Head and neck symptoms during prolonged simulated weightlessness

Microgravity exposure induces cephalad fluid shifts in astronauts, but systematically collected data on head and neck-related symptoms remain limited and largely anecdotal. We therefore performed a retrospective pooled analysis of nine mid- and long-duration six-degree head-down tilt bed rest campaigns, an established analog of microgravity-induced fluid shifts, conducted at a single research center between 2016 and 2025. Medical records of 101 healthy participants were screened for head and neck-related symptoms before, during and after bed rest. Symptoms were classified and graded using the Common Terminology Criteria for Adverse Events version 6.0, and event frequency, duration, severity and timing were analyzed with phase-specific, exposure-adjusted metrics. A total of 412 head and neck-related events were documented. During head-down tilt, 84 of 101 participants (83.2%) experienced at least one event, compared with 35 participants (34.7%) during the 15-day baseline phase preceding bed rest; overall, 91 participants (90.1%) were affected. Exposure-adjusted event rates were higher during head-down tilt (7.08 events per 100 participant-days) than during the baseline phase (3.56) or the recovery phase (2.04), and this pattern was present in all nine campaigns. Headache and vertigo were the most frequent manifestations, followed by nasal, ocular, otologic and dermatologic symptoms. Onset was analyzed separately for head-down tilt days 1–30, covered by all participants, and days 31–60, covered only by the 60-day campaigns. New events continued to occur during the second month, although at a lower rate than during the first (5.80 versus 8.21 events per 100 participant-days). With the exception of one prolonged tinnitus case, all symptoms had resolved by the end of the recovery phase. Head and neck symptoms were thus common during head-down tilt bed rest, and most events were mild and self-limiting. In the absence of a control group without bed rest, and with heterogeneous countermeasure protocols across campaigns, these descriptive findings do not establish causality. They provide a systematic reference for the design of controlled analog studies of otorhinolaryngological, ocular and dermatological responses to simulated microgravity.