Impulses from the proprioceptors of the joints and muscles are essential to all reflex, postural, and volitional movements. Those from the neck are of special importance in relating the position of the head to the rest of the body. The sense organs listed previously are connected with the cerebellum and pathways in the brainstem, particularly the vestibular nuclei and, via the medial longitudinal fasciculi, with the ocular motor nuclei. These cerebellar and brainstem structures are the important coordinators of the sensory data and provide for postural adjustments and the maintenance of equilibrium. They are the basis of the mechanisms whereby the perceptions of one’s self (the body schema) and one’s surroundings (the environmental schema) are matched. Accordingly, any disease that disrupts these neural mechanisms may give rise to vertigo and disequilibrium. The interdependence of the two schemata (self and environment) is ascribed to the fact that the various sense organs—retinal, labyrinthine, and proprioceptive—are usually activated simultaneously by any body movement.
One element of the sense of stable equilibrium derives from the ability to match visual and positional information during motion. Through reflex mechanisms, we come to see objects as stationary, while we are moving (mainly the ocular fixation reflex) and moving objects as having motion when we are either moving or stationary (vestibuloocular reflex). At times, especially when our own sensory information is incomplete, we mistake movement of our surroundings for movements of our own body. A well-known example is the feeling of movement that one experiences in a stationary train when a neighboring train is moving.
A factor that influences equilibrium is the effect of aging on all the afferent structures that subserve stability. The elderly may lose their balance on extending the neck, and their peripheral sensory afferents are often impaired, as are the protective postural mechanisms, making falls more frequent. A destructive lesion of one or both labyrinths may leave an elderly person permanently unbalanced, whereas a younger person largely compensates for the loss.
Clinical Characteristics of Vertigo
A careful history and physical examination usually afford the basis for separating true vertigo from the dizziness caused by near syncope, gait disorder, and anxiety. Any illusion or hallucination of motion in any plane qualifies as vertigo. The recognition of vertigo is usually not difficult when the patient states that objects in the environment have spun around or moved rhythmically in one direction or that a sensation of whirling of the head and body was experienced. A distinction is sometimes drawn between subjective vertigo, meaning a sense of turning of one’s body, and objective vertigo, an illusion of movement of the environment, but its significance is limited.
Often, however, the patient is not so explicit and a number of related experiences may be described. The feeling may be described as to-and-fro or up-and-down movement of the body, usually of the head, or the patient may compare the feeling to that imparted by the pitch and roll of a ship. Or the floor or walls may seem to tilt or to sink or rise up. In walking, the patient may have felt unsteady and veered to one side, or may have had a sensation of leaning or being pulled to the ground or to one side or another (pulsion or static tilt), as though being drawn by a strong magnet. This feeling is particularly characteristic of vertigo. Oscillopsia, a rhythmic, jerking, illusory movement of the environment, is another effect of vestibular disorder, especially if induced by movement of the head. Observant patients may actually note this rhythmic movement of the environment due to nystagmus.
Some patients may be able to identify their symptoms only when asked to compare them with the feeling of movement they experience when they come to a halt after rapid rotation. If the patient is unobservant or imprecise in descriptions, a helpful tactic is to provoke sensations by rapid rotation, or by asking the patient to stoop for a minute and straighten up; having him stand relaxed for 3 min and checking his blood pressure for orthostatic effect; and, particularly, having him hyperventilate for 3 min. Should the patient be unable to distinguish among these several types of induced dizziness or to ascertain the similarity of one of the types to his own condition, the history is probably too inaccurate for purposes of diagnosis.