external environment in relation to the person. Vertigo may be caused by dysfunction of any of the structures that are involved in sound detection or the relay of signals from the vestibular apparatus of the ear to the brain. Useful clues to localization include an analysis of effect of movement or change in position, features of the motion, abnormal symptoms or signs related to dysfunction of adjacent structures, previous attacks, nystagmus, and laboratory tests such as electroencephalography, audiometric tests, computed tomography (CT), and magnetic resonance imaging (MRI).
Seizures are triggered by the sudden and intense increase in the discharge of neurons and constitute the symptomatic expression of epilepsy. Primary seizures are of unknown etiology and are typically generalized without (petit mal) or with tonic-clonic muscle contractions (grand mal). Secondary seizures, which may be focal or generalized, result from an identifiable pathologic lesion or disease process, which may be either intracranial or extracranial. The most common intracranial lesions causing seizures are tumors, vascular lesions, head trauma, infectious diseases, congenital defects, and biochemical or degenerative diseases affecting the brain. Extracranial causes of seizures include various metabolic, electrolyte, and biochemical disturbances; fever; inborn errors of metabolism; anoxia; hypoglycemia; toxic processes; and drugs or abrupt withdrawal from drugs or alcohol.
Coma results from loss of consciousness as indicated by the complete absence of awareness of the environment or response to environmental stimuli. Confusion and stupor represent lesser degrees of impairment of consciousness. Consciousness is maintained by coordinated neural activity in both cerebral hemispheres reinforced by the reticular activating system located in the tegmentum of the brainstem. Consciousness is diminished or lost by major impairment of the reticular activating system or extensive damage to both cerebral hemispheres. The basic pathophysiologic mechanisms for loss of consciousness are (1) bilateral cerebral hemisphere disease, (2) unilateral cerebral hemisphere lesion with compression of the brainstem, (3) primary brainstem lesion, and (4) cerebellar lesion with secondary brainstem compression. These should be differentiated from nonorganic or feigned stupor.
Stroke refers to a constellation of disorders in which brain injury is caused by a vascular disorder. The 2 major categories of stroke are ischemic, in which inadequate blood flow due to thrombosis, embolism, or generalized hypoxia causes one or more localized areas of cerebral infarction, and hemorrhagic, in which bleeding in the brain parenchyma or subarachnoid space causes damage and displacement of brain structures. The clinical spectrum of focal cerebral ischemic events includes transient ischemic attacks, residual ischemic neurologic deficit, and completed infarction.
Atherosclerosis is characterized by the development of foci of intimal thickening composed of variable combinations of fibrous and fatty material and known as fibrous (atheromatous) plaques. Such lesions tend to form adjacent to branch points in arteries. The fibrous plaques may remain static, regress, progress, become calcified, or develop into complicated atheromatous lesions called dangerous or vulnerable plaques because they are responsible for clinical disease. Complications include loss of endothelial integrity, overt surface ulceration, aggregation of platelets and fibrin on the eroded plaque surface, hemorrhage in the plaque, formation of mural thrombi, embolization of plaque contents or thrombotic material or both, and total arterial occlusion by thrombus. The consequences of thrombotic occlusion are variable and unpredictable depending on the extent of disease and the amount of preexisting collateral blood flow. Thrombotic occlusion often results in tissue infarction.
Stenosis or occlusion of a carotid artery accounts for a high percentage of strokes. The most common location for atherosclerosis in the carotid system is at the bifurcation of the common carotid artery into the internal and external carotid arteries. Atherosclerotic stenosis at the origin of the common carotid artery is rare, but aortic arch arteritis (Takayasu disease) can occlude the proximal common carotid artery and other aortic branches. The extracranial pharyngeal portion of the internal carotid artery is usually spared of atherosclerosis but is subject to fibromuscular dysplasia and medial dissection. Atherosclerosis can affect the siphon portion of the carotid artery and the site of bifurcation of the internal carotid artery into anterior and middle cerebral arteries after it has begun its intracranial course. Ischemia in the internal carotid territory can lead to visual field defects, language defects, and hemiparesis or hemiplegia.
Collateral circulation occurs by blood flow from the vasculature supplied by one major blood vessel into the vascular branches of another major blood vessel through small vascular channels that connect the two systems. Occlusion of the internal carotid artery can be partially ameliorated through collateral circulation. The major extracranial pathways of collateral circulation are anastomoses between the ophthalmic artery and branches of both external carotid arteries. The major intracranial pathways of collateral circulation are the anastomoses formed by the circle of Willis at the base of the brain. The amount of collateral circulation is determined by the specific anatomy of the vascular connections and the extent and distribution of vascular disease.
The internal carotid artery bifurcates within the cranial cavity into the anterior cerebral artery (which supplies the anterior paramedian cerebral hemisphere) and the larger middle cerebral artery (which supplies the lateral cerebral hemisphere and most of the basal ganglia) after giving origin to the ophthalmic, anterior choroidal, and posterior communicating artery branches. The middle cerebral artery contributes penetrating lenticulostriate branches that arise from its horizontal main stem and trifurcates near the lateral cerebral (sylvian) fissure into major superior and inferior trunks and a small anterior temporal artery. Occlusion of the main stem of the middle or anterior cerebral artery or their superficial branches is usually caused by an embolus from the heart or the proximal vessels, particularly the internal carotid artery.
Atherosclerosis involves large- and medium-sized cerebral arteries, whereas hypertension produces disease of small penetrating arteries of the brain. Progressive arteriolosclerosis develops in the small vessels. Hyaline and fibrinoid material thickens the wall and obliterates the lumen. The lacunae (holes), the small, round lesions deep in the brain parenchyma, are commonly found in the brain at autopsy. Some lesions are clinically significant. A small infarct in the base of the pons or internal capsule can produce a pure motor hemiplegia with contralateral weakness of the face, the arm, and the leg but no sensory, visual, or intellectual defects. Other lesions can produce pure sensory strokes. Lacunar lesions in the pons can produce several syndromes, including hemiparesis coupled with ataxia.
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