The Effect of Mobile Stroke Unit Care on Functional Outcomes: An Application of the Front-door Formula

BACKGROUND: The Berlin-based B_PROUD study was designed to assess the effect of mobile stroke unit (MSU) dispatch among ischemic stroke and transient ischemic attack (TIA) patients without contraindications to reperfusion treatments. However, a large proportion of patients for whom the MSU was dispatched did not ultimately receive MSU care. We estimated the causal effect of additional MSU care on 3-month functional outcomes among B_PROUD patients for whom an MSU was dispatched. METHODS: We used data from the B_PROUD study (1 February 2017-8 May 2019). Given the presence of exposure-outcome unmeasured confounding, we used the front-door formula to identify the distribution of modified Rankin scale (mRS) outcomes under two hypothetical interventions: (1) receiving additional MSU care and (2) only receiving conventional care. We considered the time from dispatch to thrombolysis as the full mediator and adjusted for exposure-mediator and mediator-outcome confounding. We used a parametric estimator to estimate the common odds ratio (cOR) and 95% bootstrapped confidence intervals (CI). RESULTS: We included in total 768 ischemic stroke/TIA patients with MSU dispatch. The MSU was canceled for 180 (23%) patients, whereas 588 (77%) received MSU care. The unadjusted association between the care group and mRS favored conventional care (cOR = 1.7; 95% CI = 1.2, 2.3); however, after applying the front-door formula, the mRS distribution favored MSU care (cOR = 0.88; 95% CI = 0.81, 0.96). CONCLUSIONS: Receiving MSU care was associated with better functional outcomes than conventional care only, compatible with the hypothesized beneficial effect of MSU care on poststroke outcomes, among stroke and TIA patients without contraindications to reperfusion treatments.