Role of posturography in the management of vestibular patients

Role of posturography in the management of vestibular patients JOSEPHM. FURMAN,MD, PhD,Pittsburgh, Pennsylvania

Computerized dynamic posturography uses a force platform that can both translate and rotate. When combined with visual stimuli, it can be used to determine the relative importance of the various sensory inputs critical for balance, namely vision, somatosensation, and vestibular sensation. Studies to date have shown several patterns of abnormalities on posturography, the best recognized being the vestibular pattern. Many patients with a high suspicion of having a peripheral vestibular abnormality [based on other information) have such a pattern. Moreover, few normal subjects have such abnormalities. Although computerized dynamic posturography does not help in localizing a lesion or determining its etiology, such testing does provide a functional measure that can be helpful in understanding a patient,s ability to properly use vestibular system information. Posturography is helpful in the management of patients with suspected vestibular disorders. In addition to suggesting the presence of a vestibular system abnormality, particularly an uncompensated peripheral vestibular deficit, a bilateral peripheral vestibular deficit, or a central vestibular abnormality, computerized dynamic posturography may help guide the clinician regarding choice of treatment and counseling of patients. Further studies are required to determine the ultimate role of computerized dynamic posturography in the management of patients with vestibular disorders. [OTOLARYNGOLHEADNECKSURG1995;112:8-15,]

R

[Wosturography has existed for several centuries. 1 Stationary force platforms, equipped with strain gauges, have been used to record postural sway too small to be observed unaided. 2 The use of force platforms to record sway during simple, quiet standing has not proved to be particularly useful clinically when assessing patients with vestibular disorders. Recently, the field of posturography has been advanced by the development of computerized dynamic posturography, wherein a force platform is mechanized so that it can either move in a horizontal plane, that is translate, or rotate out of the horizontal plane, that is pitch the subject either forward or backward. 3-6 Computerized dynamic posturography has been combined with visual stimuli as a means of

From the Departmentof Otolaryngology,Universityof Pittsburgh School of Medicine. Presented at the Clinical Applications of Vestibular Science, Universityof California,Los Angeles Schoolof Medicine,Los Angeles, Calif., Feb. 12-13, 1994. Received for publication July 14, 1994; accepted July 15, 1994. Reprint requests: Joseph M. Furman, MD, PhD, Eye and Ear Hospital of Pittsburgh, 203 Lothrop St., Suite 500, Pittsburgh, PA 15213. Copyright© 1995by the AmericanAcademyof OtolaryngologyHead and Neck SurgeryFoundation,Inc. 0194-5998/95/$3.00 + 0 23/1/59193

determining the relative importance of the various sensory inputs critical for balance: namely, vision, somatosensation, and vestibular sensation. When compared with other tests currently available for vestibular function assessment, computerized dynamic posturography is unique in that it assesses "balance" rather than attempting to assess peripheral or central vestibular function more directly. Although stability during upright stance undoubtedly depends on vestibular function and, more specifically, vestibulospinal function to a large extent, stability also depends on other sensory inputs including vision and somatosensation and on the motor control system, especially that concerned with the lower extremities and the trunk. Because of its recent popularity for the assessment of patients with suspected vestibular disorders, its wide availability, and the burgeoning literature on the topic, this discussion will focus on the role of computerized dynamic posturography, as conducted with the EquiTest platform (NeuroCom International, Inc., Clackamas, Ore.). The EquiTest platform can both translate and rotate. During testing, a computer monitors the forces generated by the patient on the platform, including vertical forces and horizontal plane (shear) forces. The testing software supplied by NeuroCom International, Inc. allows

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two broad categories of tests: (1) recording of responses to small brief movements of the support surface, either translations or rotations; and (2) recording of postural sway during various combinations of sensory inputs. These two types of tests have been called the motor control test (formerly the movement coordination tests) and sensory organization test. The motor control test uses a total of nine forward and nine backward translations of three different magnitudes. Also, five sequential platform rotations are delivered in the toes-up and then the toes-down direction. The forces generated during these maneuvers are analyzed by computer. Patient responses are compared with responses from an age-appropriate group of normal subjects. The sensory organization test uses six conditions: (1) platform stable with eyes open with a stationary visual surround, (2) platform stable with eyes closed, (3) platform stable with eyes open with a moving visual surround, (4) platform moving with eyes open with a stationary visual surround, (5) platform moving with eyes closed, and (6) platform moving with eyes open with a moving visual surround. For the two conditions in which the visual surround is moving (3 and 6) and for the three conditions in which the platform is moving (4, 5, and 6), the movement of the visual surround and/or the platform is coupled to the sway of the patient in an attempt to "stabilize" the visual surround or platform rotations, thereby providing a nearly null or at best distorted input from that sensation (Fig. 1). The sensory organization test is the portion of computerized dynamic posturography that is most useful in the assessment of patients with suspected vestibular disorders. The sensory organization test was designed with the idea that by providing reduced or distorted sensory information from the visual system and somatosensory system, patients could be forced to rely on their vestibular sensations to maintain upright balance. In this manner, conditions 5 and 6 assess how patients use vestibular information when it is the only available input providing reliable information. Why use computerized dynamic posturography? Table 1 lists the potential uses of computerized dynamic posturography. Indeed, the same questions could be asked about any vestibular laboratory tests: How does posturography aid in diagnosis? Specifically, does posturography provide information that allows localization of a lesion or lateralization of a lesion? Does posturography provide information useful to improving the choice of treatment for particular patients? Does posturography assist in the long-term treatment of patients with vestibular

Table t. Potential uses for p o s t u r o g r a p h y Diagnosis Dysfunction Localization Lateralization Cause Choice of therapy Management

disorders? These questions are implicit in any discussion of the role of computerized dynamic posturography. The material below reviews the current status of this test on the basis of published material. Although some conclusions can be drawn regarding the role of computerized dynamic posturography in the assessment of patients with suspected vestibular disorders, because computerized dynamic posturography is such a new technique, additional time and effort on the part of the scientific community will be required to determine its ultimate place in the human vestibular laboratory. POSTUROGRAPHY AND VESTIBULAR DISORDERS-RESULTS FROM MEDICAL LITERATURE

Several authors have studied large populations of patients suspected of having vestibular disorders and noted, as would be expected from the design of the sensory organization test, that patients with ongoing vestibular disorders have abnormal postural sway during conditions 5 and 6. In a study by Goebel and Paige, 7 subjects were tested immediately after the cessation of rotation. That is, subjects were tested during an ongoing vestibulo-ocular asymmetry with computerized dynamic posturography. These subjects had great difficulty standing on conditions 5 and 6 but were otherwise normal. This pattern (i.e., abnormal sway on conditions 5 and 6) is considered suggestive of a vestibular deficit. Various other patterns aside from the "5, 6" pattern also have been discussed in the literature and are shown in Fig. 2. Figure 2A illustrates the typical "5, 6" pattern seen in patients with acute vestibular imbalance. The vestibular pattern has been observed in patients with ongoing vestibulospinal deficits. This includes patients with recent unilateral peripheral vestibular damage and patients with bilateral peripheral vestubular deficits, s-l~ Other vestibular-like patterns have been described, such as the "5" pattern, wherein sway is abnormal only with the platform moving and the patient closing the eyes. 16 The significance of this isolated "5" pattern is uncertain.

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1 Norm= vision. Fixed support.

2

Absent vision. Fixed support,

3

Sway-referenced vision. Fixed support,

4

Normal vision.

Sway-referenced support,

5 Absent vision.

Sway-referenced support

6

Sway-reforenced vision and support.

rl

il Fig. t. Six conditions of sensory organization test for computerized dynamic posturography. [Reprinted with permission from NeuroCom International, Inc., Clackamas, Ore.)

Another pattern, the "4, 5, 6" pattern has been labeled "surface dependence" or "combined visualvestibular deficit." Patients with the "4, 5, 6" pattern are unable to stand when somatosensation is distorted, despite having the opportunity for normal visual and vestibular inputs. Another pattern that has been described is that of an "aphysiologic" response, wherein patients appear to do better on paradigms that logically should be more difficult. For instance, if a patient performs below normal limits on conditions 1 and 2 but performs well on conditions 5 and 5, this is considered an "aphysiologic" response. This may reflect poor cooperation on the patient's part or a deliberate attempt to produce abnormal test results. Another pattern that has been described is that of a "severe" abnormality of postural sway, wherein patients have great difficulty maintaining balance in all of the conditions. After unilateral peripheral vestibular injury, a process known as compensation begins wherein patients are able to make use of information from a single labyrinth as adequate vestibular input to the ocular motor, spinal motor, and perceptual systems. The process of compensation depends on a patient's age, general neurologic status, and level of physical activity-specifically, activities that include combined visual, vestibular, and somatosensory inputs. Several studies have suggested that after successful compensation, posturography test results normalize and patients lose their "5, 6" pattern, and they may, in fact, have normal postural sway. 8'9 Fetter et al. 8 followed the time course of recovery after unilateral peripheral vestibular injury with posturography. Some of their data are shown in Fig. 3 and suggest

that 2 to 3 weeks after loss of unilateral peripheral vestibular function, most patients lose their "5, 6" pattern. Thus it has been suggested that posturography can provide valuable information regarding the status of compensation for a peripheral vestibular deficit. The "4, 5, 6" pattern suggests not only that patients are unable to rely on vestibular information but also that they have difficulty using visual information because they are dependent on a stationary support surface. This type of pattern has been reported in patients who complain of imbalance and dysequilibrium, including persons with peripheral vestibular deficits. 12 Moreover, the surface-dependent pattern has been reported by some to be more common in elderly patients than in young patients. The aphysiologic patterns on posturography always should lead to some doubt on the part of the physician regarding the reliability of the result. Although some unusual patients may for some reason be able to balance themselves better under more adverse circumstances, logically, if a patient has difficulty standing on a fixed surface surrounded by a stationary visual surround, it is difficult to explain how this patient could then be able to stand normally on a moving surface without proper visual cues. Some patients with balance disorders are unable to undergo computerized dynamic posturography testing because they are unable to stand without assistance or because they have a positive Romberg test result in which they fall with eyes closed (the equivalent of condition 2), Such patients are not candidates for computerized dynamic posturography. However, many patients with severe abnormali-

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Fig. 2. Patterns of abnormalities on computerized dynamic posturography. Results shown were obtained with the EquiTest.The six sensory conditions refer to those shown in Fig. t. The ordinate refers to equilibrium score (i.e., peak-to-peak sway amplitude with t00 equal to t2 degrees of sway and 0 equal to a fall). A, Vestibular pattern. Note that the patient swayed excessively on conditions 5 and 6. B, Surface-dependent pattern. Thispattern is also known as a combined visual-vestibular pattern. Note that the patient fell on conditions 4, 5, and 6, all of which are characterized by inaccurate somatosensory information because of a sway-referenced support surface. C, Severe pattern. Note the excessive sway on all conditions with a very low composite score. D, Nonspecific pattern. Because sway on condition 6 was within normal limits and all other conditions were associated with excessive sway, this patient should be suspected of having produced an aphysiologic result, possibly on the basis of poor cooperation.

ties of postural control are able to stand, although with increased sway amplitudes, without actually falling. These persons produce "severe" patterns because they often sway on all of the test conditions. It is important to know the sensitivity and specificity of laboratory tests. In the case of posturography and its role in the management of patients with vestibular disorders, it is important to know the likelihood of an abnormal result when a patient truly has a vestibular disorder (i.e., a true positive or "hit") and the likelihood of an abnormal result when a patient has a normal vestibular system (i.e., a false positive). To reliably determine sensitivity and specificity of a test, it is absolutely essential that there be a "gold standard," that is, a means by which

a definite answer regarding normality can be made. Then the test in question can be compared against the "right answer." Unfortunately, in the field of vestibular disorders there is no gold standard. Rather, experienced clinicians, making use of history, physical examination, and a medley of laboratory tests, render their best judgment regarding diagnosis. In the case of posturography testing, to make a determination of sensitivity and specificity, studies must rely on other measures that are of questionable reliability. Simply stated, without a gold standard, which is unavailable, the true sensitivity and specificity of computerized dynamic posturography cannot be determined. Rather, posturography testing can only be compared with expert

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(Figure and figure legend adapted with permission from Fetter M, Diener HC, Dichgans J. J Vestib Res 1991;t:373-83.}

opinion based on other information. Several studies have addressed the issue of sensitivity and specificity of computerized dynamic posturography using expert opinion as the "gold standard" with regard to both the sensory organization test and the motor control test. Hamid et al. 12 have indicated that the sensory organization test has a sensitivity of 95% and fase-positive rate of only 5%. Voorhees 1° has indicated a 45% sensitivity for computerized dynamic posturography for patients with peripheral vestibular deficits. Keim 17 has indicated that few patients with vestibular disorders have abnormal motor control test results. As noted earlier, all such estimates are plagued by their inability to compare dynamic posturography against a gold standard. Nonetheless, many patients with a high suspicion of having peripheral vestibular abnormalities (on the basis of other information) have abnormalities on the sensory organization portion of dynamic posturography. Moreover, few normal subjects have such abnormalities. Also, few patients with vestibular abnormalities have difficulty on the motor control test. An obvious question that can be asked regarding computerized dynamic posturography is whether it adds significantly to the laboratory assessment of patients suspected of having vestibular system abnormalities. Is electronystagmography alone sufficient? Is electronystagmography together with rotational testing sufficient? Does computerized dynamic posturography provide significant additional information regarding the diagnosis, choice of treat-

ment, and management of patients with suspected vestibular system abnormalities to warrant its expense? Several studies have compared the rate of abnormalities on computerized dynamic posturography with the rate of abnormalities on electronystagmography and/or rotational testing. The results from five selected studies of this type are shown in Table 2. These studies all have the same problem: there is no gold standard for making a diagnosis of a vestibular system abnormality. Indeed, these studies provide descriptive statistics and are primarily based on the initial complaint in the history or, at best, the assessment by a single physician regarding whether an abnormality is in the peripheral or in the central vestibular system. As noted in Table 2, nearly half of those patients with suspected vestibular abnormalities or with dysequilibrium of unknown cause have abnormal posturography results. A somewhat lower percentage of these persons having abnormalities on conventional electronystagmography. In two studies that specifically assessed the relative proportion of patients with normal electronystagmography or rotation who also had abnormal "sensory organization" on posturography, 18'I9 approximately one in four patients fit this pattern. Thus it could be argued that computerized dynamic posturography does indeed provide information in many cases not provided by other test modalities. Results from computerized dynamic posturography may disagree with the results from other vestibular laboratory testing for several reasons. Posturography assesses the vestibulospinal and postural control systems, whereas other vestibular laboratory tests rely on the vestibulo-ocular reflex and the ocular motor system. Thus it would be expected that there would be a group of patients with abnormalities on posturography with normal test results otherwise. Moreover, both caloric testing and rotational testing depend on the horizontal semicircular canals, whereas vestibular influences on postural sway probably depend on the vertical semicircular canals and the otolith organs. Thus posturography tests a different aspect of vestibular function from that assessed by electronystagmography and rotational testing. Indeed, the lack of a one-to-one correspondence between posturography and other vestibular tests supports the idea that computerized dynamic posturography augments the laboratory assessment of patients suspected of having vestibular system abnormalities. Posturography has been correlated with subjective scores on a dizziness handicap inventory. 2° A study by Jacobson et al. has indicated that "condi-

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Table 2. Comparison of posturography with other vestibular tests [selected studies] % Abnormal CDP [SOT]

Patient type Peripheral (n = 52) No vertigo (n = 49) Vertigo (n = 110) Balance disturbed

% Abnormal ENG or rotation

Total

Agree

Disagree

Total

Agree

Disagree

Reference

Notes

38 39 53 71

15 8 25 -

23 31 28 -

37 16 38 42

15 8 25 -

21 8 13 -

Asai et al, ~8 Goebel and Paige 19

Rotation ENG

Keim ~7

ENG

45 49 37

-

-

28 42 46 (ENG); 54 (rotation)

V o o r h e e s 1°

ENG

Zane et al. 23

ENG and rotation

(n = 103) Peripheral (n = 112) Central (n = 53) Vertigo (n = 70)

m

CDP, Computerized dynamic posturography; SOT,sensory organization test; ENG,electronystagmography.

tion 5 of the sensory organization test is most predictive of the emotional and total subscale scores (on the dizziness handicap inventory). ''2° This correlation with subjective scores on a dizziness handicap inventory implies that computerized dynamic posturography provides a useful correlate of the degree of impact that a balance disturbance has on a person's ability to function. Such a correlation supports the idea that although computerized dynamic posturography does not provide localizing or lateralizing information, or any information regarding cause, it does provide functional information regarding how well patients can use their balance and provides a suggestion about the importance of patients' balance disturbances on their activities of daily living. Several authors have addressed the issue of whether computerized dynamic posturography has any predictive value in terms of therapeutic outcome. Cass et al. 21 have indicated in a study of patients undergoing unilateral peripheral vestibulectomy with vestibular nerve section that each of their patients with abnormal preoperative posturography subsequently demonstrated abnormal late postoperative posturography. These authors believed that their results suggested that preoperative posturography may be useful in the evaluation and counseling of patients considering ablative vestibular surgery. Shepard et al., in a study of patients enrolled in a balance rehabilitation therapy program, concluded that " . . . results on dynamic posturography were . . . predictive of outcome...,,22 Again, these authors' results suggest that computerized dynamic posturography provides a functional measure that is helpful in predicting the benefit that patients may expect to receive from therapeutic intervention with physical therapy.

CONCLUSIONS

On the basis of the literature currently available, how should the following questions be answered: Why use computerized dynamic posturography? What is its role in the management of patients with suspected vestibular system abnormalities? Computerized dynamic posturography does not provide localizing or lateralizing information in the neurologic sense. Moreover, computerized dynamic posturography does not provide information regarding cause. However, testing can strongly suggest the presence of vestibular system abnormality, including a unilateral peripheral vestibular deficit for which a patient has not yet compensated or a symptomatic bilateral peripheral vestibular deficit. Computerized dynamic posturography also provides insight regarding the presence of a vestibular system deficit, regardless of localization, that renders a patient unable to properly use vestibular system information. Thus, although computerized dynamic posturography does not help in localizing a lesion or determining its cause, such testing does provide a functional measure that can be helpful in understanding a patient's ability to properly use vestibular system information. Moreover, by looking for various patterns of abnormalities, it may be learned that a patient has more than just difficulty processing vestibular sensation; they may also have difficulty with both visual and vestibular sensation, or they may have a severe abnormality of postural control. Regarding the choice of therapy, posturography may be useful in that the choice of therapy for vestibular disorders is often very much a judgment that requires the experience of the physician and a discussion with the patient about various therapeutic options. The scant results available to date suggest that posturography may assist the clinician by allowing more accurate prognostication regarding suc-

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cess. Moreover, if a patient is referred for physical therapy, results from computerized dynamic posturography can be given to the physical therapist as a means of helping to customize the treatment plan. Taken together, the literature to date regarding computerized dynamic posturography suggests that this test technique is helpful in the management of patients with suspected vestibular disorders. In addition to suggesting the presence of a vestibular system abnormality-particularly an uncompensated peripheral vestibular deficit, a bilateral peripheral vestibular deficit, or a central vestibular abnormality-computerized dynamic posturography may help guide the clinician regarding choice of treatment and counseling of patients. For example, patients who perform poorly on computerized dynamic posturography may be advised to avoid certain environments at home or at work that require balance for safety. Posturography testing may also gain a place in the long-term management of patients with vestibular system abnormalities in that testing is noninvasive and, because of its focus on functional ability, may allow the clinician to assess performance once a diagnostic workup has been completed. Computerized dynamic posturography, despite its usefulness at the present time, warrants considerable further research to solidify its role and to improve its value to the clinician evaluating patients with vestibular system abnormalities. In addition to further studies specifically aimed at enhancing our understanding of how computerized dynamic posturography assists in diagnosis, choice of treatment, and management of patients, further technologic advances should be explored. In particular, the use of sophisticated motion analysis equipment that enables measurement of the location of individual body segments may improve the ability of computerized dynamic posture platforms to modify visual and somatic sensation. Additionally, different output measures, such as sway angle based on derived center of gravity rather than on center of force and/or estimates of joint positions during postural sway, may also prove valuable. Another technologic advance that may improve computerized dynamic posturography is the ability to generate and control moving visual scenes while patients attempt to stand still. It is likely that despite any advances that are made in the field of computerized dynamic posturography, this test will remain a technique that is appropriate for assessing a patient's functional ability. Posturgraphy is unlikely to provide assistance regarding

Otolaryngology and Neck Surgery January 1995

localization of lesions, such as that provided by brain imaging, or to provide assistance regarding cause, such as that provided by blood tests or biopsies. However, because computerized dynamic posturography has the potential to inform clinicians regarding how a patient may function in various environments, this technique is likely to allow clinicians to better care for their patients with balance disorders. Computerized dynamic posturography has the potential to aid in the choice of therapy and in the counseling of patients regarding activities that may put them at risk of injury. Further studies are re, quired to determine the ultimate role of computerized dynamic posturography in the management of patients with vestibular disorders. The author thanks Dr. Lewis Nashner for his helpful comments. REFERENCES

1. Baron JB. History of posturography. In: igarashi M, Black FO, eds. Vestibular and visual control on posture and locomotor equilibrium. Basel: Karger, 1983:54-9. 2. Terekhov Y. Stabilometry as a diagnostic tool in clinical medicine. Canadian Medical Association Journal 1976;115: 631-3. 3. Diener HC, Dichgans J, Guschlbauer Bi Bacher M. Role of visual and static vestibular influences on dynamic posture control. Human Neurobiology 1986;5:1065-113. 4. Allure JHJ, 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 (Stockh) 1988;106:252-63. 5. Nashner LM, Black FO, Wall C. Adaptation to altered support and visual conditions during stance: patients with vestibular deficits. J Neurosci 1982;2:536-44. 6. Nardone A, Giordano A, Corra T, Schiepati M. Responses of leg muscles in humans displaced while standing. Brain 1990; 113:65-84. 7. Goebel J, Paige G. Posturography following rotation: a model of posture control during vestibular dysfunction. OTOLARYNGOLOGYHEADNECKSURG1990;102:722-6. 8. Fetter M, Diener HC, Dichgans J. Recovery of postural control after an acute unilateral vestibular lesion in humans. J Vestib Res 1991;1:373-83. 9. Parker S, Krebs D, Gill K, Riley P. Varying sway-referencing gain to quantify measurement of standing balance in patients with bilateral vestibular hypofunction. In: Woollacott M, Horak F, eds. Posture and gait: control mechanisms. XIth International Symposium of the Society for Postural and Gait Research. Portland: University of Oregon Books, 1992:315-8. 10. Voorhees R. The role of dynamic posturography in neurotologic diagnosis. Laryngoscope 1989;99:995-100L 11. Freyss A, Semont A, Vitte E, Freyss M. Dynamic body stabilization-Equitest Smart System in patients with bilateral vestibular caloric areflexia. In: Woollacott M, Horak F, eds. Posture and gait: control mechanisms. XIth International Symposium of the Society for Postural and Gait Research. Portland: University of Oregon Books, 1992:292-5. 12. Hamid M, Hughes G, Kinney S. Specificity and sensitivity of

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13. 14.

15.

16.

dynamic posturography: a retrospective analysis. Acta Otolaryngol (Stockh) 1991;suppl 481:596-600. Bowman C, Mangham C. Clinical use of moving platform posturography. Seminars in Hearing. 1989;10:161-71. Cyr D, Moore G, Moller C. Clinical application of computerized dynamic posturography. Ear Nose Throat J 1988; 67(suppl):36-47. Herdman SJ, Sandusky AL, Hain TC, Zee DS, Tusa RJ. Characteristics of postural stabilityin patients with aminoglycoside toxicity. J Vestib Res 1994;4:71-80. Dickins J, Cyr D, Graham S, Winston M, Sanford M. Clinical significance of type 5 patterns in platform posturography.

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17. Keim RJ. Clinical comparisons of posturography and electronystagmography. Laryngoscope 1993;103:713-6. 18. Asai M, Watanbe Y, Ohashi N, Mizukosbi K. Evaluation of vestibular function by dynamic posturography and other

23.

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equilibrium examinations. Acta Otolaryngol (Stockh) 1993; suppl 504:120-4. Goebel J, Paige G. Dynamic posturography and caloric test results in patients with and without vertigo. OTOIARYNGOL HEAD NECKSURO1989;100:553-8. Jacobson GP, Newman CW, Hunter L, Balzer GK. Balance function test correlates of the dizziness handicap inventory. J Am Acad Audiol 1991;2:253-60. Cass S, Kartush J, Graham M. Clinical assessment of postural stability following vestibular nerve section. Laryngoscope 1991;101:1056-9. Shepard N, Telian S, Smith-Wheelock M, Raj A. Vestibular and blance rehabilitation therapy. Ann Otol Rhinol Laryngol 1993;102:198-205. Zane R, Rauhut M, Jenkins HA. Vestibular function testing: an evaluation of current techniques. OTOLARYNGOLHEAD N~CK SVRG1991;104:137-8.

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