BEST-MSU
Effect of a Mobile Stroke Unit on Functional Outcomes Among Patients With Ischemic Stroke
Clinical Question
Does prehospital care by a mobile stroke unit improve functional outcomes in ischemic stroke compared to standard EMS transport?
Bottom Line
Mobile stroke unit care was associated with a significantly greater likelihood of achieving functional independence (mRS 0–1) at 90 days compared with standard EMS transport.
Major Points
- First multicenter RCT demonstrating that mobile stroke units (MSUs) improve functional outcomes: 55.0% vs 48.1% achieved excellent outcomes (mRS 0–1) at 90 days (OR 1.30, 95% CI 1.02–1.66, P=0.04).
- Cluster-randomized design across 7 US sites — MSU availability (on-day vs off-day) determined allocation, minimizing selection bias while reflecting real-world deployment.
- Median onset-to-treatment time reduced by 36 minutes (72 vs 108 min, P<0.001) — a massive time savings translating directly to better outcomes per the 'time is brain' principle.
- MSU provided prehospital CT imaging, teleneurology consultation, and IV tPA initiation in the field — a complete acute stroke evaluation before hospital arrival.
- No increase in symptomatic ICH (2.6% MSU vs 3.0% EMS) or serious adverse events — safety comparable to standard EMS despite prehospital thrombolysis.
- Mortality trend favored MSU (10.4% vs 12.6%) though not statistically significant — the benefit was primarily in functional recovery rather than survival.
- NNT of approximately 14 for one additional patient achieving excellent outcome — compelling for a systems-level intervention.
- Diverse patient population (only 40–43% White) — one of the most racially diverse stroke trials, addressing health equity concerns.
- Cost-effectiveness remains a major barrier — MSUs cost $1–2M annually to operate, and BEST-MSU does not address whether the functional improvement justifies the healthcare system investment.
- Established the evidence base that led to expanded MSU programs across major US cities and influenced the 2024 AHA/ASA guidelines recommending MSU deployment in high-volume areas.
Design
Study Type: Prospective, multicenter, cluster-randomized controlled trial
Randomization: 1
Blinding: Outcome assessors blinded
Enrollment Period: August 2014 – September 2020
Follow-up Duration: 90 days
Centers: 7
Countries: United States
Sample Size: 617
Analysis: Adjusted logistic regression; intention-to-treat
Inclusion Criteria
- Patients with suspected acute stroke within 4.5 hours of last known well
- Eligible for IV thrombolysis
- Residing within MSU service area
Exclusion Criteria
- Patients outside MSU catchment zone or called outside MSU operating hours
- Not eligible for IV tPA administration per standard guidelines
- Non-ischemic stroke or stroke mimic (excluded from primary efficacy analysis)
- Symptom onset >4.5 hours from last known well
- Pre-existing severe disability (pre-stroke mRS >2)
- Known allergy to IV tPA or contrast dye
- Refusal to participate or inability to obtain consent
Arms
| Field | Mobile Stroke Unit (MSU) | Control |
|---|---|---|
| Intervention | Prehospital stroke care including imaging, teleneurology, and IV tPA initiation in the MSU | Standard EMS transport to hospital emergency department |
| Duration | Single prehospital intervention with 90-day follow-up | Standard care, 90-day follow-up |
Outcomes
| Outcome | Type | Control | Intervention | HR / OR / RR | P-value |
|---|---|---|---|---|---|
| Proportion of patients with modified Rankin Scale (mRS) score of 0–1 at 90 days among those with confirmed ischemic stroke | Primary | 48.1% | 55.0% | 6.90% | 0.04 |
| mRS 0–2 at 90 days | Secondary | 60.4% | 64.1% | OR 1.17 | 0.27 |
| Median onset-to-treatment time | Secondary | 108 minutes | 72 minutes | <0.001 | |
| Symptomatic ICH | Adverse | 2.6% MSU vs 3.0% EMS | |||
| Serious adverse events | Adverse | Similar between groups | |||
| Mortality at 90 days | Adverse | 10.4% MSU vs 12.6% EMS |
Criticisms
- Cluster-randomized design (MSU on-day vs off-day) may allow unmeasured confounding — patients presenting on MSU days may differ from off-days in ways not captured by baseline characteristics.
- Conducted exclusively in well-resourced US urban areas — generalizability to rural settings, lower-volume centers, or resource-limited healthcare systems is unknown.
- MSU operating cost ($1–2M/year per unit) was not addressed — cost-effectiveness analysis is critical for policy decisions about MSU deployment.
- The primary outcome was mRS 0–1 (excellent outcome) rather than mRS 0–2 (functional independence) — the mRS 0–2 endpoint did not reach significance (P=0.27), suggesting the benefit is concentrated at the top of the functional spectrum.
- Enrollment spanned 6 years (2014–2020) — practice changes during this period (thrombectomy expansion, TNK adoption) may create period effects that confound interpretation.
- COVID-19 pandemic overlap (2020) may have affected final enrollment months, EMS response patterns, and hospital protocols.
- Cannot separate the effect of faster treatment from the effect of improved triage (routing LVO patients directly to thrombectomy centers) — both contribute to MSU benefit.
- Long-term outcomes beyond 90 days were not assessed — durability of functional gains and downstream healthcare utilization unknown.
- Stroke mimics receiving tPA in the prehospital setting (inherent to the MSU model) were excluded from the efficacy analysis — real-world MSU mimic rates and consequences not fully characterized.
Funding
Patient-Centered Outcomes Research Institute (PCORI); other philanthropic and institutional support
Based on: BEST-MSU (JAMA, 2021)
Authors: James C. Grotta, Stephanie L. Yamal, Andrew D. Parker, et al.
Citation: Grotta JC, Yamal SL, Parker SA, et al. Effect of a Mobile Stroke Unit on Functional Outcomes Among Patients With Ischemic Stroke: The BEST-MSU Randomized Clinical Trial. JAMA. 2021;325(5):504–513.
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