Otolaryngology- Head and Neck Surgery

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1 Otolaryngology- OCTOBER 1997 VOLUME 117 NUMBER 4 ORIGINAL ARTICLES Posturographic evidence of nonorganic sway patterns in normal subjects, patients, and suspected malingerers JOEL A. GOEBEL, MD, ROBERT I'. SATALOFF, MD, JASON M. HANSON, MD, LEWIS M. NASHNER, SCD, DEBRA S. HIRSHOUT, MSPA, CCC-A, and CAREN C. SOKOLOW, MA, CCC-A, St. Louis, Missouri, Philadelphia, Pennsylvania, and Clackamas, Oregon During the last I0 years, computerized dynamic posturography has yielded various patterns of sway on the sensory organization test and the motor control test that have been associated with a variety of organic balance disorders. Some aspects of performance during computerized dynamic posturography, however, are under conscious control. Voluntary movements not indicative of physiologic response to balance system stimulation can also affect computerized dynamic posturography results. Quantification of nonorganic or "aphysiologic" response potterns in normal subjects, patients, and suspected malingerers is crucial to justify use of computerized dynamic posturography for identification of physiologically inconsistent results. For this purpose the computerized dynamic posturography records of 122 normal subjects, 347 patients with known or suspected balance disorders, and 72 subjects instructed to feign a balance disturbance were critically evaluated by use of seven measurement criteria, which were postulated as indicating aphysiologic sway. Each criterion was scored with a standard calculation of the raw data in a random, blinded fashion. The results of this multicenter study show that three of the seven criteria are significantly different in the suspected "malingerer" group when compared with either the normal or patient group. The relative strength of each criterion in discerning organic from nonorganic sway provides the examiner with a measure of reliability during platform posture testing. This study demonstrates that computerized dynamic posturography can accurately identify and document nonorganic sway patterns during routine assessment of posture control. (Otolaryngol Head Neck Surg 1997;I 17: ) From the Department of Otolaryngology- (Drs. Goebel and Hanson), Washington University School of Medicine; the Department of Otolaryngology-Head and Neck Surgery (Dr. Sataloff), Jefferson Medical College, Thomas Jefferson University; Neurocom, Intl. (Dr. Nashner); and the American Institute for Voice and Ear Research (Ms. Hirshout and Ms. Sokolow). Presented at the Annual Meeting of the Americ~m Academy of Otolaryngolegy-, New Orleans, La., Sept , Reprint requests: Joel A. Goebel, MD, Department of Otolaryngology-, Washington University School of Medicine, 517 S. Euclid, Box 8115, St. Louis, MO Copyright 1997 by the American Academy of Otolaryngology- Foundation, Inc /97/$ /1/73756 Distinguishing between normal, abnorrnal, and exaggerated balance function is an important element of the diagnostic workup of all patients with dizziness and balance complaints. The problem of distinguishing true physiologic and exaggerated symptoms is compounded by the fact that reliance on traditional vestibular func- tion tests fails to establish a localizing diagnosis that accounts for the patients' symptoms in a reported 30% to 50% of cases. 1 Differentiating between physiologic and 'aphysio- logic" symptoms is a significant medical-legal prob- lem 2 because complaints of dizziness and unsteadiness are frequent sequela in accidental and job-related 293

2 294 GOEBEL et al. Otolaryngology- October 1997 Visual condition Fixed Eyes closed Sway-referenced p Sway Fig. 1. The six conditions of the SOT. Conditions ] through 3 use a fixed support surface with variation of the visual field. Conditions 4 through 6 use a sway-referenced support surface with the same visual conditions as used in the first three conditions. (Reprinted with permission from Neurocom, Intl., Inc., Clackamas, Ore.) injuries such as mild traumatic brain injury 3,4 and neurotoxic chemical and drug exposure. 5,6 Identifying exaggerated symptoms is also a significant problem for treating physicians. When patients with chronic balance disorders have not received satisfactory medical attention in the past, fear, deconditioning resulting from activity restriction, and anxiety can become a major part of the problem. 7 The goal of this study was to develop an objective set of criteria by which aphysiologic/exaggerated symptoms of unsteadiness can be positively identified with a high level of confidence. Objective computerized dynamic posturography (CDP) criteria to differentiate between true physiologic and exaggerated unsteadiness have been described in previous studies Other investigators have also used fixed forceplate posturography to establish objective criteria. H,12 All of the above studies have based their criteria on known principles of balance control. Most importantly, performance should covary in relation to the difficulty of the task and that voluntary postural reactions occur at longer latencies than reflex reactions. I' C. F o r c ~ Right 1' Fig. 2. MCT. Backward translation of the platform results in forward displacement of the subject's center of gravity (Sway), which triggers a reflexive motor response as demonstrated by increased pressure on the platform sensors under the ball of the foot (C. Force). (Reprinted with permission from Neurocom, Intl., Inc., Clackamas, Ore.) METHODOLOGIC APPROACH CDP was introduced as a tool for evaluating freestanding balance during manipulation of the visual surround and support surface by Nashner et al. 13 Inherent to this technique are concepts of sensory organization and motor coordination--according to which visual, vestibular, and somatosensory inputs are compared within the central nervous system and a motor response is initiated that is appropriate to the sensory conditions and configuration of the support surface. When all senses yield correct information regarding orientation (no sensory conflicts) and the support surface is firm, the functionally appropriate motor response is predictable and generated easily. When one or more of the sensory

3 Otolaryngology- Volume 117 Number 4 GOEBfiL et al. 295 Equilibrium Score lo0~ Fall Sensory Compos~e Conditions 75 Trial 1 Trial 2 ~ Trial 3 Trial } Trial 2 Trial 3 ~+ H t:t "~ I;l --SWAY, SHEAR, AND ALIGNMENT ; i COG X-Y PLOTS Fig. 3. SOT results in a normal subject. Note consistent high scores (top), absence of rhythmic sway (bottom left), and minimal lateral sway on the COG plot (bottom right). inputs is functionally inappropriate for maintaining baiance or when the support surface is irregular or compliant, the brain must ignore the conflicting sensory cues and/or alter the pattern of motor response to avoid falling. The sensory organization test (SOT) uses six progressively more difficult test conditions illustrated in Fig. 1 to assesses a patient's ability to effectively use visual, vestibular, and somatosensory inpu~s. Previous clinical studies reported SOT patterns typical for normal subjects ~4-16 and patients with peripheral, mixed, and central vestibular system disorders 17-1'~ and those with central nervous system disorders In normal subjects and the above-documented cases of organic pathology, performance was best under the,easiest sensory condition, 1, and became progressively poorer under the most difficult sensory conflict conditions, 5 and 6. Abnormal scores under the first condition (eyes open and fixed surface) have rarely been reported in the absence of severe neurologic and musculoskeletal problems. The motor coordination test (MCT) evaiuates the patient's automatic reactions to sway induced by unexpected, brief translations (or rotation) of the support surface, as illustrated in Fig. 2. Three sizes of translation are imposed in both forward and backward directions. Small translations are at the lower threshold for eliciting automatic responses, medium translations induce brisk responses, and responses to large translations are near the maximum. Responses to platform translations commence at reflexive prevoluntary latencies, tend to be highly repeatable even in patients with impaired motor function, and are scaled in amplitude relative to the stimulus size. 23 The above-described physiologic properties of responses during CDP---sparing of the easier SOT conditions and consistency of the MCT responses--have led us to evaluate the possibility of using these findings as markers for physiologic inconsistency. To test this hypothesis, we formulated seven criteria as indicators of inconsistency and then tested the power of these criteria to differentiate among three populations of sub-

4 296 GOEBEL et al. Otolaryngology- October 1997 Sway -~ U ~ : Shear m 03 C. Force o m Right ~ Sway Shear -:~ lo Left :F C. Force Right -- ~ Sway ill -~ o Shear m Q) Left -J C. Force Right Fig. 4. MCT results in normal subject. Note overlapping nature of raw tracings following platform translation. jects: normal subjects, patients with organic balance problems and no reason for secondary gain, and normal subjects instructed to deliberately feign postural instability. METHODS AND MATERIAL This study was conducted as a multiceuter effort between the Washington University Vestibular and Oculomotor Laboratory (J. A. G., J. M. H.), Jefferson Medical College (R. T. S.), the American Institute for Voice and Ear Research (R. T. S., D. S. H., C. C. S.). and Neurocom, Intl. (L. M. N.). A total of 541 CDP records were collected from the three centers. Normal subjects (n = 122; age range, 17 to 84 years; mean age, 44.9 _ 20.7 years) were drawn from two established databases, one at Neurocom, and one at the University of Michigan (supplied by Neil Shepard, PhD). Patient files (n = 347; age range, 11 to 89 years; mean age, years) were randomly selected from the Washington University Vestibular and Oculomotor Laboratory for blinded retrospective analysis. Finally, subjects from the American Institute for Voice and Ear Research were recruited to represent the malingering group. These recruits were free of balance complaints and had no history of balance disorder. Each subject underwent physical examination, electronystagmography, and CDR Subjects with abnormal test results were excluded, and the remaining subjects (n = 72; age range, 17 to 64 years; mean age, years) were then instructed to "perform this test as if you have a balance problem." No instructions as to how severe a problem they were feigning were given. TESTING METHODS SOT and MCT protocols (EquiTest; Neurocom Intl., Inc.) were conducted in accordance with methods and subject instructions described in detail by Nashner. 24 After completion of the six SOT conditions, subjects were exposed during the MCT to the following eight sets of random onset displacements of the support surface (number per set in parenthesis): small (3), medium (3) and large (3) backward; toes up (5); small (3), medium (3), and large (3) forward; and toes down (5). Figure 3 shows typical SOT scores, raw shear tracings, and center of gravity (COG) plots seen in normal subjects. Note the lack of sway on the easier conditions and the paucity of lateral sway on the COG plots. Figure 4 demonstrates the normal motor response to three small, medium and large backward translations. Note the similarity of force and sway responses during repeated exposures and the tendency for response sizes to increase with the larger stimuli. In contrast, Fig. 5 shows the SOT scores (5A), SOT raw tracings (5B), COG plots (5C), and MCT tracings (5D) from a suspected malingerer. Note the lower scores on SOT 1 and 2 compared with 5 and 6, the large amplitude sway without falling, the lateral sway, and the variability of the MCT tracings with platform translations. From these observations, seven criteria believed to be consistent with aphysiologic response to CDP were defined and applied to each data record: 1.Substandard performance on SOT 1. Score equals number of points below norm for the best trial of SOT 1. Enter zero if above norm. 2.Lower scores on SOT 1 and 2, higher scores on SOTs 5 and 6. Using best individual trials, score equals [(Score 1 - Norm 1) + (Score 2 - Norm2)] - [(Score 5 - Norms) + (Score 6 - Norm6) ]. 3.Repetitive large-amplitude anteroposterior sway without falling. Score equals the average number

5 Otolaryngology- Volume 117 Number 4 GOEBEL et al Equilibrium Score Normal Vision SwayfRef Surface 140g00 I~0g~0 75 Absent Vision SwayRef Surface 50,4,Z4a 25 SwayRef Vision "~ SwayRef Surface A Fall Composite Conditions 53 C 10 degrees 14 = 332 Sway ~4~40g,4,4~, Shear B D Fig. 5. SOT and MCT results in a suspected malingerer. A, Low SOT 1 and 2 scores with normal SOT 6, B, Large-amplitude sway without falling on SOT 4, 5, and 6. C, Excessive lateral sway without failing on SOT 4, 5, and 6 (top, middle, bottom). D, Excessive sway with nonoverlapping tracings during platform translations. of anteroposterior sways that exceed 7.5 degrees for each trial of SOTs 4, 5, and 6 without a fall. 4.Excessive lateral sway without falling. Score equals the average number of lateral sways that exceed degrees from the patier, t's center of gravity for each trial of SOTs 4, 5, and 6 without a fall. 5.Excessive variability on SOTs 1 and 2. Score equals average of standard deviations for all trials of SOTs 1 and Exaggerated motor responses to small platform translations. Score equals average number of degrees of sway across trials for small forward and small backward displacements. 7. Inconsistent motor responses to small and large, forward and backward platform translations. Score equals number of tests with at least two of three "concordant" trials per test. Maximum score equals 4. For criteria 1, 2 and 5, numeric values were extract- ed directly from the CDP record. For criteria 3, 4, and 6, a scoring template was created to overlay the tracings in order to obtain a score (Fig. 6). Only criterion 7 required a qualitative judgment by the scorer that was not derived numerically or guided by a template. The numeric scores for each subject were derived from blinded records scored at each location by the same scorer (J. M. H.) and entered into a master data file. For criteria 1, 3, 4, 6, and 7, a numeric cutoff was used to dichotomize the data for analysis because the score distributions for these criteria were highly skewed. For criterion 2, the data were transformed and subsequently analyzed by a Kruskal-Wallis test to distinguish the three groups, with a p value of 0.05 tal~en as significant. After the initial analysis of the data for criterion 5, it became obvious that the lack of sufficient trials among the normal and patient groups for SOTs 1 and 2 made it impossible to compare variability between trims between groups; further analysis of this criterion was not made.

6 298 GOEBEL et al. Otolaryngology- October 1997 Antero-Posterior Sways I.5 degrees (peak-to-peak) Lateral Sways 10 8 MCT: Largest Amplitude Sway degrees Fig. 6. Scoring template for criterion 3 (Antero-posterior sways), criterion 4 (Lateral sway), and criterion 6 (MCT: Largest amplitude sway). For further detail, see text. Table 1. Percentage of subjects with positive criteria for aphysiologic sway Criterion Cutoff Normals Patients Malingerers Sensitivity Specificity 1 0 0% 5% 72% 72% 96% % 25% 31% 31% 80% % 75% 57% 57% 40% % 5% 32% 32% 95% 7 4 1% 7% 72% 72% 95% Sensitivity and specificity calculations are noted for each criterion separately. RESULTS The results of the study are summarized in Table 1 and displayed graphically in Fig. 7. When analyzed separately, three of the six criteria (criterion 5 was eliminated because of insufficient data) separated the normal and patient groups from the group instructed to feign instability with a high degree of specificity. Criterion 1, which examined sway under SOT condition 1, was a strong indicator of aphysiologic response because both the normal and patient groups scored at or above the lower limit of normal performance in more than 95% of the cases, whereas feigners tended to sway more. Criterion 6 was a strong indicator of aphysiologic response because only those individuals instructed to feign instability swayed greater than two degrees in response to the small forward and backward translations. Finally, criterion 7 was a strong indicator because the group feigning instability demonstrated proportionately much more discordant sway, different sway patterns from one trial to the next, during repeated medium and large translations. None of the remaining three criteria as scored in this study was able to provide an indication of aphysiologic sway with a high degree of confidence. When results from multiple criteria were combined in Table 2, separation of normal and patient groups from the aphysiologic group instructed to feign instability was nearly perfect. When criteria 1 and 7 are seen in the same subject, the false-positive rate of incorrectly identifying a patient or normal subject as a malingerer was reduced to zero while maintaining a sensitivity of 57%. A different combination of criteria (1 or 6 or 7) increased the sensitivity of correctly identifying the aphysiologic sway subjects to 89% while lowering the

7 Otolaryngology- Volume 117 Number 4 GOEBEL et al. 2~ l 7 50-] j ~,,N I Sensitivity - 72% I Specificity - 96% ~ 75-1!5 q 25-~ 0 t -100 i 7.~ 75- I 1 7 5o-] I I > Norm Deviation below Norm (CDP score) Difference in CDP scores Sensitivity - 31% Specificity - 80% J I I -- I -- I -- I -- I ~ I A I i 7 > Average number of sways Sens: tivity - 57% I I ~ I -- [ i I ~ I i I ~ 7 > Average number of sways Sensitivity - 32% ~ 501~7~ 250"~ ~ Specificity-95% ' 7 I I I -- I -- I -- I -- I -- I -- i t I ~I~ Average sway (degrees) %-.I " ~! Specificity-95%Sensitivity-72% 50 g 2 ~ i. ~ ~ ~... ~ 4 3 t M t 2 [] 1-0 Number of concordant trials Fig. 7. Graphic presentation of criteria 1, 2, 3, 4, 6, and 7 in normal subjects, patients, and malingering recruits (criterion 5 eliminated). Data expressed in percentage of subjects scoring at a given level. Double vertical line represents 95% confidence limit used for all criteria except number 2 (see text for details). Note the high specificity of criteria 1,6, and 7. specificity to 88%, A compromise combination of criteria (1 or [6 and 7]) yielded a rule with a sensitivity of 75% and a specificity of 96%. DISCUSSION This study demonstrates the utility of CDP in distinguishing organic from nonorganic sway patterns. The criteria used were derived from examination of sway patterns in normal subjects where resolution of sensory conflict and typical motor responses to support surface movement are expected (Fig. 3A). Further observations from patient files led to the hypothesis that performance on the easier SOT conditions I and 2 is reproducible and almost always normal even in cases of severe balance dysfunction. Furthermore, repeated large-amplitude swaying without falls is unlikely and, if present, is directed in the anteroposterior rather than lateral direction. In addition, there is minimal sway response to

8 300 GOEBEL et al. Otolaryngology- October 1997 Table 2. Percentage of subjects with multiple positive criteria for aphysioiogic sway Combination Normals Patients Malingerers Sensitivity Specificity I 1 & 6 0% 1% 28% 28% 99% 1 & 7* 0% 0% 57% 57% 100% 6 & 7 1% 1% 28% 28% 99% 1 & 6 & 7 0% 0% 25% 25% 100% 1 or 6 2% 9% 76% 76% 93% 1 or 7 1% 12% 88% 88% 91% 6 or 7 2% 12% 76% 76% 91% 1 or 6 or 7* 2% 15% 89% 89% 88% (1 & 6) or 7 23% 8% 75% 75% 94% 1 & (6 or 7) 0% 1% 60% 60% 99% 1 or (6 & 7)* 1% 6% 75% 75% 96% (1 or 6) & 7 1% 1% 60% 60% 99% (1 or 7) & 6 1% 2% 31% 31% 98% (1 & 7) or 6 2% 5% 64% 64% 95% Sensitivity and specificity calculations are shown for all combinations of criteria 1,6, and 7. *Optimal combinations (see Results section). small platform perturbations and typical patterns to the sway responses to both small and large platform translations. Therefore it was believed that these criteria might be helpful to detect additional nonorganic voluntary sway within the records of patients with no pathology or exaggeration of symptoms. This study supports the use of multiple criteria in distinguishing aphysiologic from normal and true pathologic sway patterns with a high degree of reliability. Cevette et al. 8 recently reported a high degree of discrimination of aphysiologic from normal and true pathologic sway in a similarly designed study in which 22 suspected malingerers were compared with agematched normal subjects and patients with vestibular dysfunction. They were able to discriminate the aphysiologic group from the age-matched normal and patient groups with a 95.5% accuracy using two criteria based on the SOT data set alone. The first of two useful criteria in the Cevette study was based on greater internal variability among trials of the same sensory conditions in the aphysiologic patients. We were unable to test a variability criterion (number 5 in our study) because of insufficient patient data. The second criterion in the Cevette study was based on the aphysiologic group performing significantly better than the patients under conditions 5 and 6 and poorer under conditions 1 and 2. This criterion is similar to criterion 2 in our study, which was not sensitive in discriminating aphysiologic subjects. Two explanations for this difference are possible. First, normal subjects asked to deliberately feign instability in our study may have used different techniques than the actual patients suspected of malifigering in the Cevette study. Second, we may have reduced the discriminating power of our criterion 2 by including only the best SOT score for each sensory condition. In the Cevette study, results of all three trials were used in the analysis. Two prior studies used static posturography to discriminate between normal, true pathologic, and aphysiologic sway. Uimonen et al. 12 reported little success in discriminating aphysiologic sway using static posturography measurements of sway velocity, sway area, and the Romberg quotient (eyes open versus eyes closed sway). They concluded that static posturography was no better than observations of trained observers. Their result is at variance with our criterion 1 and Cevette's criteria findings, both of which indicated increased sway in the aphysiologic groups under sensory condition 1. Guidetti 11 reported good discrimination by correlating anteroposterior and lateral components of sway. Although these two components were not correlated in normal subjects and true pathologic cases, these components were correlated in individuals exaggerating their sway. We believe that the sway response in our group of subjects feigning balance disturbance is likely representative of the sway response of patients with nonorganic sway on CDR In selected cases of patients suspected of exaggerated sway for secondary gain, a similar pattern of SOT and MCT abnormalities was noted (Fig. 5). This finding supports the hypothesis that the nonorganic sway represents voluntary sway superimposed on the physiologic response. It is important to note, however, that both organic and nonorganic sway may exist in the same patient, and the presence of certain aphysiologic criteria may not negate the validity of the entire test. There are a number of possible reasons that four of the seven criteria failed to separate physiologic from aphysiologic performance on CDR For criterion 2 the

9 Otolaryngology- Volume 117 Number 4 GOEBEL et al 301 use of the best trial for conditions 1, 2, 5, and 6 rather than the average of all trials per condition most likely prevented this calculation from being useful. As noted above, Cevette et al. 8 believed that better performance on harder trials combined with poorer perfbrmance on the easier tests using average scores per condition was a useful way to identify aphysiologic patterns. It may also be that the aphysiologic group used voluntary sway throughout all trials and therefore did score poorer on the more difficult tests. This criterion is being reassessed with other parameters for analysis of the raw data. For criterion 3 (anteroposterior sway > "7.5 degrees) and criterion 4 (lateral sway > 2.5 degrees), the problem may lie in the magnitude of the sway designated as excessive. These figures (7.5 degrees anteroposterior, 2.5 degrees lateral) were derived from observations in normal subjects and patients as the "outer limits" most individuals could sway without actually falling. Most likely, these limits are too broad, and if more narrow sway angles were used, the distinction between normal subjects, patients, and malingerers could be made. Reanalysis with narrower limits is presently under way. As noted before, criterion 5 was excluded because of the lack Of repeated trials in most of the patient group trials. Variability on repeated tests was noted by Cevette et al. 8 as a significant finding in his nonorganic group, and we believe this criterion would have been useful if repeated trials of conditions 1 and 2 were available in our patient group. It is important to note the need for caution in applying the information from this study. Most importantly, this study attempted to identify not one but rather multiple indicators of nonorganic performance, and the criteria were therefore designed to minimize false-positive identification of a patient's response as aphysiologic. Taken in isolation, however, perhaps an occasional patient exhibits one "positive" criterion for nonorganic sway when in fact the problem is physiologic. One limitation of this study is the lack of patients with severe neurologic or musculoskeletal disorders in the patient population. As noted, the charts were selected at random and were primarily from patients with vestibular dysfunction and postural instability without progressive central neurologic deficit. If, indeed, more severely affected patients were included, perhaps a decrease in SOT conditions 1 and 2 scores because of disease would have been noted. One could argue, however, that such patients' diseases could be diagnosed easily by other clinical criteria and would rarely if ever be confused with the nonorganic cases. Another pitfall is the assumption that all nonorganic sway is malingering. A variety of anxiety disorders and psychological/psychiatric disorders, including conversion reaction and somatization disorders, could influence postural sway during CDP with no conscious effort by the patient for secondary gain. With the above considerations, it appears from this study that CDP is a useful tool to document patterns of aphysiologic sway in cases of nonorganic balance dysfunction. Discriminant analysis theo~2 predicts that, as long as limits on individual criterion can be set sufficiently high to minimize false-positive rates, the best isolation of aphysiologic sway patterns will be achieved by including both the more and less sensitive criteria. This principle is perhaps especially true in light of the results of other aphysiologic studies, suggesting that a number of different strategies may be used to exaggerate symptoms of unsteadiness. However, responses on CDP must be interpreted with caution and in context with the patient's history and physical examination. Finally, nonorganic sway patterns occur in a variety of psychological conditions in addition to malingering, and therefore these patterns cannot by themselves be used to discern intention. We thank Nell Shepard, PhD, for inclusion of raw data for the normal group and Doug Fishel and Kevin Rosen for their assistance with data acquisition. We also thank Keith Dunnigan, PhD, for assistance with the statistical analysis. REFERENCES 1. Hain TC. Treatment of vertigo. The Neurologist 1995;1: Whiplash and dizziness, American College of Legal Medicine Comminique & Newsbriefs 1992:9, 3. 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10 302 GOEBEL et al. Otolaryngology- October 1997 Cesarani A, Alpini D, editors. Aspetti medico-legali dei disturbi dell'equilirio. Milano: Bi & Gi Editori, 1991:163-78, 12. Uimonen S, Laitakari K, Kiukaanniemi H, Sorri M. Does posturography differentiate malingerers from vertiginous patients? J Vestib Res 1995;5: Nashner LM, Black FO, Wall C. Adaptation to altered support and visual conditions during stance: patients with vestibular deficits. J Neurosci 1982;2: Jackson RT, Epstein CM. Effect of head extension on equilibrium in normal subjects. Ann Otol Rhinol Laryngol 1991;100: Peterka RJ, Black FO. Age-related changes in human posture control: sensory organization tests. J Vestib Res 1990;1: Wolfson L, Whipple R, Derby C, et al. A dynamic posturography study of balance in healthy elderly. Neurology 1992;42: Asai M, Watanabe Y, Ohashi N, Mizukoshi K. Evaluation of vestibular function by dynamic posturography and other equilibrium examinations. Acta Otolaryngot (Stockh) Snppl 1993;504: Keim RJ. Clinical comparisons of posturography and electronystagmography. Laryngoscope 1993;103: Lipp M, Longridge NS. Computerized dynamic posturography: its place in the evaluation of patients with dizziness and imbalance. J Otolaryngol 1994;23: Jackson RT, Epstein CM, De l'aune WR. Abnormalities in posturography and estimations of visual vertical and horizontal in multiple sclerosis. Am J Otol 1995;16: Nelson SR, DiFabio RP, Anderson JH. Vestibular and sensory interaction deficits assessed by dynamic platform posturography in patients with multiple sclerosis. Ann Otol Rhinol Laryngol 1995;104: Tian J-R, Herdman S J, Zee DS, Folstein SE. Postural stability in patients with Huntington's disease. Neurology 1992;42: Diener HC, Horak FB, Nashner LM. Influence of stimulus parameters on human postural responses. J Neurophysiol 1988;59: Nashner LM. Computerized dynamic posturography. In: Jacobson GP, Newman CW, Kartush JM, editors. Handbook of balance function testing. Chicago: Mosby-Year Book, 1993: BOUND VOLUMES AVAILABLE TO SUBSCRIBERS Bound volumes of Otolaryngology- are available to subscribers (only) for the 1997 issues from the Publisher, at an individual cost of $89.50 ($ for Canadian, $ for international subscribers) for Vols. 116 (January-June) and 117 (July- December). Shipping charges are included. Each bound volume contains subject and author indexes, and all advertising is removed. Copies are shipped within 60 days after publication of the last issue in the volume. The binding is durable blue buckram with the Journal name, volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, Westline Industrial Drive, St. Louis, MO , USA; phone (800) or (314) Subscriptions must be in force to qualify. Bound volumes are not available in place of a regular Journal subscription.