Porth's Essentials of Pathophysiology, 4e

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Nervous System

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the environment in motion (i.e., objective vertigo) or the person may be in motion and the environment stationary (i.e., subjective vertigo). Persons with vertigo frequently describe a sensation of spinning, “to-and-fro” motion, or falling. Vertigo should be differentiated from light-headedness, faintness, or syncope. 74–77 Presyncope, which is character- ized by a feeling of light-headedness or “blacking out,” is commonly caused by postural hypotension (see Chapter 18) or a stenotic lesion in the cerebral circulation that limits blood flow. An inability to maintain normal gait may be described as dizziness despite the absence of objective vertigo. The unstable gait may be caused by dis- orders of sensory input (e.g., proprioception), peripheral neuropathy, or gait problems, and usually is corrected by touching a stationary object such as a wall or table. Vertigo can result from central or peripheral vestibular disorders. Vertigo due to peripheral vestibular disorders tends to be severe in intensity and episodic or brief in duration. In contrast, vertigo due to central vestibular causes tends to be mild and constant and chronic in duration. Motion sickness is a form of normal physiologic vertigo. It is caused by repeated rhythmic stimulation of the vestibular system, such as that encountered in car, air, or boat travel. Vertigo, malaise, nausea, and vomiting are the principal symptoms. Autonomic signs, including lowered blood pressure, tachycardia, and excessive sweating, may occur. Hyperventilation, which commonly accompanies motion sickness, produces changes in blood volume and pooling of blood in the lower extremities that lead to postural hypotension and sometimes to syncope. Some persons experience a variant of motion sickness, complaining of sensing the rocking motion of the boat after returning to ground. This usually resolves after the vestibular system becomes accustomed to the stationary influence of being back on land. Disorders of peripheral vestibular function occur when signals from the vestibular apparatus are distorted, as in benign paroxysmal positional vertigo, or are unbalanced by unilateral involvement of one of the vestibular organs, as in Ménière disease. The inner ear is vulnerable to injury caused by fracture of the petrous portion of the temporal bones; by infection of nearby structures, including the middle ear and meninges; and by blood-borne toxins and infections. Damage to the vestibular system can occur as an adverse effect of certain drugs or from allergic reactions to foods. The aminoglycosides (e.g., streptomycin, gentamicin) have a specific toxic affinity for the vestibular portion of the inner ear. Alcohol can cause transient episodes of vertigo. The cause of peripheral vertigo remains unknown in approximately half of the cases. Severe irritation or damage of the vestibular end- organs or nerves results in severe balance disorders reflected by instability of posture, ataxia, and falling Disorders of Peripheral Vestibular Function

Direction of spin

Horizontal canals

Left ear

Right ear

Direction of endolymph movement

Hair displacement

Nerve discharge

Slow

Slow

Nystagmus

Fast

Fast

drifting of eye movement in the opposite direction, thus stabilizing the binocular fixation point. This pattern of slow–fast–slow movements is nystagmus (Fig. 38-22). Clinically, the direction of nystagmus is named for the fast phase of nystagmus. Spontaneous nystagmus that occurs without head movement or visual stimuli is always pathologic. It seems to appear more readily and more severely with fatigue and to some extent can be influenced by psychological factors. Nystagmus due to a CNS pathologic process, in contrast to vestibular end-organ or vestibulocochlear nerve sources, seldom is accompanied by vertigo. If present, the vertigo is mild. Nystagmus eye movements can be tested by caloric stimulation or rotation (to be discussed). Vertigo Disorders of vestibular function are characterized by a condition called vertigo, in which an illusion of motion occurs. With vertigo, the person may be stationary and FIGURE 38-22. Effect of spinning a subject clockwise. On acceleration, the endolymph in the horizontal ducts will lag behind with respect to movement of the duct wall.The hairs of cristae will be displaced to the left. In the left semicircular duct, hair displacement is away from the kinocilium, leading to decreased nerve discharges below the resting level. On the right, hair displacement is toward the kinocilium, leading to an increase in nerve discharge above the resting level. (From Sekurt FE. Basic Physiology for the Health Professions. 2nd ed. Boston, MA: Little, Brown; 1982:140.)