Validation of a next-generation sensory organization test in adults with and without vestibular dysfunction

Abstract

BACKGROUND:

The traditional Sensory Organization Test (T-SOT) is a gold standard balance test; however, the psychometric properties of assessing sensory organization with a virtual-reality-based posturography device have not been established.

OBJECTIVE:

Our overall aims were to assess the criterion, concurrent, and convergent validity of a next-generation Sensory Organization Test (NG-SOT).

METHODS:

Thirty-four adults (17 vestibular-impaired) participated. We compared the area under the curve (AUC) for receiver operator characteristic (ROC) analysis for the T-SOT and NG-SOT composite scores. Between-group and between-test differences for the composite and sensory analysis scores from each SOT were assessed using Wilcoxon Rank Sum tests. Additionally, we ran Spearman correlations between the NG-SOT composite score and outcomes of interest.

RESULTS:

The AUCs for the NG-SOT and T-SOT were 0.950 (0.883, 1) and 0.990 (0.969, 1) respectively (p = 0.168). The median composite, vision, and visual preference scores were lower on the NG-SOT compared to the T-SOT; whereas, the median somatosensory score was higher on the NG-SOT compared to the T-SOT. Associations between the composite score and patient-reported or performance-based outcomes ranged from poor to strong.

CONCLUSIONS:

The NG-SOT is a valid measure of balance in adults. However, the results of the NG-SOT and T-SOT should not be used interchangeably.

Keywords

Vestibular disorders adults sensory organization test validity

1 Introduction

The Sensory Organization Test (SOT) is established as an important outcome measure for vestibular rehabilitation (VR) that provides unique data regarding the individual and integrated influences of somatosensory, visual, and vestibular inputs on balance [5, 6]. An overview of the theoretical basis for the SOT [56] and guidelines for using the SOT in the management of patients with vestibular disorders [52] are published elsewhere.

Judgements about performance on the SOT are based on comparing the amplitude of the displacement of the center of gravity (COG) during each test trial with a set of theoretical limits of stability in the sagital plane. In general, measures of COG displacement are reported to be adequately reliable [61] and are often used to describe impaired balance in individuals with vestibular disorders [7 , 55]. More specifically, the SOT has been shown to be a valid measure of balance in younger [21, 38] and older [34] healthy adults, as well as those with a history of vertigo [30], peripheral [24, 32] and central vestibular dysfunction [67].

The SOT is not intended to localize vestibular lesions, rather it is designed to be a test of functional ability [28]. Thus, it is not surprising that the SOT has been found to have over 90% specificity in those with vestibular dysfunction but low sensitivity in this population [20]. Additionally, only modest correlations have been found between the SOT and clinical tests of static standing; multiple full-body kinematic and kinetic measures of gait [23]; and timed tests of gait [23]. Further, the SOT does not correlate strongly with diagnostic tests based on the functioning of the vestibular-ocular reflex, such as caloric and rotational chair testing [23]. These findings led Evans and Krebs to question the role of the SOT in measuring vestibulospinal function [23]. Similarly, Mallinson and colleagues found no correlation between performance on the SOT and abnormalities on either ocular or cervical vestibular evoked myogenic potential testing [47]. The discrepancies between performance on the SOT and abnormalities on laboratory-based vestibular function tests do not negate the usefulness of the SOT, rather these findings suggest that each of these tests are measuring different aspects of balance function and that all are potentially useful in patient management [47].

Recently, a next-generation posturography device was developed that includes a modernized version of the SOT. To date, limited data regarding the validity of this next-generation test (NG-SOT) have been published [65]. The options for validating the NG-SOT include conducting a direct comparison with the traditional version of the SOT (T-SOT) to assess whether similar results are obtained for each test.

In this study, we aimed to demonstrate the criterion, concurrent, and convergent validity of the NG-SOT. We anticipated the NG-SOT composite score would show criterion validity by correctly classifying healthy and vestibular-impaired adults. We also believed the NG-SOT would show concurrent validity compared to the T-SOT as composite scores for both systems would not be significantly different. Additionally, we sought to explore convergent validity by examining relationships between NG-SOT scores and patient-reported and performance-based outcome measures.

2 Methods

2.1 Participants

All participants were required to be between 20 and 79 years old; speak English as their primary language; have no impairment in self-reported cognitive function that prevented independent living; have no history of neurological (including concussion), musculoskeletal (including whiplash), vision, or pain conditions that would preclude accurate testing; be able to stand for 20 minutes without sitting and walk 6.096 meters without assistance; be able to withhold certain as-needed medications that may affect performance (See Supplemental Digital Content 1) and abstain from drinking alcohol for 48 hours prior to the study visit; and not be pregnant or planning to become pregnant within three months of being enrolled in the study. Eligible adults also had at least 75% of full, active range of motion in the ankles, knees, and hips; normal bilateral lower-extremity strength; and normal bilateral lower extremity sensation. The 95% confidence interval (CI) from age-based normative data for the Five-times Sit-to-stand Test (FTSTST) [9] was used as a threshold for normal lower extremity strength and the presence of protective sensation on the plantar surface of the feet based on Semmes-Weinstein filament testing [26] was our criteria for normal lower extremity sensation.

Healthy participants were recruited from the University of Wisconsin (UW)-Madison and UW Health systems and were enrolled if they met the above criteria and had no history of a vestibular condition. Vestibular-impaired participants who had been seen previously or were currently being seen for VR were recruited from clinics within the UW Health system. Each of these participants had a diagnosis of unilateral or bilateral vestibular dysfunction that was documented by laboratory-based, vestibular function tests (See Supplemental Digital Content 2). Performance of a home exercise program for VR by these individuals was permitted while participating in this study.

2.2 Participant safety

Studies show VR assessment techniques are safe, well-tolerated, and associated with only infrequent and relatively mild adverse reactions [31, 58]. However, we anticipated potential adverse events (AE) may include vertigo, nausea, vomiting, fear of falling, loss of balance, headache, joint pain, muscle soreness, or emotional distress. Mild dizziness (self-reported intensity ≤3/10) lasting ≤15 minutes was an expected, temporary reaction; thus, only prolonged dizziness (>15 minutes) and/or severe dizziness (self-reported intensity ≥7/10) were to be reported as an AE. Falls and any injury that might occur while performing tests were to be reported as a serious adverse event (SAE).

2.3 Study procedures

The UW-Madison Health Sciences Institutional Review Board approved this research which was conducted in accordance with the provisions of the Declaration of Helsinki [4]. All study data were collected and managed using the Research Electronic Data Capture (REDCap) system [33], a secure web-based application. A telephone screening was conducted prior to enrollment, during which one study visit was scheduled for those who were eligible. All participants underwent a comprehensive clinical examination (See Supplemental Digital Content 2).

2.3.1 Outcome measures

The Activities-specific Balance Confidence Scale (ABCS) [59], Dizziness Handicap Inventory (DHI) [39], Visual Vertigo Analogue Scale (VVAS) [19], and Vestibular Activities and Participation Measure (VAPM) [3] were administered within 48 hours of the start of the study visit. The DHI for healthy participants was scored “0/100” if they had not experienced dizziness and these individuals did not complete the VAPM, since this is a disease-specific measure. All participants were required to complete the ABCS and VVAS. In accordance with published recommendations for validating electronic versions of paper outcome measures [15], we examined the validity of the electronic versions of the patient-reported outcome measures by conducting a cognitive debriefing and usability assessment after testing. We asked each person questions regarding what they were thinking about as they answered the questions on these patient-reported outcomes and relating to their experience of using the REDCap system.

2.3.1.1. Activities-specific Balance Confidence Scale (ABCS): The ABCS [59] consists of 16 questions for which the respondent answers how confident he or she feels about their balance in specific situations. Respondents rate their confidence in not becoming unsteady or falling using a scale of 0% to 100% with 0% indicating no confidence and 100% indicating complete confidence in the ability to maintain balance. The average score on the ABCS, for which higher scores indicate greater balance-related confidence, is reported in the analysis.

2.3.1.2. Dizziness Handicap Inventory (DHI): The DHI [39] consists of 25 questions regarding how often dizziness is experienced in specific functional situations or has impacted physical or emotional well-being. Each question is answered as dizziness is always, sometimes, or never a factor. Responses are scored as “yes” = 4 points, “sometimes” = 2 points, and “no” = 0 points. The maximum total score is 100 points with higher scores indicating higher levels of perceived handicap associated with dizziness. The DHI total score correlates with functional performance when stratified as mild (0–30), moderate (31–60), and severe (>60) [70].

2.3.1.3. Visual Vertigo Analogue Scale (VVAS): The VVAS [19] consists of nine items, each relating to a specific symptom-provoking situation, including walking down supermarket aisles, being a passenger in a car, and being under fluorescent lights. For the visual analog scale score associated with each question, there are two anchors, 0 representing no dizziness and 10 representing extreme dizziness or activity avoidance. We report the average score for the VVAS, which ranges from 0 to 100 with higher scores indicating greater severity of visually-induced dizziness. The internal consistency and validity [19], as well as the responsiveness [18] of the VVAS has been established for persons with visually-induced dizziness.

2.3.1.4. Vestibular Activities and Participation Measure (VAPM): The VAPM [2] consists of 34 items for which participants indicate their level of difficulty performing. Items include focusing attention, moving from sitting to standing, walking on different surfaces, operating a vehicle, shopping, and maintaining a job. Responses are scored as “none” = 0 points, “mild” = 1 point, “moderate” = 2 points, “severe” = 3 points, or “unable to do” = 4 points. Respondents may skip questions that are not applicable. We report the average scores which are calculated based on responses to each question that is answered. The VAPM possesses very good reliability and validity in persons with vestibular and balance disorders [3].

2.3.1.5. Functional Gait Assessment (FGA): The FGA [71] consists of 10 gait-related items. Each item is scored on an ordinal scale of 0–3 points; thus, the optimal score is 30. Participants walked in their preferred, flat-soled shoes along a 6.096-meter path in a clinic hallway with markings that facilitate appropriate scoring embedded in the floor. Data collection was standardized using a stopwatch for timing, a designated obstacle for the step over obstacle task, and consistently using the same set of stairs. The FGA total score is reported in our analyses. The psychometric properties of the FGA are described elsewhere [71].

2.3.1.6. Instrumented 2-minute Walk Test (i2MWT): The i2MWT [48] was used to assess gait by using three body-worn movement monitors that combine several sensors, including a 3-axis accelerometer, a 3-axis gyro, a 3-axis magnetometer, and a temperature sensor within a single package (Mobility Lab, APDM, Inc., Portland, OR, USA; software version 2). One sensor was placed on the torso at the level of the intervertebral disc between the fourth and fifth lumbar vertebrae using the strap provided and the remaining sensors were secured on the dorsum of each foot on top of the participant’s shoes. The i2MWT was administered per the manufacturer’s standardized protocol. Markings outlining dedicated walkways are incorporated into the flooring where the testing was conducted. Participants walked in flat-soled shoes back and forth along a 10-meter walkway at their preferred speed and they performed 180 degree turns in their preferred direction at either end.

2.3.1.7. Sensory Organization Testing: We used random assignment to determine the order of administration for the T-SOT and NG-SOT. A member of the study team who was not aware of the participant’s group placement (BCH) conducted the randomization and revealed it at the time of each participant’s enrollment to the research assistant (CRG) who conducted all testing. Each SOT was conducted in separate private examination rooms dedicated to vestibular and balance testing and treatment using the methods as described by the respective developers. The T-SOT was performed on a NeuroCom SMART Balance Master 6.0 computerized dynamic posturography (CDP) device (NeuroCom International, Inc, a Division of Natus, Clackamas, OR, USA; Equitest, software version 5.07) under overhead recessed LED lighting (See Fig. 1A). We administered the NG-SOT using a Bertec Balance Advantage CDP device (Bertec, Corp, Columbus, OH, USA; Balance Advantage software version 1.0.0, build 1045) under indirect recessed incandescent lighting (See Fig. 1B). A standardized script was utilized to provide instructions to participants. Foot position was marked with painter’s tape once the participant’s bare feet were initially placed on the force platform. We anticipated that each test would take 20 minutes on average, not including rest breaks which were to be provided as needed. In order to minimize possible carry-over effects from one SOT to another, the FGA and i2MWT were conducted during the interim period between the two SOTs.

Fig. 1

Computerized Dynamic Posturography Devices. 1A: NeuroCom SMART Balance Master 6.0 device (NeuroCom International, Inc, a Division of Natus, Clackamas, OR, USA. 1B: Bertec Balance Advantage device (Bertec, Corp, Columbus, OH, USA.

The test protocol for the NG-SOT is standardized by the manufacturer and is conceptually like that for the T-SOT. The T-SOT protocol is published elsewhere [51]; however, a brief overview follows. Balance is assessed during 3 trials of 6 equilibrium conditions (Table 1). Performance on each trial is reported as an equilibrium score which is expressed as a percentage ranging from 0% to 100%. We calculated mean scores for each equilibrium condition. These scores reflect a person’s maximal anterior-posterior COG displacement (in degrees) relative to his or her theoretical anterior-posterior limits of stability. Greater stability is indicated by higher equilibrium scores. Falls result in a score of 0% for that trial. The composite score, which is an overall measure of performance, is calculated as a weighted average using the mean equilibrium scores for conditions 1 and 2 and each equilibrium score from conditions 3 through 6. Estimates of the influence of somatosensory, vision, and vestibular input, as well as visual-vestibular integration on balance are reported as corresponding sensory analysis scores: somatosensory, vision, vestibular, and visual preference (Table 2).

Table 1

Description of the SOT Equilibrium Conditions

Condition	Vision	Support Surface	Visual Reference
1	Eyes Open	Stable	Stable
2	Eyes Closed	Stable	(N/A)
3	Eyes Open	Stable	Sway-referenced
4	Eyes Open	Sway-referenced	Stable
5	Eyes Closed	Sway-referenced	(N/A)
6	Eyes Open	Sway-referenced	Sway-referenced

SOT = Sensory Organization Test, N/A = not applicable.

Table 2

The Derivation of the SOT Sensory Analysis Scores

Sensory Analysis Scores	Ratio of Equilibrium Scores
Somatosensory	Condition 2 / Condition 1
Vision	Condition 4 / Condition 1
Vestibular	Condition 5 / Condition 1
Visual Preference	Conditions 3 + 6 / Condition 2 + 5

SOT = Sensory Organization Test.

2.4 Statistical considerations

The primary outcome of this study was the composite score of the NG-SOT. We planned to test for differences between the healthy and vestibular-impaired groups in terms of the NG-SOT composite score with a two-tailed, two sample t-test with a significance level of 0.05 to assess criterion validity. We based our power calculation on a Cohen’s d effect size (ES) of 1.0, which, if shown, would confirm the validity of the NG-SOT composite score. The assumption was that the standard deviation of the NG-SOT would be near that of the T-SOT (∼18) [72]. Our sample provided 80% power to detect an ES of 1.0.

Planned secondary analyses included testing for differences between groups for each performance score of the NG-SOT with two-sample t-tests and evaluating concurrent validity through paired t-tests between the NG-SOT and T-SOT composite score, as well as the equilibrium and sensory analysis scores. Since the data collected did not meet the assumption of normality, Wilcoxon Rank Sum tests were used in analyses relating to between-test and between-group comparisons. Additionally, we employed Spearman’s method for correlational analyses aimed at determining the convergent validity between the NG-SOT composite score and the ABCS average score, DHI total score, VVAS average score, VAPM average score, FGA total score, and all measures from the i2MWT. All secondary analyses were considered hypothesis generating and conducted at an unadjusted significance level of 0.05.

All analyses were conducted using “R” for statistical computing version 3.5 [1]. We followed the intention to treat principle and conducted sensitivity analyses of missing data before the final analyses.

3 Results

3.1 Demographics

The characteristics of the study population and scores for specific outcome measures are presented in Table 3. The vestibular-impaired adults were significantly older than the healthy adults; however, the gender ratio and average body mass index for each group were not significantly different. Based on post-hoc analysis, age was not a significant factor for between-group differences (Supplemental Digital Content 3). Compared to the healthy adults, a greater proportion of vestibular-impaired adults reported a history of at least one fall in the 6 months prior to enrolling in the study. The vestibular-impaired group had worse scores on all patient-reported and performance-based outcome measures. No AEs and SAEs occurred during this study. One subject requested to rest between trials during NG-SOT testing and they chose to remain standing on the posturography device during this break.

Table 3
Study Population Characteristics

Healthy (n = 17) Impaired (n = 17) P-value

Demographics

Age (Years)^∧ 39.3 (11.2) 61.1 (10.2) <0.001

Gender (Female)^* 13 (76.5%) 9 (52.9%) 0.28

BMI^∧ 26.0 (4.4) 25.3 (4.7) 0.68

> = 1 fall in prior 6 months^* 1 (5.9%) 6 (35.3%) 0.09

Medication Usage^*

Dizziness as a side-effect 2 (11.8%) 5 (29.4%) 0.14

Anti-vertigo 0 (0.0%) 2 (11.8%) 0.48

Sedatives 0 (0.0%) 0 (0.0%) 1.0

Narcotics or barbiturates 0 (0.0%) 1 (5.9%) 1.0

Patient-reported Measures‘

ABCS Average Score 99.0 (99.0–100.0) 89.0 (81.0–95.0) <0.001

DHI Total Score 0.0 (0.0-0.0) 28.0 (22.0–36.0) <0.001

VVAS Average Score 0.0 (0.0-0.0) 15.4 (0.0–33.0) <0.001

VAPM Average Score N/A (N/A-N/A) 0.7 (0.4–1.0) N/A

Performance-based Measures‘

Horizontal DVAT (degradation in visual acuity) 1.0 (1.0–2.0) 5.0 (3.0–7.0) <0.001

Vertical DVAT (degradation in visual acuity) 1.0 (1.0-1.0) 4.0 (3.0–7.0) <0.001

FTSTST (time) 6.9 (6.2–8.5) 9.6 (8.8–11.1) <0.001

FGA Total Score 30.0 (29.0–30.0) 24.0 (20.0–26.0) <0.001

	Healthy (n = 17)	Impaired (n = 17)	P-value
Demographics
Age (Years)^∧	39.3 (11.2)	61.1 (10.2)	<0.001
Gender (Female)^*	13 (76.5%)	9 (52.9%)	0.28
BMI^∧	26.0 (4.4)	25.3 (4.7)	0.68
> = 1 fall in prior 6 months^*	1 (5.9%)	6 (35.3%)	0.09
Medication Usage^*
Dizziness as a side-effect	2 (11.8%)	5 (29.4%)	0.14
Anti-vertigo	0 (0.0%)	2 (11.8%)	0.48
Sedatives	0 (0.0%)	0 (0.0%)	1.0
Narcotics or barbiturates	0 (0.0%)	1 (5.9%)	1.0
Patient-reported Measures‘
ABCS Average Score	99.0 (99.0–100.0)	89.0 (81.0–95.0)	<0.001
DHI Total Score	0.0 (0.0-0.0)	28.0 (22.0–36.0)	<0.001
VVAS Average Score	0.0 (0.0-0.0)	15.4 (0.0–33.0)	<0.001
VAPM Average Score	N/A (N/A-N/A)	0.7 (0.4–1.0)	N/A
Performance-based Measures‘
Horizontal DVAT (degradation in visual acuity)	1.0 (1.0–2.0)	5.0 (3.0–7.0)	<0.001
Vertical DVAT (degradation in visual acuity)	1.0 (1.0-1.0)	4.0 (3.0–7.0)	<0.001
FTSTST (time)	6.9 (6.2–8.5)	9.6 (8.8–11.1)	<0.001
FGA Total Score	30.0 (29.0–30.0)	24.0 (20.0–26.0)	<0.001

These data are reported as ^∧= mean (standard deviation) from two-tailed t-tests, ^* = N (%) from chi-square tests, or ‘ = median (interquartile range) from Wilcoxon Rank Sum tests (as appropriate for the distribution of each data type). BMI = body mass index, ABCS = Activities-specific Balance Confidence Scale, DHI = Dizziness Handicap Inventory, VVAS =Visual Vertigo Analogue Scale, FTSTST = Five-times Sit-to-stand Test, DVAT = Dynamic Visual Acuity Test, FGA = Functional Gait Assessment, and N/A = not applicable.

3.2 Criterion validity

We assessed the discriminate ability of each version of the SOT by modeling the sensitivity (true positive rate) and specificity (false positive rate) of the composite score from each test using receiver operator characteristics (ROC) analyses. The area under the curve (AUC) (95% CI) for the composite score on the NG-SOT and T-SOT were 0.950 (0.885, 1) and 0.990 (0.969, 1) respectively but were not significantly different (p = 0.166) (Fig. 2). In addition, we calculated an optimal cut-point for the NG-SOT composite score of 82.4 points using Youden’s index. This threshold for discriminating between healthy and vestibular-impaired adults had 94% sensitivity and specificity.

Fig. 2

Receiver Operator Characteristic Curves for the SOT Composite Scores. SOT = Sensory Organization Test. The area under the curve for the receiver operator characteristic analysis of the ability to discriminate vestibular-impaired participants from healthy participants using the SOT composite score is plotted as the true positive rate versus the false positive rate in 2A for the next-generation SOT and in 2B for the traditional SOT.

3.3 Concurrent validity

The vestibular-impaired group had greater postural sway than the healthy group during all equilibrium conditions on both versions of the SOT (Table 4). Additionally, the vision, vestibular, and composite scores from both versions of the SOT for the vestibular-impaired group were lower than those of the healthy group (Table 5).

Table 4
Summary of Results for the SOT Mean Equilibrium Scores

SOT Component Version Healthy (n = 17) Impaired (n = 17) P-value^*

Mean Equilibrium 1 T-SOT 96.3 (95.3–97.0) 95.0 (94.0–95.7) 0.009

NG-SOT 94.0 (93.0–94.7) 91.7 (90.7–93.0) 0.01

P-value^∧ <0.001 <0.001

Mean Equilibrium 2 T-SOT 93.3 (92.0–93.7) 90.7 (87.3–91.7) 0.031

NG-SOT 93.7 (91.0–94.7) 90.7 (85.7–92.0) 0.008

P-value^∧ 0.499 0.783

Mean Equilibrium 3 T-SOT 93.3 (92.3–95.0) 90.0 (87.3–91.7) 0.002

NG-SOT 93.0 (91.7–93.3) 89.7 (85.0–92.3) 0.011

P-value^∧ 0.232 0.743

Mean Equilibrium 4 T-SOT 91.7 (90.3–92.3) 84.7 (74.7–87.0) <0.001

NG-SOT 86.7 (82.0–87.0) 73.3 (62.3–78.0) <0.001

P-value^∧ <0.001 0.002

Mean Equilibrium 5 T-SOT 78.0 (74.7–81.7) 50.0 (37.0–61.7) <0.001

NG-SOT 77.3 (75.0–83.0) 50.3 (0.0–62.3) <0.001

P-value^∧ 0.973 0.903

Mean Equilibrium 6 T-SOT 81.7 (77.0–84.0) 57.7 (0.0–63.3) <0.001

NG-SOT 76.3 (70.7–80.3) 52.7 (34.3–62.0) <0.001

P-value^∧ 0.046 1.0

SOT Component	Version	Healthy (n = 17)	Impaired (n = 17)	P-value^*
Mean Equilibrium 1	T-SOT	96.3 (95.3–97.0)	95.0 (94.0–95.7)	0.009
	NG-SOT	94.0 (93.0–94.7)	91.7 (90.7–93.0)	0.01
	P-value^∧	<0.001	<0.001
Mean Equilibrium 2	T-SOT	93.3 (92.0–93.7)	90.7 (87.3–91.7)	0.031
	NG-SOT	93.7 (91.0–94.7)	90.7 (85.7–92.0)	0.008
	P-value^∧	0.499	0.783
Mean Equilibrium 3	T-SOT	93.3 (92.3–95.0)	90.0 (87.3–91.7)	0.002
	NG-SOT	93.0 (91.7–93.3)	89.7 (85.0–92.3)	0.011
	P-value^∧	0.232	0.743
Mean Equilibrium 4	T-SOT	91.7 (90.3–92.3)	84.7 (74.7–87.0)	<0.001
	NG-SOT	86.7 (82.0–87.0)	73.3 (62.3–78.0)	<0.001
	P-value^∧	<0.001	0.002
Mean Equilibrium 5	T-SOT	78.0 (74.7–81.7)	50.0 (37.0–61.7)	<0.001
	NG-SOT	77.3 (75.0–83.0)	50.3 (0.0–62.3)	<0.001
	P-value^∧	0.973	0.903
Mean Equilibrium 6	T-SOT	81.7 (77.0–84.0)	57.7 (0.0–63.3)	<0.001
	NG-SOT	76.3 (70.7–80.3)	52.7 (34.3–62.0)	<0.001
	P-value^∧	0.046	1.0

These data are reported as the median (interquartile range). SOT = Sensory Organization Test, T-SOT = traditional Sensory Organization Test, NG-SOT = next-generation Sensory Organization Test. The ^*p-value is from a Wilcoxon Rank Sum test comparing the healthy and vestibular-impaired groups within each SOT version. The ^∧P value is from a Wilcoxon Rank Sum test comparing the performance of healthy and vestibular-impaired participants on the T-TOT versus the NG-SOT.

Table 5

Summary of Results for the SOT Composite and Sensory Analysis Scores

Score	Version	Healthy (n = 17)	Impaired (n = 17)	P-value^*
Somatosensory	T-SOT	96.0 (95.0–98.0)	95.0 (93.0–97.0)	0.135
	NG-SOT	100.0 (97.0–101.0)	99.0 (94.0–100.0)	0.171
	P-value^∧	0.001	0.053
Vision	T-SOT	95.0 (94.0–97.0)	89.0 (84.0–91.0)	<0.001
	NG-SOT	91.0 (90.0–93.0)	80.0 (68.0–84.0)	<0.001
	P-value^∧	0.001	0.003
Vestibular	T-SOT	82.0 (78.0–84.0)	52.0 (39.0–65.0)	<0.001
	NG-SOT	82.0 (80.0–88.0)	63.0 (0.0–71.0)	<0.001
	P-value^∧	0.36	0.566
Visual Preference	T-SOT	102.0 (100–105)	101.0 (93.0–105)	0.448
	NG-SOT	98.0 (95.0–101)	99.0 (94.0–100.0)	0.782
	P-value^∧	0.021	0.428
Composite	T-SOT	87.5 (85.0–88.5)	74.1 (56.7–77.0)	<0.001
	NG-SOT	84.9 (81.5–86.1)	67.9 (54.9–74.8)	<0.001
	P-value^∧	0.014	0.547

The healthy and vestibular-impaired groups had greater postural sway during equilibrium conditions 1 and 4 on the NG-SOT compared to the T-SOT (Table 4). Additionally, the healthy group was also more unstable during equilibrium condition 6 on the NG-SOT compared to the T-SOT (Table 4) and only healthy participants had lower composite, somatosensory, and visual preference scores on the NG-SOT compared to the T-SOT (Table 5). We also found that vision scores were lower on the NG-SOT compared to the T-SOT within each group (Table 5).

3.4 Convergent validity

Correlational analysis was used to evaluate the convergent validity of the NG-SOT (Table 6). The composite score showed a strong, positive association with the ABCS average score and a strong, negative association with the DHI total score. Moderate, negative associations were found between the composite score and the VVAS and VAPM average scores. The analysis also revealed moderate, negative associations between the composite score and the time to complete the FTSTST and degradation in visual acuity during the horizontal and vertical dynamic visual acuity tests (DVAT) from the clinical examination. In addition, a strong, positive association was seen between the composite score and the FGA total score.

Table 6
Correlational Analysis to Assess Convergent Validity of the NG-SOT

Patient-reported Outcome Measures N Rho (95% CI)

ABCS Average Score 34 0.71 (0.49, 0.85)

DHI Total Score 34 –0.80 (–0.9, –0.64)

VVAS Average Score 34 –0.59 (–0.78, –0.32)

VAPM Average Score 17 –0.61 (–0.84, –0.18)

Performance-based Outcome Measures

Non-instrumented Tests

Horizontal DVAT (degradation in visual acuity) 34 –0.69 (–0.83, –0.46)

Vertical DVAT (degradation in visual acuity) 34 –0.69 (–0.83, –0.45)

FTSTST (time) 34 –0.59 (–0.77, –0.31)

FGA Total Score 34 0.73 (0.52, 0.86)

Instrumented 2-minute Walk Test

Cadence 30 0.39 (0.03, 0.66)

Velocity 30 0.51 (0.18, 0.74)

Cycle Time 30 –0.39 (–0.66, –0.04)

Double Support 30 –0.50 (–0.73, –0.17)

Lateral Step Variability (Left) 30 –0.60 (–0.79, –0.3)

Lateral Step Variability (Right) 30 –0.57 (–0.77, –0.26)

Stance (Left) 30 –0.26 (–0.56, 0.11)

Stance (Right) 30 –0.44 (–0.69, –0.1)

Step Duration (Left) 30 –0.43 (–0.68, –0.08)

Step Duration (Right) 30 –0.37 (–0.65, –0.02)

Stride Length (Left) 30 0.51 (0.18, 0.73)

Stride Length (Right) 30 0.57 (0.26, 0.77)

Swing (Left) 30 0.26 (–0.11, 0.56)

Swing (Right) 30 0.44 (0.1, 0.69)

Trunk ROM (Coronal) 30 0.51 (0.19, 0.74)

Trunk ROM (Sagital) 30 –0.25 (–0.56, 0.12)

Trunk ROM (Transverse) 30 –0.01 (–0.37, 0.35)

Turn Angle 30 0.33 (–0.03, 0.62)

Turn Velocity 30 0.63 (0.36, 0.81)

Turn Duration 30 –0.61 (–0.8, –0.32)

Turn (Number of Steps) 30 –0.15 (–0.49, 0.22)

Patient-reported Outcome Measures	N	Rho (95% CI)
ABCS Average Score	34	0.71 (0.49, 0.85)
DHI Total Score	34	–0.80 (–0.9, –0.64)
VVAS Average Score	34	–0.59 (–0.78, –0.32)
VAPM Average Score	17	–0.61 (–0.84, –0.18)
Performance-based Outcome Measures
Non-instrumented Tests
Horizontal DVAT (degradation in visual acuity)	34	–0.69 (–0.83, –0.46)
Vertical DVAT (degradation in visual acuity)	34	–0.69 (–0.83, –0.45)
FTSTST (time)	34	–0.59 (–0.77, –0.31)
FGA Total Score	34	0.73 (0.52, 0.86)
Instrumented 2-minute Walk Test
Cadence	30	0.39 (0.03, 0.66)
Velocity	30	0.51 (0.18, 0.74)
Cycle Time	30	–0.39 (–0.66, –0.04)
Double Support	30	–0.50 (–0.73, –0.17)
Lateral Step Variability (Left)	30	–0.60 (–0.79, –0.3)
Lateral Step Variability (Right)	30	–0.57 (–0.77, –0.26)
Stance (Left)	30	–0.26 (–0.56, 0.11)
Stance (Right)	30	–0.44 (–0.69, –0.1)
Step Duration (Left)	30	–0.43 (–0.68, –0.08)
Step Duration (Right)	30	–0.37 (–0.65, –0.02)
Stride Length (Left)	30	0.51 (0.18, 0.73)
Stride Length (Right)	30	0.57 (0.26, 0.77)
Swing (Left)	30	0.26 (–0.11, 0.56)
Swing (Right)	30	0.44 (0.1, 0.69)
Trunk ROM (Coronal)	30	0.51 (0.19, 0.74)
Trunk ROM (Sagital)	30	–0.25 (–0.56, 0.12)
Trunk ROM (Transverse)	30	–0.01 (–0.37, 0.35)
Turn Angle	30	0.33 (–0.03, 0.62)
Turn Velocity	30	0.63 (0.36, 0.81)
Turn Duration	30	–0.61 (–0.8, –0.32)
Turn (Number of Steps)	30	–0.15 (–0.49, 0.22)

NG-SOT = next-generation Sensory Organization Test, Activities-specific Balance Confidence Scale, DHI = Dizziness Handicap Inventory, VVAS = Visual Vertigo Analogue Scale, FTSTST =Five-times Sit-to-stand Test, DVAT = Dynamic Visual Acuity Test, and FGA = Functional Gait Assessment.

4 Discussion

To our knowledge, this study is the first to assess the criterion, concurrent, and convergent validity of the NG-SOT in adults with and without vestibular dysfunction. Our data demonstrates that the NG-SOT and the T-SOT have excellent criterion validity. We also established that the NG-SOT shows concurrent validity with its predecessor based on a comparison of composite scores across both tests for vestibular-impaired participants. Additionally, we found that the NG-SOT shows good convergent validity through moderate to strong associations with patient-reported and performance-based outcome measures frequently administered to patients with vestibular dysfunction.

4.1 Criterion validity

The excellent sensitivity and specificity associated with the optimal cut-score for the NG-SOT composite score of 82.4 points in this study population far exceeds those of early studies involving the T-SOT which were assessed by DiFabio in a historical review [20]. None of the studies included in DiFabio’s analysis utilized the AUC from ROC methods to determine an optimal cut-score for the T-SOT. Furthermore, most of the studies examined employed retrospective methods and did not include a healthy control group. It is these differences in approach which likely explain the discrepancies between our results and the older literature involving the T-SOT.

More recent studies using the T-SOT have produced mixed results regarding the ability to discriminate populations. There is evidence supporting [57, 68] and questioning [63] the ability of the T-SOT composite score to discriminate fallers and non-fallers who are community-dwelling older adults. Other data suggests that performance during condition 6 [11] and the composite score [69] may discriminate between non-fallers and frequent fallers. Additionally, raw data pertaining to the COG displacement during the T-SOT has been shown to have good sensitivity and specificity for discriminating fallers from non-fallers in a population of people living with multiple sclerosis [42]. There is also support for the ability to discriminate between higher and lower levels of disability related to multiple sclerosis based on the T-SOT composite score [36]. Thus, the discriminate abilities of the NG-SOT depend on the population being studied, which aspects of the test are used to assess criterion validity, and the statistical methods being utilized.

4.2 Concurrent validity

4.2.1 Comparison of performance on the NG-SOT versus the T-SOT

While vestibular-impaired and healthy adults showed similar patterns of performance on both versions of the SOT, when analyzing our data across both groups, we noted important differences in how participants scored on one test versus the other. Participants were overall less stable during the NG-SOT compared to the T-SOT. The most dramatic differences in performance from one test to the other were seen for conditions 1 and 4, during which healthy and vestibular-impaired participants were significantly less stable on the NG-SOT. Additionally, healthy participants were also significantly less stable during condition 6 on the NG-SOT compared to the T-SOT. The instability observed in these equilibrium conditions is further reflected in the somatosensory and vision scores which are derived from the equilibrium scores.

The differences that we observed between performance on the NG-SOT and T-SOT are consistent with and extend those reported by Trueblood and colleagues and underscore the need for new normative data for the NG-SOT [65]. In the study by Trueblood and colleagues, the concurrent validity of the NG-SOT was assessed by comparing actual performance on the NG-SOT to published normative data for the T-SOT in a population of healthy adults between the ages of 20 and 69 years old. Participants in their study were also shown to be less stable throughout conditions 1, 4, and 6 on the NG-SOT compared to the T-SOT norms. In contrast, we administered both tests to each participant after randomizing the order of testing and allowed for rest and other assessments to be conducted between the SOT tests.

4.2.2 Factors that potentially influence performance on the NG-SOT

We believe the psychophysical properties of the virtual environment used for the NG-SOT, such as the lack of a horizon and the quality of the virtual imagery, are important factors in the discrepancies in performance on this test compared to the T-SOT. It is well-established that the presence of visual references in the environment increase postural stability [49, 50]. Further, perception of the reality and stability of the visual cues in a virtual environment also influence postural control [16]. Unlike the enclosure used with the T-SOT, there are minimal horizontal references that are useful for spatial orientation within the field-of-view inside the enclosure used for the NG-SOT. The only reference to upright that is available for gravity estimation is the vertically-oriented, luminous, virtual tunnel that is projected on to the enclosure. A paucity of visually-orienting cues would be expected to impact performance during conditions 1 and 4 in which the virtual visual environment is stable. Further, a dearth of visually-orienting cues would have a more pronounced impact on performance during condition 4, when the surface is sway-referenced and the availability of orienting sensory cues is further reduced, compared to condition 1. This is supported by the fact that both healthy and vestibular-impaired participants had significantly greater postural sway during conditions 1 and 4 on the NG-SOT compared to the T-SOT.

Consistent with these conclusions, the NG-SOT somatosensory score was likely influenced by the removal of a disorienting visual environment with transition from condition 1 (eyes open, stable image, and stable surface) to condition 2 (eyes closed, stable surface). Likewise, the vision score appears to have been influenced by difficulty using the available, but minimally-effective, visual cues for balance during condition 4 (eyes open, stable visual environment, and sway-referenced surface). Additionally, the finding that healthy participants had lower visual preference scores on the NG-SOT compared to the T-SOT is consistent with their poorer performance during condition 6 on the NG-SOT compared to the T-SOT. Thus, simultaneous sway-referencing of the visual environment and the surface appears to be more destabilizing during the NG-SOT than T-SOT.

Another important consideration for understanding the effects of image quality on postural control during the NG-SOT is the contrast ratio of the virtual environment. Adaptation of vestibular reflexes has been shown to be influenced by the contrast ratio of a visual stimulus [53]. Furthermore, impaired contrast sensitivity is associated with poor balance control [43]. Thus, in order to maximize the contrast ratio of the virtual tunnel used during the NG-SOT, we conducted this test with indirect incandescent lighting rather than the direct LED lighting that was used during the T-SOT. Conducting the NG-SOT under the same bright ambient lighting used for the T-SOT would have effectively reduced the contrast ratio of the virtual tunnel, which would have had an effect equivalent to inducing impaired contrast sensitivity in those being tested. We believe our methods ensured the maximum effectiveness of the virtual environment. However, further research is needed to better characterize the effects of various aspects of the psychophysical properties of virtual environments on sensory organization testing and whether there is an optimal level and type of ambient lighting in which to conduct the NG-SOT.

The acoustical properties of the enclosure used for the NG-SOT may also impact performance. Some subjects reported being disoriented by the sound of the examiner’s voice or their own voice echoing off the NG-SOT enclosure while listening to instructions and/or while asking or responding to questions. Additionally, we observed that the internal fan that cools the NG-SOT virtual projection system, which is mounted directly above the force platform, may create either a sound localizing, masking, or white noise effect. These are important considerations because auditory information has been shown to influence postural control in healthy [13] and vestibular-impaired [22] adults. A single sound source [73] and white noise have been found to have stabilizing effects on balance, particularly during conditions of visual deprivation [60]. Masking of auditory cues during quiet standing has been shown to increase reliance on visual cues for postural control [46]. It is not yet known whether any of these effects are substantially present within the environment of the NG-SOT. However, it is plausible that standing in a disorienting sound-field may be perceived as a postural-threat, which, in turn, may have a generalized impact on performance.

4.3 Convergent validity

The strength of the association that we observed between performance on the NG-SOT and balance-related confidence as measured by the ABCS average score in this study of adults with and without vestibular dysfunction is stronger than what Cameron and colleagues found between the performance on the T-SOT and the ABCS in a population of people living with multiple sclerosis [12]. Additionally, our prospective analysis of data from 34 participants also revealed a stronger association between the NG-SOT and the severity of self-perceived disability secondary to dizziness (DHI total score) than previously identified for the T-SOT in a prospective study of 367 adults who were being seen for balance function evaluations [40], a retrospective analysis of data from 87 patients with vestibular neuritis [62], and a retrospective study of patients with vestibular schwannomas [54]. Methodological differences related to study design (population, perspective, inclusion of healthy control participants, time since onset of vestibular symptoms, sample size, and SOT version) likely explain the discrepancies between our findings and those of previous studies.

Our findings regarding the moderate associations between the NG-SOT and gait velocity, double support, lateral variability in stepping, stride length, turn velocity, and turn duration (i2MWT data) for adults with and without vestibular disorders are stronger than those of others who found weak correlations between performance on the T-SOT and instrumented and non-instrumented spatiotemporal parameters of gait in similar populations [23 , 35]. Additionally, we found a stronger association between the NG-SOT composite score and dynamic balance during gait with sensory challenges as measured by the FGA in this study population compared to prior studies that assessed the relationship between the FGA and the T-SOT in older adults and persons with Parkinson’s disease [34, 35]. Once again, methodological differences related to study design (population, perspective, and SOT version) likely explain the discrepancies between our results and those of others.

We also identified relationships between sensory organization testing and patient-reported and performance-based outcomes that have not been previously reported. Specifically, we found that performance on the NG-SOT was moderately associated with the severity of visually-induced dizziness and the extent of limitations in daily activities and participation (VVAS and VAPM average scores, respectively), These findings suggest the NG-SOT may be a useful outcome measure for persons with motion sickness and that some activity limitations and participation restrictions may be related to impaired functional utilization of sensory input for balance. Additionally, we identified moderate relationships between the NG-SOT composite score and the severity of gaze instability (degradation in visual acuity during the DVAT) and functional leg strength and anticipatory balance (time to complete the FTSTST). These associations extend the usefulness of the NG-SOT to it being a valid measure of balance in persons with impaired vision during head or body movement and those with impaired balance during transitional movements.

4.4 Limitations

In anticipation of a lengthy recruitment period, we decided to use a convenience sample rather than attempt to conduct this study using age-matched controls. This resulted in the healthy and vestibular-impaired groups being imbalanced regarding age. Age has been found to impact balance in community-dwelling, healthy adults [37 , 66] and persons with vestibular dysfunction [27 , 45]. Though age has been shown to affect T-SOT performance [10 , 34], age was not statistically meaningful for explaining the between-group differences in performance on the NG-SOT in this study population. However, this study was not powered to detect differences between specific age groups and there were only 17 participants in the overlapping age band (See Supplemental Digital Content 3). Thus, we must acknowledge the biological plausibility that age may have played a greater role in the performance of the vestibular-impaired group compared to the healthy group.

4.5 Conclusions

The NG-SOT and T-SOT have comparable ability to discriminate between vestibular-impaired and healthy adults. Thus, the NG-SOT and the T-SOT are valid measures of postural control for adults with and without vestibular dysfunction. Vestibular-impaired and healthy adults show similar patterns of performance across the NG-SOT and T-SOT; however, our data suggests that the postural challenge differs between these two tests. Therefore, results from the NG-SOT and T-SOT should not be used interchangeably. Regardless of which CDP device is used to conduct the SOT, sensory organization testing provides valuable information related to the functional use of sensory inputs for standing balance.

Conflicts of interest and sources of funding

Dr. Grove is a consultant for Wicab, Inc. Dr. Whitney is a paid speaker for Interacoustics and Medbridge. Dr. Heiderscheit has an ownership interest in NxtMile, LLC, as well as Science of Running Medicine, LLC, and is a consultant for Altec, Inc and MountainLand Rehabilitation. No other conflicts of interest are declared for any of the authors. The project described was supported by the Clinical and Translational Science Award (CTSA) program, through the NIH National Center for Advancing Translational Sciences (NCATS), grant UL1TR000427 and grant TL1TR002375. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. Additional funding was provided by the University of Wisconsin through a Research Assistantship for Dr. Grove through the Department of Surgery and a Research Grant from the Department of Orthopedics and Rehabilitation.

Footnotes

Acknowledgments

This research was made possible by the engagement of all participants and accommodations by the clinicians and staff from the UW Health Rehabilitation Clinic. The authors wish to thank Dr. Kristin E. Caldera who helped steward funding from the Department of Orthopedics and Rehabilitation, Dr. Samuel P. Gubbels who provided early predoctoral training and mentorship to Dr. Grove, and Dr. Lewis M. Nashner for his technical advice and contributions.

The supplementary material is available in the electronic version of this article: .

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