A previous study reported that the risk of developing a herniated nucleus pulposus (HNP) was higher in U.S. astronauts who had flown at least one mission, as compared with those in the NASA Astronaut Corps who had not yet flown. However, the study did not account for the cumulative effect of multiple missions. While many HNPs occurred well into astronauts’ careers or in some cases years after retirement, the higher incidence of HNPs relatively soon after completion of space missions suggests that space flight does increase the risk of HNP. To formally investigate this possibility, the investigators used a competing-risks survival model to analyze reports and dates of first HNP occurrences from 330 U.S. astronauts taking part in 745 space missions over 55 years. In this model, the outcome of interest, time from selection into the Astronaut Corps to first report of HNP is modeled as the minimum of several event times, one following each space mission plus one influenced by astronaut training and general lifestyle irrespective of participation in actual missions.
Although returning crewmembers have higher incidence of disc herniation post-flight, it is unknown if this increased incidence is related or contributes to an increased risk of injury due to dynamic loads. The aims of this study are as follows:
This study was completed using data provided by the NASA Lifetime Surveillance of Astronaut Health (LSAH) covering the period from 1959 to February 2014. The data consisted of (a) dates that U.S. astronauts were selected into the Astronaut Corps, (b) when they began and finished space flight missions, (c) basic mission characteristics such as length of mission and type of landing vehicle (Shuttle or capsule), (d) dates at which HNP diagnoses were made, and (e) basic demographic data (e.g., gender, age, weight, height). HNP diagnoses were confirmed by MRI or CT scans, myelography, operative findings, or through clinical corroboration by a neurologist or neurosurgeon. Pre-existing cases of HNP diagnosed at or before the time an astronaut was selected into the Astronaut Corps were ignored. Overall, investigators observed 51 cases of first HNP in 330 U.S. astronauts studied over time spans ranging from 0.2 to 52.0 years. In many cases, astronauts returned for yearly physical examinations even after retirement; others were lost to follow-up after retirement or were deceased. The number of missions studied per astronaut ranged from zero (those selected into the Astronaut Corps but who hadn’t yet participated in a mission) to seven comprising a total of 745 astronaut missions. After including one observation of HNP status for each astronaut just prior to his/her first mission, the resulting total number of observations for potential analysis becomes 1075. An additional 44 observations (four HNPs) were not analyzed because this data was collected after first HNPs had already occurred. In addition to HNP status, investigators also collected corresponding observations of several explanatory variables for subsequent modeling.
Of the 51 observed HNP, 19 were cervical, 3 were thoracic, and 29 were lumbar. In addition, 46 of the 51 were observed after long-duration missions, and 40 of the 51 were observed after Space Shuttle missions. Other than a small effect of age at selection, there was little evidence that mission duration, type of landing vehicle, or other demographic variables contributed to the model fit. In general the HNP risk after missions drops off sharply. However, the actual form of the distributional functions depends on the number and timing of missions, even in the no-covariate model. In a hypothetical crewmember with multiple flights, despite a sharp drop in the individual hazards related to each space flight, there would be enough carryover from one flight to the next in this hypothetical case to produce a noticeable upward trend in HNP risk as the number of flights increases.
Attribution of HNPs to Space flight
One of the most important questions addressed by this study was to separate the effects of space flight from those of the general astronaut training and lifestyle. Using point estimates of the parameters to the 40 cases of HNP observed after at least one space mission, yielded values of P ranging from 0.037 to 0.970 depending on when the HNP was reported. As expected, this probability is highest for HNP’s reported shortly after the latest mission for each astronaut.
A total of 745 out of 789 astronaut-missions took place before crewmembers’ first diagnosed HNP. Of these, 58 (7.8%) were “long-duration” and 98 (13.2%) were capsule landings. No evidence of an effect of either flight duration or landing vehicle on HNP propensity after space flight was seen. There was no strong evidence that sex, height, weight, body mass index (BMI), or a history of high-performance jet aircraft piloting had an effect on HNP risk either. However, astronauts that were older at the time of selection appeared to have higher risk (P = 0.038). In addition, pre-Shuttle astronauts had generally lower risk of HNP (P = 0.012).
No datasets exist for this study. A final report was archived.