The Niigata PPPD Questionnaire
The English translation of the Niigata PPPD Questionnaire (NPQ) is presented in Table 3. The NPQ evaluates the degree of symptom exacerbation by the three characteristic factors: upright posture/walking, movement, and visual stimulation. Each factor was assessed by four questions; thus, there were 12 questions in the questionnaire. Q3, 6, 7, and 11 pertained to upright posture/walking; Q1, 5, 9, and 12 pertained to movement; and Q2, 4, 8, and 10 pertained to visual stimulation. Each question was scored from 0 (none) to 6 (unbearable); therefore, the total score for each factor was 24, and the total score for all three factors was 72. Somatic distress due to PPPD was evaluated by the 100 mm Visual Analog Scale (VAS). Psychological distress was evaluated by the Hospital Anxiety and Depression Scale (HADS) (2), which consists of anxiety and depression subscales. Each HADS subscale was assessed by seven questions. Each question was scored from 0 (not at all) to 3 (most of the time, very often); therefore, the total score for each HADS subscale was 21, and the total HADS score was 42.
The reliability of the questionnaire was tested by Cronbach's alpha (3). The questionnaire was validated by examining the differences in the total NPQ score, score for each factor, VAS score, and HADS score between the PPPD and control groups using the Mann–Whitney U test. To compare the diagnostic capability of the factors, a receiver operating characteristic (ROC) curve was constructed and the area under the curve (AUC) for each factor was calculated. All statistical analyses were performed using SPSS version 21.0 for Windows. The significance level was set at p < 0.05.
As indicated in Table 4, the Cronbach's alpha coefficients for the total score and the scores for each factor were >0.8, with the exception of that for movement (0.75).
Figure 1 illustrates the differences in NPQ scores between the PPPD group and the control group. The total score (Fig. 1A) and the scores for each factor (upright posture/walking, movement, and visual stimulation) were significantly higher in the PPPD group than in the control group. The largest difference between the two groups was for visual stimulation (Fig. 1B).
Figure 2 presents the VAS (Fig. 2A) and HADS (Fig. 2B) scores for the PPPD and control groups. There were no significant differences in the VAS and HADS scores—total HADS scores as well as anxiety and depressive subscale scores—between the two groups.
Figure 3 presents the ROC curves for the total score and the scores for each NPQ factor. The AUC of the ROC curve and the cut-off point to identify the best sensitivity and specificity for diagnosing PPPD for each factor are also presented. The AUC for the total score was 0.780 and scores of 27 and 29 (full score: 6 × 12 = 72) had the best sensitivity (score of 27: 70%; score of 29: 68%) and specificity (score of 27: 68%; score of 29: 70%) for diagnosing PPPD. The AUC was widest for the visual stimulation factor (0.830) and a score of 9 (full score: 6 × 4 = 24) had the best sensitivity (82%) and specificity (74%) for discriminating PPPD patients from controls. The AUC was narrowest for upright posture/walking (0.684).
PPPD is classified as a functional disorder but is not a structural or psychiatric condition (1). According to recent imaging studies, functional changes in postural control mechanisms, multisensory information processing, or cortical integration of spatial orientation and threat assessment were postulated as pathophysiological mechanisms of PPPD (1,4–8). Indeed, the diagnostic criteria for PPPD (1) do not include laboratory test thresholds (e.g., vestibulo-ocular reflex gain) that are included in the diagnostic criteria of bilateral vestibulopathy (9). The diagnosis of PPPD requires a precise assessment of vestibular symptoms, exacerbating factors, and medical history. A validated questionnaire would be highly beneficial for the full and efficient evaluation of these points. Therefore, we developed a questionnaire for diagnosing and assessing PPPD severity.
Cronbach's alpha is widely used for the assessment of questionnaire reliability (3). The coefficient ranges from 0 to 1 and values of >0.8 typically indicate high reliability in terms of internal consistency (3). As indicated in Table 3, the Cronbach's alpha coefficients of the total NPQ score and the score for each NPQ factor were high (0.83–0.91), with the exception of that for the movement factor (0.75). This indicates that most NPQ factors (aside from movement) were highly reliable with high internal consistency. Regarding the slightly lower value for movement, this factor was measured using questions that could be divided into two categories: active-movement-induced and passive-movement-induced symptoms. In a clinical setting, one patient is highly susceptible to active movements, whereas the vestibular symptoms of another patient decrease with exposure to passive movements. Such inconsistency among movement questions would result in a low Cronbach's alpha coefficient for the movement factor (0.75) relative to those for other factors (>0.83).
To validate the questionnaire, the total NPQ score and the scores for each NPQ factor were compared between the PPPD group and the control group. The total score of the NPQ and the scores for each exacerbating factor of the NPQ were significantly higher in the PPPD group than in the control group (Fig. 1). In contrast, the VAS score did not differ between the PPPD and control groups (Fig. 2A). Thus, the differences in the NPQ scores between the PPPD and control groups were not due to differences in subjective severity as measured by VAS. Rather, the higher NPQ scores in the PPPD group relative to the control group suggest that this questionnaire effectively captured key PPPD characteristics and was useful for discriminating PPPD patients from control patients and for assessing the severity of PPPD.
The visual stimulation factor had the widest AUC (0.830) among the three exacerbating factors (Fig. 3). Moreover, the differences in the visual stimulation score between the PPPD and control groups were the most apparent among the three exacerbating factors (Fig. 1B), suggesting that visual stimulation was the most characteristic exacerbating factor. Notably, symptom exacerbation by upright posture/walking was also observed in the control group; therefore, the AUC of this factor was relatively narrow (0.684, Fig. 3). Similar results were reported previously in a congress abstract (10).
Regarding the diagnosing capability of the NPQ, a visual stimulation score of 9 (full score = 24) had the best sensitivity (82%) and specificity (74%) for discriminating PPPD patients from control patients. In addition, total NPQ scores of 27 and 29 (full score = 72) had the best sensitivity (score of 27: 70%; score of 29: 68%) and specificity (score of 27: 68%; score of 29: 70%) for discriminating PPPD patients from control patients. However, care should be taken when using the NPQ to diagnose PPPD. The NPQ does not include questions regarding the core vestibular symptoms (i.e., dizziness, unsteadiness, and nonspinning vertigo) or the time from onset (>3 mo), which must be assessed to diagnose PPPD. This omission was incurred because we wished to develop the NPQ for both diagnosis and the assessment of PPPD severity. The types of vestibular symptoms and the disease duration were not suitable for scoring. We assume that the NPQ may be more useful for diagnosing PPPD in patients with confirmed core vestibular symptoms and a known time from onset.
As another finding of this study, the PPPD patients (49.5 yr, range 25–79 yr) were significantly younger than the control patients (57.3 yr, range 20–82 yr). This is consistent with the findings of a previous report (11) and suggests that PPPD was not an entity of nonspecific age-related disorders. As illustrated in Figure 2B, the anxiety and depression status did not differ between the PPPD and control groups. We have reported that chronic dizziness patients with comorbid psychiatric disorders have more subjective handicaps than those without comorbidities (12). According to the diagnostic criteria, PPPD may coexist with other diseases or disorders (1). Therefore, comorbidities, such as anxiety and depression, could increase the severity of PPPD. However, the current study results suggest that the psychiatric status of PPPD patients was not necessarily more anxious or depressive than that of patients with other vestibular disorders. Thus, PPPD should be clearly discriminated from psychogenic diseases in the diagnosis of chronic dizziness.
Regarding limitations, this was a cross-sectional study that did not include a follow-up period; therefore, we procured no evidence that the questionnaire was sufficiently sensitive to assess the changes in disease severity following treatment. Additionally, we did not control for treatment when enrolling the patients—some patients were enrolled before treatment initiation, while others received medication during the study period. For the patients receiving treatment, the disease severity might have been underestimated and, thus, may have affected the score for the questionnaire. Considering these points, we suggest that the NPQ can be used best as an adjunct to clinical diagnosis; however, it is not a substitute for careful attention to history and differential diagnosis.
In the future, we would like to determine the test–retest reliability of the NPQ and to test the existence of subtypes according to the exacerbating factors using factor analysis.
In conclusion, we developed a questionnaire to aid in PPPD diagnosis and assessment. Among three exacerbating factors, visual stimulation was the most distinctive for PPPD. A visual stimulation factor score of 9 exhibited the best sensitivity (82%) and specificity (74%) for discriminating PPPD from control diseases.
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Keywords:Copyright © 2019 by Otology & Neurotology, Inc. Image copyright © 2010 Wolters Kluwer Health/Anatomical Chart Company
Cronbach's alpha; Persistent postural-perceptual dizziness; Receiver operating characteristic curve; Reliability; Validity