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Tissue Viability Thresholds in Acute Stroke

Defining thresholds of tissue viability in ischemic stroke is not quite a problem of biblical proportions, but it has become a matter reminiscent of the story of Babel. From the earliest PET studies of altered tissue hemodynamics and metabolism 2 decades ago1 through the most sophisticated multiparametric models using MRI,2 investigators have worked to determine thresholds on imaging measures of stroke patients that would distinguish regions destined to recover spontaneously, from those irreversibly injured and destined to infarct, from those threatened but potentially viable—the ischemic penumbra. The answer has been given dozens of times by dozens of investigators, and it has been a different answer each time it has been given. This observation is not meant to be dismissive of the important work that has been done toward this goal, but to underscore the difficulty in studying the problem, a difficulty compounded by the fact that no two investigative groups have approached the problem in the exact same manner. The articles of Oppenheim and colleagues in the current3 and recent4 issues of Stroke are excellent examples of MRI work investigating the value of single tissue parameters in predicting outcome. Their studies contained relatively large sample sizes for studies of this type. The patients were untreated and studied within 6 hours of symptom onset, the type of patients of greatest interest for a model to discriminate salvageable from unsalvageable tissue. Although single tissue parameters such as the apparent diffusion coefficient (ADC) are predictive of tissue outcome, a single parameter cannot give an absolute threshold. The evolution of ischemic injury is a dynamic process at best, approximated from a single parameter at a single imaging time point. The ADC value reflects only the ischemic history of the tissue, whereas hemodynamic parameters are indicative of the ischemic See article on page 2486