Epidemiology of swimming induced pulmonary oedema: occurrence, risk factors and outcomes

Background: Open water swimming is increasingly popular, with an estimated 4.4 million people in England participating in 2022/23. However, there is growing evidence that it is associated with a dangerous condition known as swimming-induced pulmonary oedema (SIPE). SIPE occurs when fluids from the blood leak into the lungs during swimming, causing acute shortness of breath and a cough, which may be productive of blood-tinged sputum. Symptoms usually improve when out of the water, however, SIPE can be life threatening if a swimmer is unaccompanied or cannot leave the water quickly. Despite the danger SIPE poses, awareness among outdoor swimmers, venues, event organisers and health care professionals is lacking, and there is a shortage of epidemiological evidence. I set out to investigate SIPE occurrence, risk factors, symptoms and outcomes, and explore the experiences of affected outdoor swimmers. Method: To determine the occurrence of SIPE, I carried out a cross sectional study of open water swimmers, using questionnaire surveys of the online open water swimming community and at competitive open water swimming events. Questionnaires were developed and underwent testing and validation with a small group of outdoor swimmers and a panel of experts. They collected information on shortness of breath experienced during swimming, demographics, anthropometrics, medical history, swimming habits and health behaviours. Cases of suspected SIPE were detected using a range of case definitions. To study risk factors, I carried out an unmatched case-control study using cases and controls identified from survey responses. I calculated adjusted odds ratios (ORs) for risk factors using logistic regression, controlling for age, sex and potential confounders. I investigated symptoms, outcomes and experiences of SIPE cases through a case series study. Findings: My analyses were based on a sample of 1078 outdoor swimmers. Depending on the case definition used, prevalence of SIPE was between 0.1% (95% CI 0.002 to 0.5) and 22.2% (95% CI 19.7 to 24.8) of outdoor swimmers in the UK, and incidence was between 2.8% (95% CI 0.6 to 7.9) and 14.8% (95% CI 8.7 to 22.9) of competitors per open water swimming race. The risk factors study included 107 cases and 871 controls. Significant risk factors for having ever experienced SIPE included wearing a tight wetsuit or non-tight wetsuit compared to no wetsuit (tight wetsuit: adjusted OR 6.56 (95% CI 3.50 to 12.30); non-tight wetsuit: adjusted OR 2.10 (95% CI 1.27 to 3.48)). Having an asthma diagnosis was also associated with SIPE (OR 2.70, 95% CI 1.73 to 4.21) although evidence for a causal association was limited and it is possible the SIPE case definition lacked specificity. The associations remained similar after attempts to control for potential confounding factors. Age, sex, swimming distance, maximum effort levels or a history of heart disease, hypertension or Raynaud's syndrome were not significantly associated with SIPE. The case series included 107 cases of SIPE, most of whom did not seek medical attention for their symptoms. Over a third of cases did not experience any symptoms other than shortness of breath. Chest tightness was the most common additional symptom reported. Symptoms were frequently attributed to other causes such as cold shock response, panic attack, asthma, nasal allergies or respiratory infections. Over 70% of the people who experienced symptoms suggestive of SIPE reported having previous similar episodes. Conclusions: The results suggest SIPE is a significant problem for open water swimmers, and previous studies may have underestimated its occurrence. More studies are needed that can identify milder cases of SIPE to better understand the scale of the problem and the range of symptoms involved. Wearing a wetsuit during open water swimming increases the risk of SIPE. It is important to further explore whether this is causal and what the mechanisms might be. If the association is found to be causal, it may be necessary to address wetsuit design to prevent SIPE. Increasing awareness and understanding of SIPE among outdoor swimmers, venues, event organisers and healthcare professionals is crucial for effective prevention and management.