Investigation of Mal de Debarquement Syndrome in Pilots Based on Flight Time
Article information
Abstract
Background and Objectives
This study aimed to evaluate Mal de Debarquement syndrome (MdDS) in high-risk pilots using the Istanbul MdDS Symptom Questionnaire (IMdDSSQ) and investigate the effect of flight time.
Subjects and Methods
The IMdDSSQ was administered to 150 healthy pilots, ≥18 years (37.47±11.14 years), on active duty, using a Google Forms link. The responses to the questionnaire were assessed based on flight time (<4 h and >4 h) and age group, for all subfactors.
Results
For the MdDS diagnosis subfactor of the questionnaire, 32.1% of males and 66.7% of females reported a suspicion of MdDS after flights <4 h. As flight time increased, the number of participants reporting suspected MdDS increased in both sexes. There was a significant difference between short and long flights for all subfactors of the questionnaire across all age groups and sex comparisons (p<0.05).
Conclusions
As flight time and age increased, the severity of the symptoms of MdDS increased for all subfactors. With advancing age, dizziness and the severity of intolerance to visual motion increase in long flights compared to short flights, and the quality-of-life decreases. The quality-of-life of female pilots was lower than that of male pilots.
Introduction
Mal de Debarquement syndrome (MdDS) is a condition that occurs after prolonged exposure to passive motion, particularly when the person experiences oscillation [1]. In 2020, the Bárány Society Classification Committee included MdDS in the international classification of vestibular disorders to clarify the terminology used in its clinical diagnosis and to enhance research efforts. The diagnostic criteria for MdDS are as follows: 1) non-spinning vertigo is characterized by an oscillatory perception and instability that persists most of the day; 2) symptoms appear within the first 48 hours after exposure to passive motion; and 3) when passive motion continues, symptoms temporarily decrease [2].
The most common symptom associated with MdDS includes the perception of self-motion [3]. This symptom is triggered by passive motion, such as prolonged exposure to water, air, or land transportation [1]. Symptoms tend to worsen in enclosed spaces and when individuals are sedentary, but they often decrease or may not be felt during constant movements, such as driving [4,5]. There is no standard diagnostic tool for the clinical diagnosis of MdDS; the diagnosis is typically made based on the patient’s history, with neurological, audiological, and vestibular evaluations usually within normal limits [6]. MdDS shares standard features with certain disorders that can cause dizziness or unsteadiness, and it is usually differentiated from these disorders through detailed patient history [7,8].
The only standard questionnaire designed explicitly for MdDS is the “Istanbul MdDS Symptom Questionnaire (IMdDSSQ),” created by Isik and Konukseven. The validity and reliability of this questionnaire were established as part of Isik’s master’s thesis [9]. The questionnaire consists of 18 questions that evaluate four subfactors: diagnostic criteria (DCr), dizziness characteristic (DCh), visual movement intolerance (VMI), and quality of life (QoL).
Pilots are a professional group that is constantly exposed to passive motion. In this occupation, possible MdDS symptoms are often considered part of the job, leading pilots not to seek medical attention for these complaints. However, they may report different reasons for their hospital visits and mention complaints of imbalance during their medical history. The primary aim of this study is to investigate the presence of MdDS in pilots and contribute to maintaining a regular inventory. A secondary aim is also to examine the relationship between MdDS symptoms and flight.
Subjects and Methods
Ethical consideration
This study was approved by the Institutional Ethics Committee of Istanbul Arel University, with approval number 2021/13 (protocol no E-69396709-050.06.04-188999). The research was conducted ethically, adhering to the requirements of the World Medical Association’s Declaration of Helsinki. Written informed consent was obtained from each participant for study participation and data publication.
Measurement tool
G*Power 3.1.9.7 software (Heinrich-Heine-University Düsseldorf) was used for power analysis, establishing an alpha significance level (type I error) of 0.05, a desired power value (type II error) of 0.80, and an effect size (dz) of 0.25. Based on this analysis, it was calculated that at least 134 participants should be included in the study.
Our study utilized the IMdDSSQ to evaluate pilots regarding MdDS symptoms. The IMdDSSQ administered to participants was a 5-point Likert scale, scored from 5 to 1 (Supplementary Fig. 1 and Supplementary Table 1 in the online-only Data Supplement). Hain, et al. [3] reported that a minimum exposure time of 4 hours to passive motion is required for MdDS symptoms to occur. Accordingly, the questionnaire was administered to pilots twice to assess the impact of short flights (shorter than 4 hours) and long flights (longer than 4 hours). According to the Bárány Society Classification Committee, symptoms must appear within the first 48 hours after passive motion to be classified as MdDS [2]. Therefore, individuals who agreed to participate in the study were given the IMdDSSQ via Google Forms within the first 48 hours after their flight.
The sample group was formed by connecting with aviation companies (Turkish Airlines, Anadolu Jet, Onur Air, Pegasus) and consisted of volunteer Turkish pilots. A total of 150 healthy pilots (69 female and 81 male), aged 18 years and older (37.47± 11.14 yr), who were still on active duty and had no otological, neurotological, or systemic issues, as well as no complaints of vertigo, were included in the study. The consent form is on the first page of the questionnaire, followed by the anamnesis form. At the beginning of the questionnaire, a warning was added to instruct participants to answer the questions based on the symptoms they experienced in the first 48 hours after the flight. The questionnaire was blocked from being submitted without reading the warning text and checking the acceptance box. The validity and reliability of the questionnaire were established by Işık and Konukseven. The reliability and internal consistency of the study were examined separately for flights shorter than 4 hours and flights longer than 4 hours. The internal consistency value of the questionnaire was obtained as r=0.988. As a result of the analysis, the reliability and internal consistency of the questionnaire were found to be very high (Suplementary Table 2 in the online-only Data Supplement) [10].
Statistical analysis
IBM SPSS Statistics 22.0 software (IBM Corp.) was used for statistical analyses. Cronbach’s alpha value was calculated to assess the reliability of the questionnaire. The normality distribution of the data was analyzed using the Shapiro–Wilk test. The t-test was applied when the data were normally distributed, and the Mann–Whitney U test was used when the data were not normally distributed. The Kruskal–Wallis test was employed to determine whether the means of two or more samples differed from one another. The significance level was set at p<0.05 for all analyses. For pairwise comparisons of age range differences, the Dunn-Bonferroni correction was applied. The Wilcoxon signed-rank test was performed to analyze the variations in MdDS total and subfactor scores based on flight time.
Results
The study included 150 participants: 69 females and 81 males. The average age of the males was 36.82±11.1 years, while the average age of the females was 38.23±11.1 years. Our study examined the sensation of post-travel swaying/postural instability (PTSPI) and its latency.
The IMdDSSQ is a scale consisting of four subfactors, and the first subfactor, the diagnostic criteria, has a structure created as a result of the Bárány Association Consensus document [2]. According to the results obtained by the participants from this section, they are classified as non-MdDS, suspicious MdDS, and MdDS. When the flight duration, sex, and age factors are compared with the scores obtained from this section, the flight duration is the primary effective factor in increasing MdDS symptoms and reaching a significant level in terms of MdDS diagnosis. It was seen that sex is the second most effective factor, and the increase in age is third in terms of MdDS diagnostic criteria (Tables 1 and 2). Additionally, the participant’s responses to the IMdDSSQ were analyzed based on the total score of the questionnaire and four subfactors. Comparative analyses were conducted while considering the age and sex characteristics of the participants.
Evaluation of IMdDSSQ subfactors by sex in short and long flights
Comparison of PTSPI sensation between age groups
Comparison of PTSPI sensation and latency based on sex
According to the responses provided to the IMdDSSQ by pilots participating in the research, 70.6% reported experiencing PTSPI symptoms. When examined by sex, 49.4% of male participants and 95.7% of female participants reported feeling PTSPI symptoms. These results indicate that MdDS symptoms are more prevalent among female pilots.
Based on participants’ responses to questions about how long after the flight PTSPI symptoms appear (latency), the average time for male pilots to feel PTSPI was 39.12±7.0 minutes, while for female pilots, it was 25.53±15.3 minutes. The findings suggest that the latency of PTSPI symptoms is shorter in the female population.
Comparison of PTSPI sensation and latency based on age groups
Participants were divided into four age groups: 18–27, 28–37, 38–47, and 48+. Our findings revealed no significant difference between the 18–27 age group and the 28–37 age group, nor between the 38–47 age group and the 48+ age group (p> 0.05). However, PTSPI was found to be more common in individuals over the age of 37 compared to those aged 37 and under (p<0.05). This finding indicates that older pilots experienced PTSPI more frequently (Table 2). In our statistical analysis to determine whether age affects the latency of PTSPI symptoms, no significant difference was found between the 18–27 age group and the 28–37 age group, and the time taken for individuals in the 38–47 age group to feel swaying or poor posture after travel was significantly shorter than that of the 18–27 age group and the 28–37 age group. The latency of PTSPI symptoms was significantly lower in individuals over 48 years compared to all other age groups (p<0.05). According to our findings, as pilots age, they begin to feel PTSPI symptoms earlier after their flights (Table 3).
Comparison of PTSPI latency between age groups
Evaluation of sex-based subfactors regarding flight time
When evaluating the DCr subfactor for short and long flights by sex, female pilots are found to be more likely to be diagnosed with MdDS compared to male pilots. Additionally, we investigated whether there were significant differences in MdDS diagnoses between male and female pilots within the groups for short and long flights. A significant difference was observed between short and long flights for male and female pilots regarding MdDS diagnoses (p<0.05). This result indicates that as flight time increases, the number of female and male pilots diagnosed with MdDS increases (Table 1).
When evaluating the DCh subfactor for short and long flights by sex, female pilots experience more severe dizziness than male pilots. We examined whether there were significant differences in the diagnosis of dizziness between male and female pilots within the groups for short and long flights. A significant difference was noted between short and long flights for diagnosing dizziness in male and female pilots (p<0.05). This finding suggests that as flight duration increases, the severity of dizziness in both female and male pilots increases (Table 1).
It was examined whether there were significant differences in VMI diagnoses between male and female pilots within the groups for short and long flights. A significant difference was found between short and long flights in the diagnosis of VMI for both male and female pilots (p<0.05). When evaluating the VMI subfactor for short and long flights by sex, the severity of VMI in female pilots is higher than in male pilots. These results indicate that as flight time increases, the VMI severity for female and male pilots also increases (Table 1).
When the QoL subfactor for short and long flights is evaluated by sex, female pilots have a lower QoL score than their male counterparts. Additionally, we investigated whether there were significant differences in QoL between male and female pilots in the groups for short and long flights. A significant difference was found between short and long flights regarding the rate of life diagnoses for both male and female pilots (p<0.05). Accordingly, as flight time increases, the QoL scores for both female and male pilots decrease (Table 1).
Evaluation of age-based subfactors regarding flight time
When evaluating the DCr subfactor for short and long flights by age, it was found that pilots are more likely to be diagnosed with MdDS as they age. A significant difference in MdDS diagnoses was observed between short and long flights across all four age groups (p<0.05). As the flight duration increased, the participants’ scores on the MdDS diagnostic criteria subfactors also increased. Therefore, as the flight duration lengthened, pilots experienced MdDS symptoms more intensely and met the diagnostic criteria (Table 4).
Evaluation of IMdDSSQ subfactors by age in short and long flights
When the DCh subfactor for short and long flights is evaluated by age, pilots experience more severe dizziness with increasing age. No significant difference was found between the 18–27 age group and the 28–37 age group for short and long flights (p>0.05). However, there is a significant difference between short and long flights in the 38–47 and 48+ age groups (p<0.05). Therefore, as flight duration increases with age, the severity of dizziness also increases (Table 4).
When evaluating the VMI subfactor for short and long flights, it was found that pilots experience more severe VMI as they age. No significant difference was noted between the 18–27 age group and the 28–37 age group for short and long flights (p>0.05). In contrast, significant differences in the VMI were found in the 38–47 and 48+ age groups between short and long flights (p<0.05). These results indicate that as flight time increases with age, the VMI severity of pilots also increas-es (Table 4).
When the QoL subfactor for short and long flights is evaluated by age, it was found that pilots’ QoL scores decrease as they age. Additionally, significant differences were observed within the group regarding QoL between short and long flights across all age groups (p<0.05). These results suggest that as flight time increases, pilots’ QoL scores decrease in all age groups (Table 4).
There is a significant difference between the MdDS total scores for flights shorter than 4 hours and those longer than 4 hours (p<0.05). According to these findings, the longer the flight duration, the higher the severity of MdDS symptoms reported by pilots (Tables 5 and 6).
Frequency distribution of IMdDSSQ according to subfactors in short and long flights
Correlation for short and long flights on all subfactors of the IMdDSSQ
Comparison of MdDS and factors regarding flight time and age groups
Variations in MdDS total and subfactor scores based on flight time were analyzed. Flights shorter than 4 hours had significantly lower total MdDS scores and subscale scores compared to flights longer than 4 hours (p<0.001) (Table 7).
Comparison of MdDS and IMdDSSQ subfactors by flight time
The comparison of MdDS and subfactors by age range was conducted. No statistically significant differences were observed between the 18–27 and 28–37 age groups for MdDS or any of its in any condition (p>0.05) (Fig. 1).
Comparison of MdDS and subfactors by age groups.*p<0.05; **p<0.01. <4 hours: flight less than 4 hours. >4 hours: flight more than 4 hours. MdDS, Mal de Debarquement syndrome.
A linear mixed model analysis was performed to evaluate the effects of flight time, sex, age, and post-travel swaying instability on MdDS and subfactors.
In the diagnostic criteria subfactors, individuals with flights shorter than 4 hours exhibit significantly lower MdDS symptoms compared to those with flights exceeding 4 hours (β=-2.660, p<0.001). Regarding sex, male participants report significantly lower diagnostic criteria scores than females (β=-2.781, p<0.001). Age is positively associated with MdDS severity, indicating that older individuals experience more severe symptoms (β=0.144, p<0.001). Additionally, PTSPI latency is negatively correlated with diagnostic criteria scores (β=-0.033, p<0.001) (Table 8).
Linear mixed model analysis of MdDS and IMdDSSQ subfactors
In the dizziness characteristics subfactor, individuals who have flown for less than 4 hours report significantly lower dizziness scores compared to those with longer flight durations (β=-2.307, p<0.001). Male participants demonstrate significantly lower dizziness scores compared to female participants (β=-2.953, p<0.001). The positive relationship between age and dizziness scores indicates that older individuals experi-ence greater dizziness severity (β=0.123, p<0.001). PTSPI latency exhibits a significant negative association with dizziness scores (β=-0.032, p<0.001) (Table 8).
In the visual motion intolerance subfactor, participants with flights shorter than 4 hours demonstrate lower scores than those with longer flights (β=-1.807, p<0.001). Males report significantly lower visual motion intolerance scores compared to females (β=-2.238, p<0.001). The positive association with age (β=0.102, p<0.001) suggests that older individuals exhibit higher visual motion intolerance severity. The relationship between PTSPI latency and visual motion intolerance is not statistically significant (β=-0.007, p=0.063) (Table 8).
In the QoL, participants with shorter flight times report significantly lower symptom severity than those with longer flights (β=-4.047, p<0.001). Males experience significantly fewer negative impacts on QoL compared to females (β=-4.374, p<0.001). The positive association between age and QoLimpairment (β=0.208, p<0.001) suggests that older individuals experience greater declines in QoL due to MdDS. Additionally, PTSPI latency is negatively associated with QoL scores (β=-0.032, p<0.001) (Table 8).
The MdDS total score, which provides a comprehensive measure of symptom severity, indicates that participants with flights shorter than 4 hours exhibit significantly lower total MdDS scores compared to those with longer flights (β=-10.820, p<0.001). Males report significantly lower total MdDS scores than females (β=-12.190, p<0.001). The positive association between age and total MdDS severity (β=0.569, p<0.001) suggests that older individuals experience more severe MdDS symptoms. Additionally, PTSPI latency is negatively associated with total MdDS scores (β=-0.110, p<0.001) (Table 8).
Discussion
While no peer-reviewed studies specifically investigating MdDS symptoms, incidence, and prevalence in pilots have been found in the literature, pilots’ working conditions expose them to passive motion, posing potential occupational risks related to MdDS. To fill this gap, the IMdDSSQ was administered to healthy pilots on active duty. The questionnaire’s reliability coefficient was examined for pilots at risk for MdDS, and its reliability and internal consistency were found to be highly dependable (Supplementary Table 2 in the online-only Data Supplement). This study indicates that the questionnaire can be used to diagnose MdDS in pilots.
Hain, et al. [3] stated that MdDS requires a minimum of 4 hours of exposure to passive motion. Referring to this study, we examined the answers given by pilots to the questionnaire regarding flights with durations of less than 4 hours and those longer than 4 hours, categorizing them into separate groups.
Our study adds to the literature by presenting MdDS findings in the same individuals for short and long flights. Cha and Cui [11] noted that contrary to a study by Hain, et al. [3], at least 2 hours of exposure to passive motion is necessary for MdDS to occur. In our study, suspicious MdDS findings were also observed in pilots over 37 on flights shorter than 4 hours. These findings align with the research of Cha and Cui [11].
There is a significant difference between flights less than 4 hours and those over 4 hours in the subfactors of DCr, DCh, VMI, and QoL. Accordingly, as flight time increased, more pilots exhibited MdDS symptoms. These findings suggest that flight time plays a significant role in relation to MdDS. In the literature, no studies by Hain, et al. [3] or Cha and Cui [11] specifically addressed flight time. There is no definitive limit for flight time concerning the MdDS threshold [2].
A significant difference was found between males and females for the subfactors of DCr, DCh, VMI, and QoL in flights less than 4 hours and those longer than 4 hours. Our findings indicated that female pilots were more likely to be diagnosed with MdDS than male pilots, which aligns with existing literature. Most studies on MdDS, including those related to motion sickness and vestibular migraine, emphasize that MdDS is more prevalent in the female population [12-19]. Mucci, et al. [12] suggested that the higher incidence of MdDS symptoms in females may be linked to gonadal hormones and that hormonal fluctuations could contribute to an increase in these symptoms.
To understand whether there is a change based on age in the subfactors, the participants’ ages were examined in groups 18–27, 28–37, 38–47, and 48+. In the DCr, DCh, VMI, and QoL subfactors, the number of pilots exhibiting MdDS symptoms increases with age. While this result primarily suggests age-related changes, it is also important to consider that as age increases, the duration of time spent practicing the profession extends, leading to more flight. Consequently, it remains unclear whether the increase in MdDS symptoms is due to agerelated changes or a higher number of flights.
It is known that MdDS is more prevalent in the female population. The results from our study demonstrated the occurrence of MdDS-related symptoms, regardless of sex, in the population exposed to passive motion, such as pilots. These symptoms were thought to be related to age. The literature has reported that MdDS symptoms are more common in middleaged and older females. In line with this literature, our findings support that MdDS symptoms increase with age in females. Furthermore, these results indicate that MdDS symptoms also increase with age in the male population, in addition to what has been reported in the literature [20].
In our study, when analyzing the PTSPI sensation between sexes, 40 males and 66 females reported experiencing PTSPI, while 41 males and three females indicated they did not. These findings suggest that females experience higher rates of PTSPI than males. When examining PTSPI sensation among age groups, individuals aged 38 years and over show more PTSPI than those under 38 years. These findings indicate that the sensation of PTSPI increases with age. There is currently no peer-reviewed study in the literature that compares the distribution of PTSPI between sex and age groups.
Cha, et al. [2] reported that MdDS symptoms occur within the first 48 hours after travel. Among the participants in our study, the average time for males to start feeling symptoms was 39.12±7.06 minutes, while females began to feel symptoms at an average time of 25.53±15.37 minutes. These results reveal that females experience the onset of PTSPI in a shorter time than males. According to the literature, symptoms appear more quickly in our study [1,2]. It is thought that the quicker onset of symptoms may be due to pilots being an occupational group that is constantly exposed to passive motion.
When analyzing those who reported feeling PTSPI among age groups, it was observed that the latency for individuals in the 48+ age group (20.89±13.27 min) was significantly lower than for the 18–27 age group (41.17±8.01 min), the 28–37 age group (41.53±8.26 min), and the 38–47 age group (32.29± 13.05 minutes). These findings indicate that the time to start feeling PTSPI decreases with age. No similar study was found in the literature that examined the latency of PTSPI between sex and age groups.
According to the MdDS diagnostic criteria established by the Bárány Society Classification Committee, MdDS symptoms must continue for at least 48 hours [2]. While symptom duration was not evaluated in our study, it is important to note that all the pilots continued their active professional lives, meaning that less than 48 hours typically elapsed between their two trips. Despite this, MdDS symptoms were observed in the pilots participating in our study. This may be due to the nature of the profession, as pilots are constantly exposed to passive motion. Additionally, all of our participants exhibited the first three diagnostic criteria, suggesting they may indeed have MdDS.
Limitation of study
The limitations of the study include the fact that the number of female pilots who continue their active professional lives is less than that of males, which results in an inability to achieve numerical equality between sexes.
Additionally, all the pilots in our study maintained active professional careers, with the majority experiencing intervals of less than 48 hours between flights. Therefore, we acknowledge that the duration of symptoms could not be evaluated for participants due to their ongoing professional commitments and frequent flights. In subsequent studies, the evaluation of the duration of symptoms may offer significant insights into the existing literature on MdDS.
The findings of the present study are based on survey data. It is recommended that future studies encompass a more extensive sample of flight attendants and encompass a broader evaluation of clinical assessments, including vestibular function tests and fMRI scans. Such an approach is likely to yield valuable insights that will further enrich the existing MdDS literature.
Conclusion
Females exhibit a higher incidence of MdDS symptoms for all subfactors than males. Regardless of sex, the number of pilots diagnosed with MdDS increased with age and flight time. In terms of quality of life, male pilots were found to have a better quality of life than female pilots. However, regardless of sex, the quality of life related to MdDS declined as age increased.
Additionally, female pilots reported a higher sense of PTSPI, and both sexes experienced PTSPI at a higher rate and in a shorter duration as age progressed and flight time increased.
MdDS is a disorder characterized by the onset of symptoms within 48 hours after travel, with these symptoms persisting for at least 48 hours. All the pilots in our study maintained active professional careers, with the majority experiencing intervals of less than 48 hours between flights. Despite this, further investigation is needed into the reasons for the occurrence of MdDS symptoms in pilots.
Supplementary Materials
The online-only Data Supplement is available with this article at https://doi.org/10.7874/jao.2024.00521.
Supplementary Table 1.
Subfactor items and scoring
Supplementary Table 2.
Reliability and internal consistency analysis of the MdDS questionnaire and its subscales in short and long flights and overall
Supplementary Fig. 1.
A: Informed Consent Form, B: Patient Anamnesis Form, C: Istanbul Mal de Debarquement Syndrome Symptom Questionnaire (IMdDSSQ).
Notes
Conflicts of Interest
The author has no financial conflicts of interest.
Author Contributions
Conceptualization: all authors. Data curation: Çağla Aydın. Investigation: Çağla Aydın. Methodology: Emel Uğur, Çağla Aydın. Project Administration: Bahriye Özlem Konukseven. Supervision: Emel Uğur, Bahriye Özlem Konukseven. Visualization: Çağla Aydın. Writing—original draft: Emel Uğur, Çağla Aydın. Writing—review & editing: all authors. Approval of final manuscript: all authors.
Funding Statement
None
Acknowledgments
We would like to express our sincere gratitude to Erdogan Onur Eroglu for his technical support.