Dizziness and other sensations of imbalance are, along with headache, back pain, and fatigue, among the most frequent complaints in medicine (Kroenke and Mangelsdorff). The significance of these complaints varies greatly. For the most part they are benign, but there is always the possibility that they signal an ominous neurologic disorder. Diagnosis of the underlying disease demands that the complaint of dizziness be analyzed correctly—the nature of the disturbance of function being determined first and then its anatomic localization. This approach to neurologic diagnosis is invaluable in the patient whose main complaint is dizziness.
The term dizziness is applied by the patient to a number of different sensory and psychic experiences—a feeling of rotation or whirling as well as nonrotatory swaying, weakness, faintness, light-headedness, or unsteadiness. Blurring of vision, feelings of unreality, syncope, and even petit mal or other seizure phenomena may be called “dizzy spells.” These experiences fall into four categories: (1) vertigo, a physical sensation of motion of self or the environment; (2) near syncope, a sensation of faintness; (3) disequilibrium, a disorder of imbalance of stance or gait; and (4) ill-defined light-headedness, or “giddiness,” a symptom that often accompanies anxiety. Hence, close questioning of the patient as to how he is using the term dizziness is a necessary first step in clinical work.
Several mechanisms are responsible for the maintenance of a balanced posture and for awareness of the position of the body in relation to its surroundings and to gravity. Continuous afferent impulses from the eyes, labyrinths, muscles, and joints inform us of the position of different parts of the body. In response to these impulses, the adaptive movements necessary to maintain equilibrium are carried out. Normally, we are unaware of these adjustments because they operate largely at a reflex level. The most important of the afferent impulses are the following.
1.
Visual information from the retinae and possibly proprioceptive impulses from the ocular muscles, enable us to judge the distance of objects from the body. This information is coordinated with sensory information from the labyrinths and neck (see the following text) to stabilize gaze during movements of the head and body.
2.
Impulses from the labyrinths, which function as highly specialized spatial proprioceptors and register changes in the velocity of motion (either acceleration or deceleration) and position of the body in relation to the gravitational vertical. The cristae of the three semicircular ducts sense angular acceleration of the head in the three planes of roll, pitch and yaw, and the maculae of the saccule and utricle sense linear acceleration and gravitational pull. In each of these structures, displacement of sensory hair cells is the effective stimulus. In the semicircular ducts, this is accomplished by movement of the endolymphatic fluid, which, in turn, is induced by rotation of the head. In the utricle and saccule, the hairs are displaced by the movement of the otoliths in response to gravity, thus generating a force that displaces the otoliths. This end organ is a force transducer that converts the generated force into neural impulses that are conducted down the vestibular nerve to the vestibular nuclei. In either case (angular and linear acceleration), the force causes depolarization of the nerve terminals and initiation of impulses in the vestibular nerve, with the production of two main reflex responses: the vestibuloocular, which stabilizes the eyes, and the vestibulospinal, which stabilizes the position of the head and body.
3.