Does Stroke Associated Pneumonia play an important role on risk of In-hospital mortality associated with severe stroke? A four-way decomposition analysis of a national cohort of stroke patients.

Background
Severe strokes, and stroke associated pneumonia (SAP) have long been associated with poorer patient health outcomes, for example in-hospital mortality. However, it is unclear what role SAP plays in the risk of in-hospital mortality associated with a severe stroke at admission.

Methods
Using the Sentinel Stroke National Audit Program data on stroke admissions (2013-2018) in England and Wales, we modelled the ‘total’ effect for severe stroke on risk of in-hospital mortality. Through four-way decomposition methodology, we broke down the ‘total’ observed risk into four components. The direct ‘severity on outcome only’ effect, the pure indirect effect of severity mediated via SAP only, the interaction between severity and SAP when mediation is not present and when mediation via SAP is present.

Results
Of 339,139 stroke patients included, 9.4% had SAP and 15.6% died in hospital. Of SAP patients 45% died vs 12% of non-SAP patients. The risk ratio for in-hospital mortality associated with severe vs mild/moderate stroke (i.e. total effect) was 4.72 (95% 4.60,4.85). Of this, 43% increased risk was due to additive SAP interaction, this increased to 50% for ‘very severe’ stroke. The remaining excess relative risk was due to the direct severity on outcome effect only i.e. there was no evidence here for a mediation effect via SAP.

Conclusion
SAP was associated with a higher mortality in severe stroke patients. Prioritising SAP prevention in severe stroke patients may improve in-hospital survival. Our results suggest that in severe stroke patients avoiding SAP might result in an up to 43% reduction in mortality.