Key Points
Question
Are there changes in the opacification of the paranasal sinuses or mastoid air cells associated with symptoms of head congestion and facial fullness frequently experienced by astronauts during spaceflight?
Findings
This cohort study of 35 astronauts found that long-duration (>30 days) spaceflight on the International Space Station, but not short-duration spaceflight on the Space Shuttle, was associated with an increased likelihood for the development of mastoid effusions. No change in the opacification of paranasal sinuses was noted after long-duration or short-duration spaceflight.
Meaning
The development of mastoid effusions during spaceflight might be associated with several Earth-based mechanisms.
Abstract
Importance
Head congestion is one of the most common somatic symptoms experienced by astronauts during spaceflight; however, changes in the opacification of the paranasal sinuses or mastoid air cells in astronauts have not been adequately studied.
Objectives
To quantify preflight to postflight changes in the opacification of the paranasal sinuses and mastoid air cells in Space Shuttle astronauts and International Space Station (ISS) astronauts and to assess whether there are differences between the 2 groups of astronauts.
Design, Setting, and Participants
This cohort study examined preflight and postflight head magnetic resonance images (MRIs) of 35 astronauts who had participated in either a short-duration (≤30 days) Space Shuttle mission or a long-duration (>30 days) ISS mission and had undergone both preflight and postflight MRI. Images were obtained before and after spaceflight. Images were evaluated by 2 neuroradiologists blinded to which mission each astronaut had flown and to which images were preflight or postflight images.
Exposure
Spaceflight on the Space Shuttle or the ISS.
Main Outcomes and Measures
Measured outcomes included preflight to postflight changes in Lund-Mackay scores for the paranasal sinuses and in scores grading mastoid effusions.
Results
Most astronauts in both the Space Shuttle group (n = 17; 15 men; mean [SD] age at launch, 47.7 [3.1] years) and the ISS group (n = 18; 14 men; mean [SD] age at launch, 48.6 [4.7] years) exhibited either no change or a reduction in paranasal sinus opacification as seen on postflight MRI scans (Space Shuttle group: 6 [35.3%] had no sinus opacification before or after spaceflight, 5 [29.4%] had less sinus opacification after spaceflight, 3 [17.6%] had the same amount of sinus opacification before and after spaceflight, and 3 [17.6%] had increased paranasal sinus opacification after spaceflight; ISS group: 8 [44.4%] had no sinus opacification before or after spaceflight, 4 [22.2%] had less sinus opacification after spaceflight, 1 (5.6%) had the same amount of sinus opacification before and after spaceflight, and 5 [27.8%] had scores consistent with increased paranasal sinus opacification after spaceflight). Long-duration spaceflight (ISS group) was associated with an increased risk of mastoid effusion relative to short-duration spaceflight (relative risk, 4.72; 95% CI, 1.2-18.5). Images were obtained a mean (SD) 287.5 (208.6) days (range, 18-627 days) prior to and 6.8 (5.8) days (range, 1-20 days) after spaceflight. Astronauts had undergone either a mean (SD) of 13.6 (1.6) days of spaceflight on the Space Shuttle (17 astronauts) or 164.8 (18.9) days on the ISS (18 astronauts).
Conclusions and Relevance
This study found that exposure to spaceflight conditions on the ISS is associated with an increased likelihood for the formation of mastoid effusions. There was no association between exposure to spaceflight conditions and changes in paranasal sinus opacification. The limitations of this study include lack of information concerning medical history and mission-specific operational experience for individual astronauts. Further studies are indicated to determine the cause and composition of the mastoid effusions.
This cohort study examines preflight to postflight changes in the opacification of the paranasal sinuses and mastoid air cells in Space Shuttle astronauts and International Space Station astronauts and determines whether there are differences between the 2 groups.
Introduction
Long-duration spaceflight on the International Space Station (ISS) subjects the human body to conditions not normally experienced on Earth, including exposure to higher levels of radiation, the closed environment of the space station with unique atmospheric conditions and microbial ecosystems, and microgravity. Microgravity, or weightlessness, is a condition in which the forces of acceleration experienced by astronauts on a spacecraft in orbit around the Earth essentially counterbalance the force of gravity, resulting in the sensation of weightlessness. In microgravity, there is a loss of the normal head-to-foot hydrostatic pressure gradient experienced during the upright posture on Earth, which results in a cephalad fluid shift.1 As a result, astronauts experience facial edema and report subjective symptoms of nasal stuffiness and head pressure.2
Although head and sinus pressure is one of the most common somatic symptoms noted by astronauts during spaceflight,3 to our knowledge, there are few data concerning any physiological changes of the paranasal sinuses or mastoid air cells. The National Aeronautics and Space Administration (NASA) has avoided the use of computed tomographic imaging for astronauts to limit the dose of radiation that astronauts receive, given the radiation exposure they experience during spaceflight; however, as part of routine medical protocol, all astronauts undergo preflight and postflight magnetic resonance imaging (MRI) of the head. Therefore, the purpose of this study was to review these head MRI scans obtained before and after either short-duration missions (≤30 days) on the Space Shuttle or long-duration missions (>30 days) on the ISS to document the rate of occurrence of paranasal sinus and mastoid air cell opacification during spaceflight.
Methods
Study Design
All demographic data and head MRI scans for this cohort study were obtained from the NASA Lifetime Surveillance of Astronaut Health Program. The study was approved by the institutional review boards at the NASA Johnson Space Center and the Medical University of South Carolina. All astronauts signed written informed consent for use of their data, and the manuscript was reviewed by the NASA Lifetime Surveillance of Astronaut Health Office to ensure astronaut anonymity.
Study Participants
Study participants included all NASA astronauts who, at the time of data collection from NASA, met the following 2 inclusion criteria: the astronaut (1) had flown on either the Space Shuttle or the ISS and (2) had undergone both preflight and postflight MRI of the head. The practice of routinely performing preflight and postflight MRI on all astronauts is relatively new and began in 2009. Therefore, from 2009 until the time of data collection for this study, only 35 NASA astronauts had undergone both preflight and postflight MRI per routine medical protocol.
Imaging Protocols and Interpretation of Images
The paranasal sinuses and mastoid air cells were evaluated using T2-weighted sequences. Magnetic resonance imaging scans were performed on either a 1.5-T Philips Intera system (n = 14) (Philips Medical Systems; imaging parameters: repetition time, 3.6 seconds; echo time, 80 milliseconds; field of view, 240 × 240 mm; and slice thickness, 4 mm) or a 3-T Siemens Verio system (n = 21) (Siemens Healthcare; imaging parameters: repetition time, 3.2 seconds; echo time, 409 milliseconds; field of view, 250 × 250 mm; and slice thickness, 1 mm). However, for each astronaut, the preflight and postflight MRI scans were performed on the same MRI scanner using the same imaging parameters.
Interpretation of the MRI scans was performed by 2 experienced neuroradiologists (M.U.A. and M.V.S.) in blinded fashion. For each astronaut, the preflight and postflight images were displayed on side-by-side computer monitors. To blind the neuroradiologists, the MRI scans for the Space Shuttle astronauts vs the ISS astronauts were presented in randomized order. In addition, for each astronaut, the presentation of the preflight vs postflight images on the right vs left computer monitor was also randomized.
Mucosal thickening of the paranasal sinuses was graded using the Lund-Mackay system, which scores each sinus (maxillary, anterior ethmoid, posterior ethmoid, frontal, and sphenoid sinuses) according to the following scale: 0 (no opacification), 1 (partial opacification), or 2 (complete opacification).4 The ostiomeatal complex is scored as 0 (not occluded) or 2 (occluded). The scores for both the right and left sides are summed, for total Lund-Mackay scores ranging from 0 to 24 for each astronaut.
Mastoid effusions were graded by percentage of opacification of the mastoid air cells stratified into thirds (Table 1). This grading system was developed for the purpose of this study and has not been validated. The intention was to create a grading scale that the 2 neuroradiologists could use to identify obvious and substantial differences in the amount of mastoid opacification. Each side (ie, right and left) was scored separately and then summed, for total scores ranging from 0 to 6 for each astronaut. The 2 neuroradiologists first scored the images independently and then met to reach a concordance for any readings with an initial lack of agreement.
Table 1. Grading Scale Used for Mastoid Effusions.
| Gradea | Description |
|---|---|
| 0 | No air cell opacification |
| 1 | 1%-33% of air cells opacified |
| 2 | 34%-66% of air cells opacified |
| 3 | 67%-100% of air cells opacified |
The right and left sides are graded separately and then summed for a total maximum score of 6.
Statistical Analysis
Duration of spaceflight and previous spaceflight experience for both groups of astronauts are presented as mean (SD) number of days, and age at launch as mean (SD) number of years. Sex distributions within the groups are presented as numbers and percentages. Preflight to postflight changes in paranasal sinus mucosal thickening and mastoid effusion scores (summed across the left and right sides) are described as median values and interquartile ranges and evaluated with 95% CIs and Cohen r effect sizes. Relative risk and 95% CIs were used to describe the risk for mastoid effusion scores between short-duration and long-duration flight.
Results
Duration of Spaceflight
The participants included 35 NASA astronauts who had flown missions on either the Space Shuttle (n = 17) or the ISS (n = 18). Preflight MRI scans were acquired a mean (SD) of 287.5 (208.6) days (range, 18-627 days) before flight, and postflight MRI scans were acquired a mean (SD) of 6.8 (5.8) days (range, 1-20 days) after flight. Astronauts who had flown missions on the ISS spent more days in space (mean [SD], 164.8 [18.9] days) than astronauts who had flown missions on the Space Shuttle (mean [SD], 13.6 [1.6] days). Between the 2 groups, there were no meaningful differences in age at launch, previous spaceflight history, or sex (Table 2).
Table 2. Characteristics of Astronauts.
| Characteristic | Space Shuttle (n = 17) | International Space Station (n = 18) |
|---|---|---|
| Spaceflight duration, mean (SD), d | 13.6 (1.6) | 164.8 (18.9) |
| Age at launch, mean (SD), y | 47.7 (3.1) | 48.6 (4.7) |
| Previous spaceflight experience, mean (SD), d | 77.4 (96.7) | 34.6 (56.4) |
| Female sex, No. (%) | 2 (11.8) | 4 (22.2) |
Paranasal Sinus Mucosal Thickening
Most astronauts in the Space Shuttle and ISS groups exhibited either no change or a reduction in paranasal sinus opacification. In the Space Shuttle group, comparing the Lund-Mackay scores for each astronaut before and after spaceflight, we found that 6 astronauts (35.3%) had no sinus opacification before or after spaceflight, 4 (22.2%) had less sinus opacification after spaceflight, 3 (17.6%) had the same amount of sinus opacification before and after spaceflight, and 5 (27.8%) had scores consistent with increased paranasal sinus opacification after spaceflight (Figure 1A; eTable 1 in the Supplement). In the ISS group, comparing the Lund-Mackay scores for each astronaut before and after spaceflight, we found that 8 astronauts (44.4%) had no sinus opacification before or after spaceflight, 5 (27.8%) had less sinus opacification after spaceflight, 1 (5.6%) had the same sinus opacification before and after spaceflight, and 4 (22.2%) had scores consistent with increased paranasal sinus opacification after spaceflight (Figure 1A; eTable 1 in the Supplement). At the group level, concerning the change in paranasal sinus opacification from before to after spaceflight, there was no meaningful change for the Space Shuttle (median difference, 0.00; 95% CI, −1.00 to 0.50; Cohen r = 0.15) or ISS groups (median difference, 0.00; 95% CI, −1.00 to 1.00; Cohen r = 0.03) (Figure 1A).
Figure 1. Change in Lund-Mackay Scores From Before to After Spaceflight for Individual Space Shuttle and International Space Station (ISS) Astronauts.
A, The change preflight to postflight in Lund-Mackay scores for individual Space Shuttle and ISS astronauts. In the graphs, participants are ordered based on preflight Lund-Mackay Scores, from lowest to highest. The Lund-Mackay scores were 0 before and after spaceflight for Space Shuttle astronauts 1 through 6 and for ISS astronauts 1 through 8. At the group level, there was no meaningful change in Lund-Mackay scores from preflight to postflight for either group. B, The change preflight to postflight in mastoid effusion scores for individual Space Shuttle and ISS astronauts. In the graph, participants are ordered based on preflight mastoid effusion scores, from lowest to highest. Mastoid effusion scores were 0 before and after spaceflight for Space Shuttle astronauts 1 through 8 and for ISS astronauts 1 through 4. At the group level, the ISS group showed a meaningful increase in score from preflight to postflight, whereas the Space Shuttle group showed no change. Individual astronaut numbers do not necessarily correspond to the same individual astronaut numbers as in panel A. Horizontal lines indicate medians, dots indicate outliers, and diamonds indicate means.
Mastoid Effusions
At baseline, the opacification of the mastoid air cells was similar between the Space Shuttle and ISS groups (Table 3). Comparing the scores for each astronaut before and after spaceflight in the Space Shuttle group, we found that 8 astronauts (47.1%) had no mastoid air cell opacification before or after spaceflight, 5 (29.4%) had opacification before spaceflight that was unchanged after spaceflight, 2 (11.8%) had less mastoid air cell opacification after spaceflight, and 2 (11.8%) had scores consistent with increased mastoid air cell opacification after spaceflight (Figure 1B; eTable 2 in the Supplement). Comparing the scores for each astronaut before and after spaceflight in the ISS group, we found that 4 astronauts (22.2%) had no mastoid air cell opacification before or after spaceflight, 3 (16.7%) had opacification before spaceflight that was unchanged after spaceflight, 1 (5.6%) had less mastoid air cell opacification after spaceflight, and 10 (55.6%) had scores consistent with increased mastoid air cell opacification after spaceflight (Figure 1B; eTable 2 in the Supplement). Concerning the change in mastoid air cell opacification from before to after spaceflight, there was no meaningful change for the Space Shuttle group (median difference, 0.00; 95% CI, 0.00-0.00; Cohen r = 0.06) but a moderate effect in the ISS group (median difference, 1.00; 95% CI, 0.5-1.00; Cohen r = 0.45) (Figure 1B). Long-duration spaceflight in the ISS group was associated with an increased risk of mastoid effusion relative to short-duration spaceflight in the Space Shuttle group (relative risk, 4.72; 95% CI, 1.2-18.5) (eTable 3 in the Supplement). The signal intensity of the effusions was isointense to cerebrospinal fluid on the T2-weighted images in all cases (Figure 2). No surrounding inflammatory changes were noted in the adjacent soft tissues.
Table 3. Number of Astronauts With Mastoid Effusions Before Spaceflight According to Mastoid Grading Score.
| Group | No. of astronauts | Effusion grading score | |||
|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | ||
| International Space Station | 18 | 9 | 6 | 3 | 0 |
| Space Shuttle | 17 | 9 | 5 | 3 | 0 |
Figure 2. Axial T2-Weighted Magnetic Resonance Images Demonstrating the Development of Mastoid Effusions in an International Space Station Astronaut.
Yellow arrowheads indicate mastoid effusions.
Discussion
Here we document the occurrence of the opacification of the paranasal sinuses and mastoid air cells in astronauts after spaceflight. Despite the common report of sinus pressure among astronauts, most did not develop or experience worsening of preexisting paranasal sinus mucosal thickening. This finding is inconsistent with reports of medication use by astronauts because head congestion is the most common reason ISS astronauts3 and the third most common reason Space Shuttle astronauts5 reported regular or repeated medication use during spaceflight. For example, in a study of 24 ISS astronauts, 55% of the astronauts reported treating congestion or allergy symptoms using onboard over-the-counter decongestants, and 38% of the astronauts treated these symptoms repeatedly.3 The reason for the discrepancy between symptoms perceived by astronauts and imaging findings is unknown. However, medication use by astronauts during a spaceflight mission is not systematically tracked by NASA, and studies rely on astronauts self-reporting medication use during postflight mission debriefings and flight physician notes from private medical conferences to determine the relative frequency of onboard medication use.3,5,6 Therefore, it is highly likely that medication use is underreported.6
Instead, we found that ISS, but not Space Shuttle, astronauts had developed or experienced increased mastoid air cell opacification as seen on postflight MRI scans. This opacification likely represented simple mastoid effusions, given the lack of hypointensity on the T2-weighted images and inflammatory findings in the adjacent soft tissues,7,8 although postcontrast imaging was not performed.
Although mastoid effusions are a common incidental finding on MRI scans, occurring in approximately 21.8% of the population,8 they can also be indicative of underlying pathologic conditions, including eustachian tube dysfunction, eustachian tube obstruction (lymphatic tissue hypertrophy, upper respiratory inflammation, or neoplasia), vascular disease,8 cerebrospinal fluid fistula into the middle ear or mastoid, venous congestion, and dural venous sinus occlusion.9 Here we review common Earth-based causes for the development of mastoid effusions and speculate on the potential importance of each mechanism for astronauts.
Eustachian Tube Dysfunction
The most common terrestrial cause of mastoid effusions is eustachian tube dysfunction.10 On Earth, gravity plays a role in the function of the eustachian tube. In the horizontal position, venous tissue pressure increases around the eustachian tube, reducing air flow from the middle ear by up to two-thirds that experienced in the upright position.11 During spaceflight, venous congestion from cephalad fluid shifts may compromise eustachian tube function,12 altering the eustachian tube’s drainage of physiological fluid produced by mastoid mucosa.
Normally, pressure changes due to changes in altitude are offset by the eustachian tube opening to equalize pressure in the mastoid air cells with surrounding ambient pressure.13 However, it has been shown in aircraft pilots with eustachian tube dysfunction that a lack of pressure equalization can result in middle ear barotrauma with injury and inflammation of the middle ear mucosa,13 which could be manifested on MRI scans as a mastoid effusion. Divers with eustachian tube dysfunction can also experience compression barotrauma during descent.14 Although the atmospheric pressure on board the ISS is 14.7 psi, astronauts experience drastic changes in barometric pressure during extravehicular activities (EVAs) or water immersion exercises during training12; in the setting of a chronically compromised eustachian tube opening due to venous congestion, astronauts may be predisposed to experience middle ear barotrauma. For Space Shuttle EVAs, the whole cabin was depressurized to 10.2 psi; thus, crew members not participating in an EVA were also subjected to change in cabin pressure.15 Astronauts participating in the EVA experienced a pressure of 4.3 psi within their space suits.15 Also, in preparation for EVAs, astronauts breathe pure oxygen, thus exposing the middle ear cleft to enriched oxygen, which alters gas exchange across the mucosa and can lead to middle-ear oxygen absorption syndrome.11,12
Eustachian Tube Obstruction
Sinusitis and middle and upper respiratory inflammation are known common causes of middle ear effusion, perhaps by blocking the eustachian tube with inflammatory tissue or hypertrophic adenoids or via retrograde flow of secretions through the eustachian tube.10,16 Many unique factors of the ISS environment predispose astronauts to respiratory inflammation and infection. The ISS is an enclosed environment, and air is recirculated after removal of particulates by high-efficiency particulate air filtration. Sources of particulate matter include cargo, water, experiments, experimental animals, food, crew members, and visitors to the ISS.17 In microgravity, owing to the lack of gravitational settling, particulates remain airborne until they are trapped by ISS filters.17 These particulates include bioaerosols created from coughing or sneezing or during speech, increasing the risk of person-to-person transmission of viruses and other microbial agents.18 Bacterial and fungal growths have been found on ISS surfaces and within free-floating condensates.19 Dry ambient air on the ISS may result in decreased mucociliary transport and possible obstruction of the paranasal sinus passages, which generates prime conditions for the formation of a bacterial infection.20 In addition, astronauts face many stressors (altered circadian rhythms, strenuous workloads, and radiation exposure) that make them susceptible to altered immune system function.21 However, actual infection rates are low, perhaps owing to prelaunch quarantine procedures. This finding is in concordance with the fact that we found no meaningful differences in paranasal sinus opacities after spaceflight. However, the Lund-Mackay system is specific for sinus opacification, rather than nasal mucosal inflammation, which can also be reflective of active infection.4
Venous Sinus Pathologic Characteristics
Venous sinus congestion and the consequent lack of adequate venous drainage could generate conditions favorable for the formation of mastoid effusion. The mastoid emissary veins, which drain the posterior auricular veins and the occipital venous plexus, terminate into the transverse and sigmoid sinus system, with the direction of flow normally extracranial to intracranial.22 Stagnancy of this venous collection system may increase upstream hydrostatic venous pressure and lead to the formation of a transudative fluid edema in the area of the mastoid air cells.23
An extreme pathologic example of this process of stagnancy of the venous drainage is lateral venous sinus thrombosis. The lateral venous sinus system includes the sigmoid sinus, which runs adjacent to the inner wall of the mastoid process, and the transverse sinus, which collects blood from the mastoid area.23 Lateral venous sinus thrombosis can result in hyperemia and edema of the mastoid mucosa, with transudation of fluid into the mastoid air cells.22
In the case of microgravity exposure, it has been proposed that the loss of the normal gravity-dependent hydrostatic pressure gradient with resultant cephalad fluid shifts may lead to venous congestion in the head and neck.24 A lack of gravity to promote head and neck venous drainage could conceivably increase backflow pressure to the level of the mastoid emissary veins and may cause a transudative effusion, similar to the mechanism responsible for mastoid effusions seen after transverse sinus thrombosis. Supporting this mechanism, Marshall-Goebel et al25 recently reported the identification of internal jugular vein thrombosis in 2 ISS astronauts by onboard ultrasonography.
Limitations
This study has some limitations owing to the lack of data on the medical history and mission-specific operational experience of individual astronauts. First, beyond basic demographic information and the assurance that the astronauts were in good physical condition to meet medical requirements for flight, data on each astronaut’s medical history, such as allergy status, or any preflight history of asthma or sinonasal disease were not available for this study. However, the incidence of preexisting allergic conditions is likely very low because astronaut candidates are screened for a history of allergic conditions,3 although there is emerging evidence suggesting that altered immune responses occur during spaceflight (including the occurrence of atypical allergic symptoms in crew members who do not have allergies on Earth).21 Furthermore, data on each astronaut’s in-flight sinonasal symptoms or use of decongestants were not available for this study.
Second, many operational factors may be associated with the development of paranasal sinus or mastoid opacification in these 2 astronaut cohorts. Previous spaceflight history and aeronautical backgrounds vary among the astronauts. Training and in-orbit activities such as EVAs vary among astronauts and may be associated with differences in paranasal or middle ear disease. The experience of returning to Earth also varies among the astronauts. During landing, astronauts returning on the Space Shuttle experienced a more gradual gravitation load primarily oriented along the body’s craniocaudal axis.26 Astronauts who returned on the Russian Soyuz spacecraft experienced higher gravitation forces oriented primarily across the body front to back.26 Future studies should examine the role each of these factors may play in the development of sinus and mastoid opacification.
Third, the Lund-Mackay system assesses only sinus opacification. To our knowledge, there is not a currently validated system for evaluating the nasal cavity or turbinate mucosal thickening. Astronauts may experience nasal, sinus, or head congestion and request decongestants if they have mucosal swelling in the nasal cavity or turbinates (ie, rhinitis rather than sinusitis). This difference remains an area for further study. Similarly, the mastoid opacification system used for this study has not been clinically validated and was created by us for the purpose of characterizing meaningful changes in mastoid opacification detected on MRI scans.
Conclusions
This study’s findings indicate that, although most astronauts do not develop or experience worsening of paranasal sinus opacification between preflight and postflight MRI, long-duration spaceflight on the ISS is associated with an increased incidence of mastoid effusions. The cause of this phenomenon is likely multifactorial, but possible explanations may be reflective of the pathogenesis of several conditions associated with effusions seen on Earth. Further investigation is needed to provide a definite explanation for the development of these effusions and to determine whether these results can be applied to more than this small astronaut population.
eTable 1. Sinuses—Lund Mackay Scores
eTable 2. Mastoid Effusion Scores
eTable 3. A Contingency Table of the Number of Astronauts With New or Worsening Mastoid Effusion vs Stable or Improving Effusions for the Space Shuttle and ISS Groups
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Associated Data
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Supplementary Materials
eTable 1. Sinuses—Lund Mackay Scores
eTable 2. Mastoid Effusion Scores
eTable 3. A Contingency Table of the Number of Astronauts With New or Worsening Mastoid Effusion vs Stable or Improving Effusions for the Space Shuttle and ISS Groups