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Equilibrium is the ability to maintain orientation of the body and its parts in relation to external space. It depends on continuous visual, labyrinthine, and proprioceptive somatosensory input and its integration in the brainstem and cerebellum. Disorders of equilibrium result from diseases that affect central or peripheral vestibular pathways, the cerebellum, or sensory pathways involved in proprioception. Such disorders usually present with one of two clinical problems: vertigo or ataxia.
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Vertigo is the illusion of movement of the body or the environment. It may be associated with other symptoms, such as impulsion (a sensation that the body is being hurled or pulled in space), oscillopsia (a visual illusion of moving back and forth), nausea, vomiting, or gait ataxia.
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Vertigo must be distinguished from nonvertiginous dizziness, which includes sensations of light-headedness, faintness, or giddiness not associated with an illusion of movement. In contrast to vertigo, these sensations are produced by conditions that deprive the brain of blood, oxygen, or glucose (eg, excessive vagal stimulation, orthostatic hypotension, cardiac arrhythmia, myocardial ischemia, hypoxia, or hypoglycemia) and may culminate in loss of consciousness (syncope; see Chapter 12, Seizures & Syncope).
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The first step in the differential diagnosis of vertigo is to localize the pathologic process to the peripheral or central vestibular pathways (Figure 8-1). Certain characteristics of vertigo, including the presence of any associated abnormalities, can help differentiate between peripheral and central causes (Table 8-1).
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Peripheral vestibular lesions affect the labyrinth of the inner ear or the vestibular division of the vestibulocochlear (VIII) nerve. Vertigo from peripheral lesions tends to be intermittent, lasts for briefer periods, and produces more distress than vertigo of central origin (see next section). Nystagmus (rhythmic oscillation of the eyes) is always present with peripheral vertigo; in this setting it is usually unidirectional and never vertical. Peripheral lesions ...