Abstract and Introduction
Time is Brain! The success of acute stroke therapy is dependent on shorter delays in initiating treatment. Time delays to treatment are multifactorial, and dependent on stroke recognition, transportation to hospital emergency department (ED) from onset location, ED evaluation and finally treatment decision and implementation. The above factors received a lot of attention, including massive campaigns to improve recognition of stroke symptoms, seek immediate care, and standardization of ED acute stroke alerts and processes for management and decision-making. Prehospital care has also significantly improved and successful, in educating, mobilizing, recognizing and treating stroke by paramedics with rapid transportation to the ED for more definitive evaluation and treatment.
Rather than waiting for the patient in the ED, the Mobile Stroke Unit (MSU) brings the hospital to the patient. The MSU concept consists of bridging over the prehospital emergency transportation and ED phases, with the hope of improving patient outcomes through increasing use of thrombolysis, improving triage of patients to appropriate stroke centers, and increasing treatment with thrombectomy. The MSU potential for diagnosis and intervention has dramatically evolved since its first beginnings in 2003. It now boasts state-of-the-art equipment, allowing brain and vascular imaging, telestroke, and high-speed communications capabilities, in addition to point of care laboratory testing and potential for anticoagulation reversal. These capabilities come at the cost of larger, heavier, and expensive vehicles, which may sometimes limit their adoption and effectiveness.
Excitement about the concept and its potential, led to a proliferation of MSU adoption at several US sites, mostly in academic or large health centers, particularly in large urban areas and around the world. Credit goes to the investigators who rapidly initiated observational, comparative and randomized studies to demonstrate MSU clinical effectiveness at reducing time to treatment, and ultimately improving outcomes.
In this issue, Holodinsky et al, apply an innovative and complex conditional probability modeling study for MSU usefulness, to identify the best circumstances for MSU benefit above routine prehospital transportation and care. The authors applied their modeling to data from studies published and complemented by questionnaires sent to some MSU sites. They identified 8 theoretical potential scenarios, although admit to significant variability in the MSU standard operating criteria for dispatch and diagnostic distribution. The authors conclude that "There are very few scenarios where MSU transport predicts substantially superior outcomes to the mothership method when the MSU is dispatched from the endovascular therapy center (EVT). Outcomes using the MSU are maximized when dispatch criteria that maximize patients eligible for thrombolysis treatment are used and treatment times on the MSU are short relative to those of the EVT."
The authors provide a new perspective for modeling the effectiveness of MSUs beyond actual clinical data and trials. Consequently, however, the modeling carries limitations from the assumptions and the data available for its use, compounded by the evolution, complexity and variety in circumstances of MSU dispatch and operation, that limit the strengths of its findings. MSU dispatch pathways, geographic distribution of MSU base to patient location, to destination hospital, and workflow of EMS and MSUs are highly variable and currently not amenable to exact modeling. (Please refer to illustrative case below on limitation of abstracting and modeling MSU information). Therefore, any modeling exercise, including this one, is bound to not accurately reflect reality. The results of the model used in this study, depend entirely on the various time epochs inserted into the model in table 1, but the times used for the MSU may in fact not reflect real-life performance. The models presented in the figures show transport times of up to several hours. While this may be relevant to rural areas, most MSUs operate in urban areas where transport times are <30 minutes. In all the scenarios presented by the authors, MSUs are superior to EMS within those ranges. In rural areas, it is unlikely that MSUs would travel 3 hours or more to reach a patient. In those cases, EMS would begin transport and meet the MSU midway, again within the range of MSU benefit.
The authors may indeed be correct that there is only a minimal benefit of MSU deployment, but the field should resist inaccurate modeling and await the results of data from prospective comparative trials. The most recent study, B_PROUD 2.0 (Berlin Pre-Hospital or Usual Delivery of Acute Stroke Care), showed a significant 26% reduction in the likelihood of severe disability at 3 months with 20 minutes earlier treatment with MSU. A large study, BEST-MSU (Benefits of Stroke Treatment Delivered Using a Mobile Stroke Unit Compared to Standard Management by Emergency Medical Services), is a Phase 3, multicenter, prospective, cluster-randomized study aiming to enroll 1038 patients, with a primary outcome based on disability on the modified Rankin Scale score, with secondary outcomes of healthcare utilization and cost-effectiveness.
The study by Holodinsky et al offers a daring and provocative perspective on the potential best effectiveness of MSU. Despite its limitations, the study challenges the MSU community to define and standardize its role, investigate rigorously and systematically its effectiveness, healthcare utilization, and cost-effectiveness, in further advancing stroke care, while carving its role within the stroke systems of care.
Stroke. 2020;51(6):1637-1638. © 2020 American Heart Association, Inc.