Equilibrium and balance in the elderly.

Reprint requests: Claude P. Hobeika, MD, Director, Ear Medical Center, Inc., 6527 Colerain Ave., Cincinnati, OH 45239. Phone: [phone]; fax: [phone]; e-mail: [e-mail]

Abstract

Complaints of dizziness and disequilibrium increase with age. Sixty-five percent of individuals older than 60 years of age experience dizziness or loss of balance, often on a daily basis.

Some degree of imbalance is present in all individuals older than 60. This is the result of a generalized functional degradation. Initially, the imbalance is situational and manifests when the righting reflexes cannot meet the demands of a challenging environment, such as a slippery surface. As the functional degradation progresses, the imbalance occurs during everyday activities, independent ambulation becomes difficult, and the likelihood of falls increases. When instability is constant, the individual resorts to the use of a cane, a walker, or a wheelchair.

Introduction

In the elderly, falls often precipitate a series of events with catastrophic potential. The annual cost of the related injuries is estimated at more than $7 billion. [1]

The fear of falling is a major concern for the elderly. This fear is restrictive and constraining. It results in withdrawal, a progressive decrease in activity, and a steady decline in the quality of life and mental well-being. [2]

Orientation and balance

The maintenance of posture and the ability to move about the environment depend on orientation and balance. [1] Sixty-five percent of individuals older than 60 years of age experience dizziness or loss of balance, often on a daily basis. [3]

Orientation is the awareness of the relationship of the body and body parts to each other and to the environment in a dynamic and reciprocal interaction. It is a complex function that relies on multiple sensory input. Balance is the process by which individuals maintain and move their bodies in a specific relationship to the environment. It is an automatic and unconscious process that allows individuals to resist the destabilizing effect of gravity. Balance is essential for purposeful movement and effective communication.

Mechanisms of balance. To achieve balance, the body's center of gravity (COG) must be kept perpendicular over the center of the support base. [4] This is accomplished through the integration of information received from sensory organs and through the execution of coordinated and synchronized movements. [5] A loss of balance occurs when the sensory information about the position of the COG is inaccurate, when the execution of automatic righting movements is inadequate, or when both are present.

The postural control system receives information from receptors in the proprioceptive, visual, and vestibular systems, as well as from pressure sensors under the skin.

Somatosensory inputs. Somatosensory inputs provide information about the position of the body and body parts relative to each other and to the support surface. [6] Somatosensory inputs are the dominant sensory information for balance when the body is standing still on a fixed, firm surface. [7] They are seconded by visual information. Humans seem to rely primarily on signals from the pressure sensors in the legs and torso to maintain good balance.

Visual inputs. Vision informs about the physical environment and the relation of the body relative to that environment. Visual inputs are the primary back-ups when the somatosensory information becomes deficient. [8] They play a major stabilizing role when the support surface is precarious or compliant. [9] Clear vision depends on a stable gaze.

The vestibular system has both a sensory and a motor function:

Sensory function. The vestibular system measures the head's angular velocity and linear acceleration and detects head position relative to the gravitational axis. Head angular velocity is measured by the cristae of the semicircular canals, while the maculae of the statolabyrinth (utricle and saccule) register linear acceleration and changes in the gravitational force. Because the vestibular system senses head motion, it is less sensitive to body sway than is the visual or the somatosensory system. [10] When somatosensory and visual information are adequate, the vestibular system plays a minor role in the control of the COG position. Its role is dominant when there is a conflict between visual and somatosensory information and during ambulation. [11]

Motorfunction. The vestibular system controls muscular activity. During erect posture, it initiates transitory muscular contractions and controls muscle tone. In addition, it assists in stabilizing gaze during head and body movements by generating conjugate, smooth eye movements opposite in direction and approximately of equal velocity to head movements. [12] The vestibulo-ocular reflex stabilizes gaze during target fixation and unsuspected perturbation of head and body position. Gaze stabilization is essential for clear vision; it results from the combined effect of the vestibulo-ocular reflex on the nuclei of the extraocular muscles, neck proprioception, and the position of images on the retina.

The vestibulospinal reflex initiates the compensatory body movements necessary to maintain posture and to stabilize the head over the trunk. [4] There are positional, acceleratory, and righting vestibulospinal reflexes. [13] The positional reflexes are initiated by a change in the support surface. The acceleratory reflexes, attributed to the semicircular canals, assist in tilt detection and sway displacement. Righting reflexes tend to keep the head in an upright position and facilitate contraction of the neck receptors and the axial musculature. [14]

The vestibular-nuclear complex is located in the pons and consists of four major nuclei and seven minor ones. It processes information from the peripheral vestibular system and the visual, proprioceptive, tactile, and auditory systems. The vestibular nuclei are extensively connected to the cerebellum, to the nuclei of the extraocular muscles, and to the reticular formation in the brainstem. [15]

The cerebellum plays a prominent role in regulating the output of the vestibulospinal system through extensive reciprocal connections with the vestibular nuclei. Cerebellar lesions can result in severe postural disturbance. [16]

Pressure sensors. Located beneath the skin, pressure sensors measure the intensity of contact made by the different parts of the body with the environment. These sensors play a dominant role in the maintenance of balance as they inform about the base of support. [10]

Integration of input information. The inertial-gravitational reference provided by the vestibular system is critical to the resolution of sensory conflicts between visual and vestibular inputs and between spinal and vestibular inputs. The vestibular inputs are critical to the selection of appropriate postural movement strategies. The cerebellum and basal ganglia help to mediate visual, vestibular, and proprioceptive interactions and coordinate the proprioceptive reflexes subserving balance. [3] Information from proprioceptive, visual, vestibular, auditory, tactile, and stretch receptors in various organs is integrated to create a picture of the position and movements of the body parts relative to each other and to the environment. This picture is stored and constantly upgraded. It is the essence for all body movements and the determinant for sudden and rapid corrective motor activity.

Normal balance

The postural control system is continuously involved in changing situations. The most remarkable property of the postural control system is its ability to maintain useful functioning responses to many novel motion environments and to adapt to abnormal function in one or more of its components. [17]

During body motion, the various sensors subserving the balance function contribute conflicting information to the central nervous system. Head movements that occur while an individual is walking and scanning the horizon or running and catching a ball generate visual information that is at a variance with information supplied by other sensors. To contend with conflicting information from the various sensors, the postural control system has the ability to choose the orientationally correct information from among the visual, vestibular, and somatosensory inputs and to disregard the incorrect information. [18] This analysis and decision process is automatic and is influenced by cognition. The ability of the central nervous system to select, under changing conditions, the inputs that provide the functionally most appropriate orientations and to organize the necessary body movements is known as sensory organization. [19]

The development and fine-tuning of the mechanisms necessary for a stable equilibrium are dynamic and incremental processes that result from repeated interaction with the environment during the early developmental years and throughout life. Throughout life, an ongoing process of reflex recalibration takes place to adapt to alterations in the gravitational field and to changes in the musculoskeletal, vestibular, and visual systems. The corrective responses to disequilibrium are the product of memorized experience. [20] Postural stability is optimal between the ages of 30 and 60 years.

Normal everyday activities require constant adjustments and modulations. The two types of adjustments are 1) quick movements resulting from automatic responses based on feed-forward preprogrammed strategies [21] and 2) slower, more continuous, low-frequency movements that are highly dependent on internal and external references supplied by visual, vestibular, and proprioceptive feedback. [22]

Standing and moving (posture and locomotion). Normal activities require a balanced upright position. [23] The destabilizing effect of gravity makes in-place standing an unstable condition that requires periodic corrections in the form of spontaneous back-and-forth and side-to-side sways. [24] The limits of these spontaneous sways, known as the limits its of stability, are a function of the sway direction and are determined by the sensory conjuncture and the characteristics of the base of support. [25] The limits of stability around normal adults standing with the feet comfortably apart form an ellipse with an anteroposterior dimension of about 12.58 and a lateral dimension of about 168. [25-27]

The limits of stability, therefore, are a two-dimensional quantity defining the maximum possible COG sway angle beyond which a fall occurs. [26] In a normal individual, during in-place standing, the COG sways randomly within the limits of stability. When the COG is offset in a certain direction, as in bending forward or backward, small sway angles in the direction of that offset move the COG beyond the perimeter of the limits of stability.

The state of an individual's balance is described in terms of the angular displacement of the COG from the gravitational vertical. The larger the angle of COG sway, the greater the instability. The actual limit of stability is affected by the COG sway frequency. [27] The faster the COG sway, the smaller the limit of stability, as the body's momentum acts as an additional destabilizing force. A person using fast sway movements is closer to exceeding the limit of stability than an individual swaying slowly through a similar arc. When the COG moves beyond the limit of stability, a fall occurs unless a stumble or a rapid step repositions the base of support beneath the COG, securing additional support and enlarging the base of support. [27]

Locomotion is accomplished through the forward progression of the COG within the limits of stability. To move the body in phase with the head, there is an "ankle strategy"--that is, an acute recruitment of muscles from distal to proximal, with the ankle as a fulcrum. [20] When the COG approaches the outer limit of the base of support, a "hip strategy" is employed to maintain uprightness. Recruiting muscles from proximal to distal, the COG is moved in the opposite direction of the trunk movement. [28] A "stepping strategy" is adopted when the COG exceeds the limits of stability. [10] During locomotion, in addition to overcoming the effect of gravity, an individual also must compensate for the perturbing effects of body movements on the COG. Posture and locomotion depend heavily on cognition, alertness, motivation, and planning.

Impaired balance. Impaired balance is the result of inaccurate information about the position of the COG, inadequately executed movements to bring the COG to a balanced position, or a combination of both.

Vestibular information for body orientation is most important in environments that lack good somatosensory or visual cues for orientation. [29] When functional changes in the vestibular system are abrupt and severe, they precipitate an overwhelming disorganization of the vestibular reflexes. There is a spontaneous nystagmus, loss of gaze control with blurred vision, impaired orientation, and posture collapse. These signs of vestibular dysfunction are accompanied by vertigo (a sensation of rotation of the individual or the environment) and by manifestations of increased gastrointestinal activity, such as nausea, vomiting, and diarrhea.

Equilibrium is maintained through a flexible postural synergy. [30] When deterioration in the function of one or more systems subserving the balance function is progressive, balance remains unaffected as long as the central nervous system is able to adapt and to compensate for these functional changes. Disequilibrium is the consequence of inadequate balance function. [31] Imbalance will not manifest as long as compensation is adequate for the tasks at hand. Whenever the demands on the system exceed its functional capabilities, instability becomes evident. As functional competence continues to deteriorate, imbalance becomes more prevalent. Chronic instability occurs when the compensating strategies can no longer offset the functional decline.

Falls. By definition, to fall is to drop or to come down freely under the influence of gravity. Falls are the expression of postural collapse and result from a failure to resist the destabilizing effect of gravity. They occur whenever the righting reflexes are either insufficient or too slow to counter the force of attraction exerted by the earth's gravity on an individual. In the elderly, falls are usually the result of the accumulation of multiple chronic disabilities. Falling is a clinical entity in its own right. Falls are potentially preventable if the causative factors are recognized. [32] Diminished alertness, poor concentration, general fatigue, drug-induced sedation, and impaired situational judgment increase the likelihood of falls.

Among older people, falls are a strong predictor of placement in a skilled nursing facility. [33] Elderly individuals who have just one fall in which they are not hurt are three times as likely as other elderly people to go into a nursing home. Those who fall and are seriously injured are ten times more likely to go into a nursing home.

Aging and balance. Age-related morphologic changes occur in all body systems, including those essential for the maintenance of posture. In man, aging has been shown to be associated with a significant loss of hair cells in the vestibular sensors, [34] a decrease of primary vestibular neurons, [35] a diminution in the neuronal cell density of the cerebral cortex, and a decrease in the number of Purkinje's cells in the cerebellum. [36] In addition, there are degenerative changes in the sensory and motor systems, in the tendon receptors of the lower extremities, and in the musculoskeletal system. [37]

Aging and the vestibular system. The vestibulo-ocular reflex gain and dominant time constant decrease with age. [38] The decrease in vestibulo-ocular reflex gain, minimal until the fifth or sixth decade, quickens in later life. [39,40] This is probably the result of a combination of age-related changes in the hair cells at the center of the cristae, [34] a relatively selective loss of large-diameter primary vestibular afferents, and neuronal loss in the superior vestibular nuclei. [34,41] The superior vestibular nucleus is a major relay for the canal-ocular reflex.

Changes in the vestibulospinal reflex are difficult to assess because of functional overlap with sensory and motor functions. Distinction among vestibulospinal, visual, and somatosensory dysfunction is difficult. [42] The increased body sway seen after the age of 60 is the consequence of cumulative degenerative changes in the vestibular, proprioceptive, sensory, and musculoskeletal systems. [43] Increased body sway shrinks the limits of stability. As the COG moves rapidly, the momentum of the body acts as an additional destabilizing force.

Aging and the visual system. The visual system is of most importance in the control of balance, especially in the aged. [41] Degenerative ocular changes, such as macular degeneration and cataract, decrease the visual acuity and contribute to instability. Because vision operates slowly, when an older person loses balance, the visually guided postural reflexes do not react quickly enough to prevent a fall.

Stance. The elderly have a tendency to walk hunched forward, with the head fixed to the trunk or flexed at the neck and the eyes fixating on the ground in front of them. Such a stance places the COG in a forward position--that is, at or close to the anterior periphery of the limits of stability. In addition, this impairs orientation by limiting the visual field. The forward rotation of the head alters the position of the statolabyrinth relative to the gravitational axis. [44]

Instability

Instability manifests as an exaggeration of the COG sway and is the expression of the difficulty encountered in resisting the destabilizing effects of gravity. It is the consequence of the interaction between normally functioning and abnormally functioning components that result in functionally inappropriate and/or ineffective balance response. [45] As the destabilizing forces increase or the corrective measures become inadequate, or both, sway oscillations increase.

The amplitude of the COG sway, therefore, is representative of an individual's difficulty in achieving balance, and the amplitude and velocity of the sway are proportional to the difficulty experienced counteracting gravity.

Evaluation and management

Measurement of the COG sway. The COG sway can be measured by computer analysis of information received from a forceplate on which stands the individual to be evaluated. [46] The forceplate, or platform, is a rigid, flat surface supported by measuring devices that record the vertical forces exerted on the plate and calculate the position of the center of these vertical forces. This center represents the position of the COG or center of mass. [47] The position of this center is recorded and followed as it moves across the surface plate. The sway index expresses the degree of scatter of data about the mean center of balance.

Computerized dynamic posturography was introduced as a means to assess the vestibular, visual, and proprioceptive contributions to posture and the ability of the central nervous system to integrate sensory information. [48] During computerized dynamic posturography, the individual stands on a forceplate and measurements of the COG position are made on a static, moving, and compliant platform. Measurements are taken when the individual has his or her eyes opened and closed and after introducing conflicting visual and proprioceptive information. Computerized posturography assesses balance, but does not provide localizing or lateralizing information in the neurologic sense. It does not directly assess peripheral or central vestibular function; it is a technique for the appraisal of a patient's functional ability. [49] Platform posturography can be a predictor of falls. [50]

Evaluation of the individual with imbalance. The evaluation of patients with a vestibular disorder can be a most challenging task. The great overlap that exists between the different systems that subserve the balance function renders the interpretation of measurements of the vestibulo-ocular and the vestibulospinal reflexes difficult. Because of the effects of adaptation and habituation, these measurements do not reflect an organic loss but rather the functional loss that has remained uncompensated for at the time the measurements were made. [51]

In the field of vestibular disorders, there is no gold standard. Rather, experienced clinicians--making use of the history, physical examination, and a medley of laboratory tests--render their best judgment regarding a particular patient. [49]

In the elderly, the causes of unsteadiness and falls are multifactorial and overlapping. The approach to the management of an elderly individual with unsteadiness encompasses more than the diagnosis of the disease entity or entities that are causing the problem. Often, little can be done about the disease entities, and there usually is not a consistent relationship between anatomic abnormalities and physical signs or between physical signs and resulting function. [52]

Comprehensive management, therefore, includes:

* measurement of the functional competence of the vestibular, visual, proprioceptive, sensory, auditory, and musculoskeletal systems (imaging and/or laboratory tests may be necessary to arrive at a diagnosis)

* evaluation of gait and movements [53]

* evaluation of cognitive function and psychological characteristics

* determination of the impact of the functional loss (physiologic, functional, social, and societal) on the particular individual

Management of the elderly with imbalance. The goal of a management program is to prevent impairments by optimizing function. Balance reorganization strategies are the cornerstone of the management of balance disorders, especially in the elderly. They promote orientation, gaze stabilization, muscle strength, and joint mobility. The improvement to be expected from these exercises depends on accurate assessment of the causes of the imbalance, conceptualization of the exercises, severity of the impairments, the general physical and mental health of the patient, patient motivation, and family support.

Patients should be encouraged to incorporate these exercises into their daily routines and to use the new strategies in their everyday activities. The home environment should be made safe.

Despite all efforts, when the COG can no longer be maintained over the center of the base of support provided by the two feet, the base of support is extended with the use of a cane, walker, or a wheelchair.

Discussion

The opportunities to improve the quality of life are among the most satisfying experiences in medical practice. Too many individuals with imbalance and dizziness receive inappropriate and ineffectual care.

The nature, causes, and treatment of dizziness and imbalance puzzle many physicians. Patients with dizziness and imbalance provide a critical test of the communication skills, empathy, and knowledge of their caregivers. Dizziness and imbalance negatively affect the wellbeing of the elderly. The fear of falling is a cause of great concern and severely restricts activity.

Information about the causes and mechanisms of dizziness and imbalance should be disseminated not only to those concerned, but to the public at large. A coordinated effort on the part of the patients, their families, and the health professionals attending the patients is necessary for a successful outcome.

Counseling and balance reorganization strategies have proven successful in the management of balance dysfunction in the elderly. The following approach is recommended:

* Educate the elderly about the causes and effects of loss of balance. They need to be informed and play an active role in the management of their problems.

* Develop and promulgate treatment guidelines and protocols for effective care.

* Give the elderly access to proper care and effective therapy.

* Encourage collaboration between primary care physicians, occupational health professionals, and "balance specialists."

* Educate the public on the negative effects of loss of balance on individuals of all ages.

Conclusion

Imbalance and disequilibrium are a part of the aging process and the cause of the frequent falls encountered with advancing age. Falls can and should be prevented.

An accurate determination of the functional loss and a proper understanding of the nature of the dysfunctions will engender the appropriate medical and surgical treatments and the balance reorganization strategies needed to rework orientation and balance. When imbalance persists despite adequate therapies, a cane, a walker, or a wheelchair should be recommended.

References

(1.) National Institute on Deafness and Other Communication Disorders. Balance and the vestibular system. National strategic research plan. Bethesda, Md.: National Institutes of Health, April 1989:73-8.

(2.) Bhala RP, O'Donnell J, Thoppil E. Ptophobia: Phobic fear of falling and its clinical management. Phys Ther 1982;62: 187-90.

(3.) Sloane P, Blazer D, George LK. Dizziness in a community elderly population. J Am Geriatr Soc 1989;37:l0l-8.

(4.) Horak FB. Clinical measurement of postural control in adults. Phys Ther 1987;67:1881-5.

(5.) Paulus WM, Straube A, Brandt T. Visual stabilization of posture: Physiological stimulus characteristics and clinical aspects. Brain 1984; 107:1143-63.

(6.) Nashner LM. Sensory feedback in human posture control. Cambridge, Mass.: Massachusetts Institute of Technology, 1970.

(7.) Dietz V, Horstmann GA, Berger W. Significance of proprioceptive mechanisms in the regulation of stance. Prog Brain Res 1989;80:419-23.

(8.) Dornan J, Fernie GR, Holliday PJ. Visual input: Its importance in the control of postural sway. Arch Phys Med Rehabil 1978;59:586-91.

(9.) Lee DN, Lishman JR. Visual proprioceptive control of stance. J Hum Mov Stud 1975;l:87-95.

(10.) Keshner E, Peterson B. Frequency and velocity characteristics of head, neck, and trunk during normal locomotion. Soc Neurosci Abstr 1989;15:1200.

(11.) Begbie GH. Some problems of postural sway. In: de Reuck AVS, Knight J, eds. Myotatic, kinesthetic, and vestibular mechanisms. Boston: Little Brown & Co., 1967:80-92.

(12.) Ford FR, Walsh PB. Clinical observations upon the importance of the vestibular reflexes in ocular movements: The effect of section of one or both vestibular nerves. Bull Johns Hopkins Hosp 1936;58:80-8.

(13.) Granit R, Pompciano O, eds. Reflex control of posture and movement, vol.50. Progress in brain research. New York: Elsevier/North Holland Biomedical Press, 1979.

(14.) Nies N, Sinnott PL. Variations in balance and body sway in middle-aged adults: Subjects with healthy backs compared with subjects with low-back dysfunction. Spine 1991;16:325-30.

(15.) Ham CH. Vestibular rehabilitation. Philadelphia: F.A. Davis Co., 1994.

(16.) Shimazu H, Smith CM. Cerebellar and labyrinthine influences on singular vestibular neurons identified by natural stimuli. J Neurophysiol 1971;34:493-508.

(17.) National Institute on Deafness and Other Communication Disorders. Describing and understanding basic mechanisms of vestibular function. National strategic research plan. Bethesda, Md.: National Institutes of Health, 1989:78-81.

(18.) Nashner LM, Woollacott M, Tuma G. Organization of rapid responses to posture and locomotor-like perturbations of standing man. Exp Brain Res 1979;36:463-76.

(19.) Nashner LM. Computerized dynamic posturography. In: Handbook of balance function testing. St. Louis: C.V. Mosby Year-Book, 1993:280-305.

(20.) Horak FB, Nashner LM. Central programming of postural movements: Adaptation to altered support-surface configurations. J Neurophysiol 1986;55: 1369-81.

(21.) Diener HC, Dichgans J, Guschlbauer B, Bacher M. Role of visual and static vestibular influences on dynamic posture control. Hum Neurobiol 1986;5: 105-13.

(22.) Woollacott MH, Shumway-Cook A. Postural reflexes and aging. In: Mortimer JA, Pirozzolo FJ, Maletta GJ, eds. The aging motor system. New York: Praeger Publishers, 1982:98-119.

(23.) Bobath B. Abnormal postural reflex activity caused by brain lesions. Rockville, Md.: Aspen Publications, 1985: 1-113.

(24.) Diener HC, Dichgans J, Bacher M, Gompf B. Quantification of postural sway in normals and patients with cerebellar diseases. Electroencephalogr Clin Neurophysiol 1984;57: 134-42.

(25.) Nashner LM, McCollum G. The organization of human postural movements: A formal basis and experimental synthesis. Behav Brain Sci 1985;8:135-72.

(26.) Koozekanani SH, Stockwell CW, McGhee RB, Firoozmand F. On the role of dynamic models in quantitative posturography. IEEE Trans Biomed Eng 1980;27:605-9.

(27.) Nashner LM, Schupert CL, Horak FB, Black FO. Organization of posture controls: An analysis of sensory and mechanical constraints. Prog Brain Res 1989;80:411-8.

(28.) McCollum G, Horak FB. Parsimony in neural calculations for postural movements. In: Bloodel J, Dichgans J, Precht W, eds. Cerebellar functions. Berlin: Springer-Verlag, 1984:53-66.

(29.) Zacharias GL, Young LR. Influence of combined visual and vestibular cues on human perception and control of horizontal rotation. Exp Brain Res 1981;41:159-71.

(30.) Mergner T, Becker W. Perception of horizontal self-rotation: Multisensory and cognitive aspects. In: Warren R, Wertheim AH, eds. Perception and control of self-motion. Mahwah, NJ: Lawrence Erlbaum, 1990:219.

(31.) Weber PC, Cass SP. Clinical assessment of postural stability. Am J Otol 1993;14:566-9.

(32.) Tinetti ME, Williams TF, Mayewski R. Fall risk index for elderly patients based on number of chronic disabilities. Am J Med 1986;80:429-34.

(33.) Tinetti ME, Williams CS. Falls, injuries due to falls, and the risk of admission to a nursing home. N Engl J Med 1997;337: 1279-84.

(34.) Rosenhall U. Degenerative patterns in the aging human vestibular neuro-epithelia. Acta Otolaryngol 1973;76:208-20.

(35.) Richter E. Quantitative study of human Scarpa's ganglion and vestibular sensory epitbelia. Acta Otolaryngol 1980;90: 199-208.

(36.) Sloane PD, Baloh RW, Honrubia V. The vestibular system in the elderly: Clinical implications. Am J Otolaryngol 1989;10:422-9.

(37.) Fife TD, Balob RW. Disequilibrium of unknown cause in older people. Ann Neurol 1993;34:694-702.

(38.) Wall C, Black FO, Hunt AE. Effects of age, sex and stimulus parameters upon vestibulo-ocular responses to sinusoidal rotation. Acta Otolaryngol 1984;98:270-8.

(39.) Sharpe JA, Sylvester TO. Effect of aging on horizontal smooth pursuit. Invest Ophthalmol Vis Sci 1978;17:465-8.

(40.) Spooner JW, Sakala SM, Baloh RW. Effect of aging on eye tracking. Arch Neurol 1980;37:575-6.

(41.) Paige GD. Senescense of human visual-vestibular interactions: Smooth pursuit, optokinetic, and vestibular control of eye movements with aging. Exp Brain Res 1994;98:355-72.

(42.) Keshner EA, Allum JH. Plasticity in pitch sway stabilization: Normal habituation and compensation for peripheral vestibular deficits. In: Bles W, Brandt T, eds. Disorders of posture and gait. Amsterdam: Elsevier, 1986:289.

(43.) Allum JH, Keshner EA, Honegger F, Pfaltz CR. Indicators of the influence a peripheral vestibular deficit has on vestibulo-spinal reflex responses controlling postural stability. Acta Otolaryngol 1988;106:252-63.

(44.) Parker DE, Reschke MF, Arrott AP, et al. Otolith tilt-translation reinterpretation following prolonged weightlessness: Implications for preflight training. Aviat Space Environ Med 1985;56:601-6.

(45.) Black FO, Wall C, Nashner LM. Effects of visual and support surface orientation references upon postural control in vestibular deficient subjects. Acta Otolaryngol 1983;95:199-201.

(46.) Black FO, Wall C, Rockette HE Jr., Kitch R. Normal subject postural sway during the Romberg test. Am J Otolaryngol 1982;3:309-18.

(47.) Terekhov Y. Stabilometry as a diagnostic tool in clinical medicine. Can Med Assoc J 1976;115:631-3.

(48.) Black FO, Wall C, O'Leary DP. Computerized screening of the human vestibulospinal system. Ann Otol Rhinol Laryngol 1978;87:853-60.

(49.) Furman JM. Role of posturography in the management of vestibular patients. Otolaryngol Head Neck Surg 1995; 112:8-15.

(50.) Miller K, Hobeika CP, Sick S. Platform posturography as a predictor of falls. Proceedings of the Association for Research in Otolaryngology. St. Petersburg, Fla., 1997.

(51.) Fetter M, Diener HC, Dichgans J. Recovery of postural control after an acute unilateral vestibular lesion in humans. J Vestib Res 1990-91;l:373-83.

(52.) Tinetti ME. Performance-oriented assessment of mobility problems in elderly patients. J Am Geriatr Soc 1986;34: 119-26.

(53.) Hargreaves F, Scales JT. Clinical assessment of gait using load measuring footwear. Acta Orthop Scand 1975;46:877-95.

(a.) Director Ear Medical Center, Cincinnati.