Computerized dynamic platform posturography

TECHNOLOGY ASSESSMENT

Computerized dynamic platform posturography EDWIN M. MONSELL,MD, PhD,JOSEPH M. FURMAN, MD, PhD,SUSAN J. HERDMAN, PhD, HORSTR. KONRAD, MD, and NEILI". SHEPARD,PhD,Detroit, Michigan, Pittsburgh, Pennsylvania, Miami, Florida, Springfield, Illinois, and Ann Arbor, Michigan Computerized dynamic platform posturography is defined in this technology assessment. The review discusses what computerized dynamic platform posturography measures, what the reliability and validity of the information are, and the uniqueness of the information provided. The clinical contribution and indications for testing are discussed. There are comments on future directions for research on computerized dynamic platform posturography and a summary and conclusion. (Otolaryngol Head Neck Surg 1997;117:394-8.)

DEFINITION Postural sway has long been recognized as an important indicator of balance function. Strain gauges incorporated into force platforms have been used to record postural sway too small to be seen by the unaided observer. Such devices recently have been linked to electronic digital computers to produce computerized dynamic platform posturography (CDPP). In CDPP a force platform is mechanized and digitally controlled so that it either can move in a horizontal plane, that is, translate, or can rotate out of the horizontal plane, that is, pitch the patient either forward or backward. 1-4 CDPP has been combined with visual stimuli to determine the relative importance of vision and somatosensation to balance. Several characteristics are considered necessary for a device to qualify as CDPP in this technology assessment. The device must be capable of measuring postural sway, that is, changes in the center of the patient's applied force. The device must incorporate means of

From Henry Ford Hospital (Dr. Monsell), the University of Pittsburgh (Dr. Furman), the University of Miami (Dr. Herdman), Southern Illinois University (Dr. Konrad), and the University of Michigan (Dr. Shepard). Approved by the Board of Directors of the American Academy of Otolaryngology-Head and Neck Surgery Foundation, July 13, 1996. Reprint requests: Edwin M. Monsell, MD, PhD, Department of Research, American Academy of Otolaryngology-Headand Neck Surgery Foundation, One prince Street, Alexandria, VA 22314. Copyright © 1997 by the American Academy of OtolaryngologyHead and Neck Surgery Foundation, Inc. 0194-5998/97/$5.00 + 0 2311177659

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shifting, tilting, or otherwise perturbing the support surface or other methods of challenging the patient with unexpected movements. The device should allow at least some degree of isolation of visual and somatosensory contributions to balance so that these influences can be analyzed separately from vestibular influences. The data must be capable of quantification and signal analysis. Although most of the data on which this assessment is based come from studies of one proprietary device (Equitest, N e u r o C o m International, Clackamas, Ore.), this technology assessment pertains to any or all posturographic devices as defined herein.

TECHNOLOGY ASSESSMENT W h a t Is CDPP Used to Measure? What Kind of Information M a y Be Expected? CDPP is used to quantify the force applied by the body to a platform equipped with strain gauges. Controlled by an electronic digital computer, the device is used to measure postural sway in several test conditions. These test conditions are intended to challenge the mechanisms of postura! control. It is possible to manipulate both somatosensory and visual feedback to some extent so that their influence on balance can be altered, allowing assessment of a patient's ability to use vestibular sensations for maintaining balance. Test results are believed to relate to a patient's ability to maintain and regain balance and may reflect abnormalities of a particular sensory modality or the processing of that sensory modality in the central nervous system. Patient responses are compared with responses from an age-appropriate group of persons without symptoms

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to assess the presence of statistically defined abnormalities. Abnormalities identified by means of CDPP reflect balance dysfunction as caused by biomechanical or neurologic conditions. CDPP also may provide information about a patient's ability to adapt or habituate responses to floor movements that do not truly threaten upright postural stability. The information that can be obtained with commercially available clinical CDPP devices includes the vertical and horizontal (shear) forces generated by the patient during postural sway that is either spontaneous or a response to platform movements. Analysis of bodily movements during postural sway can be enhanced by use of motion analysis data, which include records of the position and rotation of individual body segments, or by means of analysis of electromyographic discharges. Ground reaction forces have been used to infer particular types of postural sway. Patients sway about the ankle (ankle strategy) as an inverted pendulum or flex about the hip (hip strategy). Hip strategy is associated with a predominance of shear forces, whereas ankle strategy is characterized by a predominance of changes in vertical reaction forces without much shear. If some simplifying assumptions are made, CDPP can be used to infer the extent to which these two strategies are used by a patient on the day of testing. 5 Whether these simplifying assumptions are valid has been a topic of some controversy.6 W h a t Is the Reliability Information?

and Validity

of This

CDPP has fulfilled most criteria that would be required of a reliable and valid test of postural stability. The test-retest variability has been reported. 7,8 CDPP has been shown to enable differentiation between subjects with normal and abnormal vestibular function as compared with the clinical standard of reference. 8-1° Developmental maturation of children and decline of function of elderly persons have been demonstrated.~l-14 The effects of vestibulotoxic drugs have been shown.15 Acute alteration of postural stability was demonstrated by subjecting healthy persons to rotational accelerations in a controlled experiment. Alteration of postural stability and its postflight recovery were demonstrated by space shuttle astronauts. 16 The latter work shows that CDPP can be used to measure impairment of postural stability produced by central nervous system adaptation by healthy subjects to microgravity and to measure the recovery of postural stability that occurs after adaptation to gravity. Recovery of postural control after a unilateral vestibular lesion has been measured with CDPP. 17,1s CDPP has been used to differentiate patients

with polyneuropathies from healthy persons. The effect of otitis media on the vestibular system has been demonstrated in studies with children. 19-22 The reliability of subtests of CDPP in identifying vestibular causes of abnormal postural stability has been evaluated. 8 Aphysiologic patterns of sway in which subjects perform better under more difficult than less difficult test conditions have been described. Identification of these patterns may be useful in detecting nonorganic postural dysfunction. 8,12,23 CDPP has been reported to be a useful measure of progress in vestibular rehabilitation. ~2 That CDPP can be used to measure something of clinical value is also suggested by its ability to help one predict components of a symptom inventory, such as the Dizziness Handicap Inventory.24 The clinical utility of CDPP has been supported by widespread opinion, 25-27 but is difficult to establish experimentally for several reasons. Because vestibulospinal disorders are characterized mainly by subjective sensations, they are relatively difficult to define and measure. Moreover, the measurement of such sensations is subject to large variances, in part because of the prominent emotional reactions produced by symptoms of vestibular disorders and by the dynamics of vestibular compensatory mechanisms. Demonstrating improved function or earlier return to work uniquely because of the use of CDPP would be a good criterion of validity; however, such a study would be expensive and difficult to conduct and may not be feasible. Is the Information Provided by CDPP Unique? Can the Same Information Be Obtained from Other Vestibular Laboratory Tests or Clinical Evaluations?

Etectronystagmography and rotational chair tests axe limited to assessment of visual-vestibular interactions. CDPP provides information on postural stability and motor control, which relate to balance function. The Romberg test, which is a part of a physical examination, can be a useful qualitative indication of body sway. However, to assess a patient's ability to maintain upright stance, it is helpful to have quantitative measures and to isolate each of the sensory systems (visual, vestibular, and somatosensory) that contribute to balance. When these input systems are systematically altered, increases in the challenge of the balance task are achieved. 17,28 Methods to duplicate this type of assessment in a low-tech clinical format have been reported, for example, the "foam and dome" method, or the Clinical Test for Sensory Interaction of Balance.29, 30 A recent investigation indicated that the main pat-

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terns of abnormality and normality were well identified by means of either foam posturography or CDPP; however, foam posturography did not enable recognition of subtle abnormalities and led to misidentification of some patterns of abnormality. 31 Foam tests are limited by the relative inability of findings to be quantified or standardized. CDPP appears to provide unique information that quantifies a patient's ability to use vision and somatosensation in maintaining postural stability. W h a t Does CDPP Management?

Contribute

to

Clinical

CDPP has been used to monitor improvement in postural stability during vestibular rehabilitation. 32,33 Patients with chronic vestibular deficits had improved postural stability, based on CDPP findings, after 6 weeks and after 12 weeks of vestibular exercises when compared with a group of patients performing general conditioning exercises. 31,32 Improved postural control, based on CDPP findings, also has been demonstrated among patients who performed vestibular adaptation exercises during the acute stage after unilateral vestibular loss compared with a control group of patients. 34 In both studies, the patients had statistically significant improvement in sense of stability in their usual activities (decreased disequilibrium), which was reflected in CDPP results. Differences in CDPP results among patients with chronic vestibular deficits have been shown to vary with the type of exercises patients perform. 33 It has been suggested that patients' performance on CDPP can be used to develop customized rehabilitation programs. 35'36 CDPP results also have been used as a criterion for inclusion in a vestibular rehabilitation p r o g r a m s CDPP has been used to show discouraged patients in therapy how much progress they have made, encouraging them to continue their exercises. Studies have suggested that data from CDPP can be related to the severity of vestibular or visual loss. 37,38 CDPP has been used to follow improvement in balance function after the onset of unilateral vestibular losses. 19,39 CDPP has been used to differentiate between fallers from nonfallers among elderly persons, but the results do not correlate with fear of falling or with frequency of falls. 21 The ability of CDPP to help one predict the results of treatment is not well established. CDPP performance preoperatively or on the third postoperative day did not allow prediction of recovery of postural stability (assessed by means of physical examination of gait and subjective report of disequilibrium) during the acute recovery period after resection of acoustic neuroma. 34 Among patients with chronic vestibular deficits, how-

ever, pretherapy posturography scores did allow prediction of overall therapy outcome. 12 W h a t Are Appropriate Indications for Testing?

Clinical history, physical examination, and vestibular and other laboratory tests are helpful and sometimes essential in the evaluation of balance problems. CDPP adds additional information about vestibulospinal function, stance, and sway. It does not replace other tests but supplements them. Patients in each of the following categories may be candidates for testing with CDPP: 1. Patients who are undergoing balance rehabilitation. CDPP may be the best method to evaluate patients' postural control under various conditions and to monitor progress. 32,4° 2. Patients who have symptoms of disequilibrium for whom conventional tests of vestibular function have not detected an abnormality. Because it is used to test vestibulo-oculomotor reflexes primarily mediated by the lateral semicircular canal, electronystagmography does not test many of the vestibular receptors. CDPP may be helpful when it is important to document whether an abnormality in postural control is present. It may show an abnormality for patients who have dysfunction of the other receptors that are important for balance (or their interactions) or may point to nonorganic disorders.23, 41-43 3. Patients who are being evaluated for balance impairment after trauma. 44'45 Either brain trauma or damage to the inner ear may result in disequilibrium and impaired postural stability. Posturography may help identify and characterize abnormalities of vestibulospinal function when other tests do not. 4. Disability and return-to-work assessment for patients with vestibular and neurologic disorders, such as multiple sclerosis or spinocerebellar degeneration and Parkinson's disease. 46,47 Posturography may contribute another useful measurement of dysfunction or return to normal function for these patients. 48-50 5. Patients who are receiving potentially vestibulotoxic medications, are in environments where the vestibular structures of the inner ear may be damaged, 5153 or are in environments that alter inner ear function, for example, microgravity. 17'54 It is possible that ototoxic medications or environmental toxins selectively damage vestibular sensors other than the lateral semicircular canals or the vestibulo-oculomotor reflexes. 55'56 6. Patients with a history of falls and aging patients

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with disequilibrium. Aging patients are most prone to falls and injuries related to falls. Many of these patients do not have true vertigo but instead exhibit chronic disequilibrium. CDPP may help identify deficits in balance function when the vestibulooculomotor reflexes are intact, leading to intervention.5,12,57,58 7. Patients who may have a nonorganic sensation of imbalance. These patients may have recovered balance performance yet still perceive disequilibrium or may be malingerers. 8'23,59 FUTURE DIRECTIONS

Future research on posturography may help to improve the delineation of test-retest variability. This information would help to clarify the limits of CDPP for measuring changes in postural stability. More studies that address the clinical utility of CDPP would be welcome. More needs to be understood about how CDPP relates to the functional capabilities of patients. SUMMARY AND CONCLUSIONS

CDPP is an established test of postural stability. Although some of the information provided by CDPP can be gained by means of a careful physical examination, much is unique, particularly because the data are quantitative. CDPP may be useful in the care of patients with symptoms or signs of chronic balance dysfunction when combined with the clinical history, physical examination, and data from other vestibular tests. Its value depends on patient selection, technical performance of the test, and interpretation by a knowledgeable clinician. Additional research will expand the clinical utility of CDPR REFERENCES 1. Diener HC, Dichgans J, Guschlbauer, Bacher M. Role of visual and static vestibular influences on dynamic posture control. Hum Neurobiol 1986;5:1065-113. 2. Nashner LM, Black FO, Walt C. Adaptation to altered support and visual conditions during stance: patients with vestibular deficits. J Neurosci 1982;2:536-44. 3. Allure JHJ, Keshner EA, Honegger F, Pfaltz CR. Indicators of the influence a peripheral vestibular deficit has on vestibulospinal reflex responses controlling postural stability. Acta Otolaryngol (Stockh) 1988;106:252-63. 4. Nardone A~ Giordano A, Corra T, Schiepati M. Responses of leg muscles in humans displaced while standing. Brain 1990;113:65-84. 5. Shepard NT, Schultz A, Alexander NB, Gu MJ, Boismier T. Postural control in young and elderly adults when stance is challenged: clinical versus laboratory measurements. Ann Otol Rhinol Laryngol 1993; 102:508-17. 6. Barin K. Dynamic posturography: analysis of error in force plate measurement of postural sway. IEEE Trans Biomed Eng 1992;11:52-6. 7. Ford-Smith CD, Wyman JF, Elswick LK, Fernandez T, Newton

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