METHODS

Prior to analysis, the study was reviewed and approved according to the U.S. Army Aeromedical Research Laboratory’s Human Subjects Research Protection Plan by the U.S. Army Aeromedical Research Laboratory Regulatory Compliance Office.

RESULTS

Overall, there were 663 individuals with OSA, representing 2.69% of the total number of aviators in the dataset (N = 24,568). The mean age of initial presentation of OSA was 42.62 yr. Only four of the cases were women. BMI was reported for 141 cases and ranged from 20 to 39 (M = 28.69, SD = 3.41).

The resulting distribution of waiver recommendations, suspension recommendations, and waiver continuations is displayed in Fig. 1 . Overall, 459 waiver recommendations were made for OSA. Of these, 241 were made in conjunction with other ICD codes (comorbid conditions) and 218 were made with OSA as the only diagnosis. All waiver recommendations were granted, with the exception of one, which was missing a result. Additionally, 51 suspension recommendations were made for OSA. Two recommendations were made when OSA was the only diagnosis and 49 were made with other comorbid conditions present. All suspensions were granted, except for one which was missing a decision. Finally, there were 98 waiver continuations for OSA combined with other conditions and 55 continuations where OSA was the sole diagnosis.

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For any case of OSA, the top five most common comorbid conditions are depicted in Table I . Overall, the ratio of waivers to suspensions granted in association with OSA was 9:1. With respect to the waivers, the split between those granted with OSA alone (N = 218) and those granted with OSA combined with another condition (N = 241) was roughly similar. In contrast, for the suspensions only 2 out of 51 were with OSA alone. The remaining 49 suspensions were associated with multiple other diagnoses. Notably, PTSD and depressive disorder occurred the most for suspensions when OSA was present.

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DISCUSSION

OSA is an increasingly common condition in the U.S. general population and, due to its association with an elevated BMI, is often seen with, or is contributory to, other obesity-related health problems. One might expect that a military population, with its younger demographic overall and the requirement to maintain a minimum level of physical fitness, would not see the same (and increasing) prevalence of OSA as a civilian one. Furthermore, the military aviator population with its stringent medical fitness standards and robust health surveillance requirements might be expected to show a lower prevalence, as well.

However, the Army is not immune to this trend as the prevalence of obesity has increased from 25.8% in 1989 to 37.1% in 2012, the most recent year for which data are available. ¹¹ Additionally, a study looking at longitudinal changes in BMI in British Army pilots found an average increase of 1.3 points over an individual’s flying career. ¹ Regression analysis also suggested that the change in BMI was equal to 1.62 times the number of years served as a pilot. With the increase in OSA in the general population and its known pathological correlates on the rise in the military population, it is important to understand the risks OSA poses to aviators’ performance, safety, and career longevity. Our data (2.69% overall, 2.68% for men, and 0.02% for females) does indicate lower prevalence in this Army cohort, yet the figures on increasing levels of obesity in the military generally and the wide range, and somewhat elevated mean, of recorded BMI figures (20–39, M = 28.69) for aviators demonstrates little room for complacency when regarding the potential impact of OSA in this subpopulation.

The effects of OSA on aviator performance and flight safety are well known, with the disrupted sleep and excessive daytime sleepiness having demonstrable effects on cognitive and psychomotor performance. Additionally, the increased risk of hypertension, angina, myocardial infarction, dilated cardiomyopathy, and stroke can lead to sudden incapacitation and catastrophic loss of life and aircraft. Other health effects such as the development of pulmonary hypertension and cor pulmonale from repeated nocturnal desaturations could impact career longevity too. Hence, any aviator diagnosed with OSA is disqualified from flying duties. However, current U.S. Army aeromedical policy (Aeromedical Policy Letters and Aeromedical Technical Bulletins, dated June 2021) states that waivers may be granted for rated (pilots) and nonrated (crew chief) aviators on a case-by-case basis.

OSA is diagnosed using PSG, where breathing patterns, arm and leg movements, blood oxygen levels, and the number of arousals during sleep are monitored. The most common treatment of OSA generally involves using CPAP, which acts as a pneumatic splint and prevents the upper airway from collapsing. In some milder cases a dental device which advances the jaw or holds the tongue in a different position during sleep may be used. Surgical intervention, where soft tissue from the back of the throat is removed (uvulopalatopharyngoplasty) can also alleviate symptoms for some individuals. Without treatment, the natural history of OSA will result in continued episodes of apnea and degraded sleep throughout the night, increasing daytime sleepiness, reduction in performance on skilled, multidimensional tasks (such as driving or flying), and increased risk of the associated longer term cardiovascular issues. ¹⁶ However, given that the condition is amenable to screening, diagnosis, and treatment, waivers may be granted provided adequate redress with weight loss, dental device, surgery, or CPAP use and documented resolution via PSG. There is also a requirement for continued compliance with therapy and stability of symptoms. If these requirements are not met then suspension from flying duties is the only likely outcome.

For this study, when calculating the point prevalence of OSA cases in Army aviators from the available AERO data, the count of Aeromedical Summaries (AMS) related to this diagnosis was used as the numerator. An AMS, which is collated by the attending flight surgeon or aviation medical examiner, summarizes the past medical history, the current principal complaint, and associated physical exam findings and is required for any medical condition that requires waiver or permanent medical suspension from flying duties, such as OSA. For this analysis only the first AMS related to a diagnosis of OSA (waiver recommendation, suspension, or waiver continuation) was counted for each affected individual so that the prevalence was not erroneously elevated. Other ICD-9 codes associated with the OSA cases were used to identify the five most prevalent comorbid conditions, overall and by waivered or suspended individuals.

When reviewing the waivered cases there was a roughly equitable split between those with OSA as a sole diagnosis (N = 218) and those with other conditions too (N = 241). Hypertension was a common cofinding, both in this group and overall. The link between OSA and hypertension is well established and so this observation comes as little surprise. The other prevalent conditions for this group, lumbago (N = 17) and intervertebral disc disease (N = 19), could be related to high background prevalence of back pain within the general adult population, contributions from comorbid conditions such as elevated BMI, or from factors specific to rotary-wing platforms (or a combination thereupon). ¹⁰ The main limitation with the AERO data available for this analysis is that, where comorbid conditions are present, it does not allow the primary reason(s) for the waiver to be elucidated, making it impossible to know what specifically initiated the generation of an AMS. However, we know that a confirmed diagnosis of OSA is always going to attract a waiver (at best), in line with aeromedical policy, but we cannot determine whether the comorbid condition was also a waiverable condition or simply a cofinding.

Two further common comorbid findings among the waivered OSA cases were testicular hypofunction (N = 17) and esophageal reflux (N = 17). Obesity is known to be strongly correlated with hypogonadism ¹⁴ and OSA in middle-aged men is also significantly associated with decreased testosterone secretion ¹³ and male infertility, particularly with greater OSA “exposure time” or when left untreated. ¹² The prevalence of testicular hypofunction as a comorbid diagnosis in our data concurs with the wider evidence base. Similarly, truncal adiposity is known to lead to an increase in esophageal reflux symptoms. A commonly suggested patho-genetic pathway is the increased abdominal pressure relaxes the lower esophageal sphincter and increases the risk of hiatus hernia, thus exposing the esophageal mucosa to gastric content. ⁶ Additionally, there is evidence that visceral adipose tissue is metabolically active and secretes adipokines, along with inflammatory cytokines, that may predispose to complications of esophageal reflux such as Barrett esophagus and esophageal carcinoma. ⁴

With respect to suspensions there were only two cases where OSA was the sole diagnosis and therefore, one assumes, the cause for the suspension. The remaining 49 suspension cases had other health conditions besides OSA, many of them being mental health issues (PTSD = 17 and depressive disorder = 8). For these cases it is conceivable that the poor sleep and daytime fatigue associated with OSA could have caused, contributed to, or exacerbated a mental health diagnosis, or be completely unrelated. Again, due to the data limitations described above, it is impossible to ascertain whether it was the OSA, the mental health diagnosis, or both that led to the suspensions. The authors consider that the most likely scenario is that the mental health diagnosis was the primary driver and OSA was a related or coincidental issue.

Despite this difficulty with the data set our analysis shows that OSA is common in the aviator population, albeit rates are lower than those cited in the wider civilian population, particularly females. Younger age and lower BMIs appear to be the main drivers for this difference. Whether comparative prevalence rates between aviators and civilian populations are actually similar and are not demonstrated by our data due to undiagnosed cases is doubtful, but open to conjecture. The stringent health surveillance applied to aviators renders missed diagnoses more unlikely, particularly for those who are evidently overweight. However, as OSA is usually identified via self-declared symptoms there is a possibility that some aviators could withhold such declarations due to concerns over the potential impact on their flying careers. Our data cannot delineate between these two hypotheses, but the age and BMI data suggest these are relevant factors in our different prevalence findings. What is clear from our data is that, when OSA is positively diagnosed, the appropriate aeromedical disposition actions are being applied. The fact that the majority of the cases are being waivered and there are very few cases of suspension that we know are specifically due to OSA also suggests that applied treatments appear to be effective. A few comments on prevention are warranted, as well. The concepts of comprehensive fitness and related effects upon health and readiness are not new to the military. And aircrew must all also undergo annual flight physicals, as well as periodic health assessments to address health, duty fitness, and wellness. However, a relatively novel initiative with far-reaching implications is that of the Army’s Holistic Health and Fitness (H2F) System. ⁵ H2F brings a unified and multidomain approach down to the unit level with integrated resources. While certainly not assembled specifically for OSA or related conditions, it is projected to have far-reaching positive health-related outcomes. Notably, H2F will see cadres such as physical and occupational therapists, registered dietitians, athletic trainers, and strength and conditioning coaches imbedded within units across the service, within all components, and in all geographic locations. The impacts of such large-scale investment, resourcing, and cultural change, ultimately focused on military readiness, will include health promotion and disease prevention for a number of duty and occupationally limiting medical conditions, including those associated with OSA.

As OSA is often seen in conjunction with other health diagnoses which could affect career longevity it inevitably requires robust management in the short and long term to minimize losses to the operationally available aviator pool. Further longitudinal analysis of AERO data would be useful to assess whether the prevalence of this condition is changing over time, both as a marker of treatment compliance and effectiveness in currently diagnosed individuals and to assess whether more new OSA cases are being identified, mirroring the increasing trend seen in the wider civilian population.

The main limitation associated with this study is that it assumes that health data input into the AERO database is accurate and miscoding has not taken place. The inability to elucidate primary causes for waivers or suspensions in cases with multiple diagnoses is also a weakness associated with the data available to this study. The smaller number of female aviators in the AERO database also renders health conclusions about this subgroup less authoritative.

In conclusion, our data demonstrate that OSA has a lower prevalence among U.S. Army aviators (especially women) compared to the general population, as would be expected. However, the trend of rising obesity in the military generally, as well as the extant prevalence figures for OSA presented in this paper suggest that robust health promotion measures, detailed medical surveillance, and aggressive management of cases and associated comorbid conditions by the aeromedical community remains vital if we are to maintain individual health, preserve career longevity and operational output, and improve flight safety.