Variability in the use of brain imaging and the type of diagnostic imaging used can also markedly affect estimates of the incidence and prevalence of TIAs. One study has estimated that a revision of TIA definitions to include the absence of changes on an MRI could lead to a reduction in the incidence of TIAs by {approx}30% and a resultant 7% increase in the number of cases labeled as stroke. Thus, a blend of factors related to the diagnostic process influences the ultimate diagnosis of a TIA.

Definition

Often, health professionals and the public consider TIAs benign but regard strokes as serious. These views are incorrect. Stroke and TIA are on a spectrum of serious conditions involving brain ischemia. Both are markers of reduced cerebral blood flow and an increased risk of disability and death. However, TIAs offer an opportunity to initiate treatment that can forestall the onset of permanently disabling injury.23,24

The traditional definition of a TIA was a sudden, focal neurological deficit of presumed vascular origin lasting <24 hours. The arbitrary 24-hour threshold used to distinguish TIA from stroke arose in the mid-1960s. At that time, it was assumed that transient symptoms disappeared completely because no permanent brain injury had occurred. The term TIA was applied to events lasting up to 24 hours, and the term reversible ischemic neurological deficit was applied to events lasting 24 hours to 7 days. Only symptoms enduring >7 days were thought to reliably indicate infarction and received the designation stroke. During the 1970s, it became clear that the great preponderance of events lasting 24 hours to 7 days were associated with infarction, rendering the term reversible ischemic neurological deficit obsolete, and it disappeared from standard nomenclature.

More recently, high-resolution CT and especially diffusion-weighted MRI studies have demonstrated that many ischemic episodes with symptoms lasting < 24 hours also are associated with new infarction. One third of individuals with traditionally defined TIAs exhibit the signature of new infarction on diffusion-weighted MRI. These findings highlight an inconsistency between the concept of TIA (ischemia causing symptoms but no infarction) and the traditional definition of TIA. With these observations in mind, a group of cerebrovascular physicians proposed a tissue-based, rather than time-based, definition in 2002: “transient ischemic attack (TIA): a brief episode of neurological dysfunction caused by focal brain or retinal ischemia, with clinical symptoms typically lasting less than one hour, and without evidence of acute infarction” (p 1715).

This proposed new definition has been well received. Many cerebrovascular experts endorsed the new definition, and it has been widely incorporated into the study design of major clinical trials (Warfarin-Aspirin Recurrent Stroke Study [WARSS], Randomized Evaluation of Recurrent Stroke Comparing PFO Closure to Established Current Standard of Care Treatment [RESPECT], Prevention Regimen for Effectively Avoiding Second Strokes [PROFESS], Evaluation of the STARflex Septal Closure System in Patients With a Stroke or Transient Ischemic Attack Due to Presumed Paradoxical Embolism Through a PFO [CLOSURE 1]). However, some have raised concerns. To shed additional light on key issues, individual committee members organized a pooled, patient-level data analysis integrating data from published studies of TIA and MRI.

TIA: Short-Term Stroke Risk
It has long been recognized that TIA can portend stroke, with several studies demonstrating elevated long-term stroke risk. Numerous studies also have shown that the short-term risk of stroke is particularly high, with most studies finding risks exceeding 10% in 90 days. Risk is particularly high in the first few days after TIA, with most studies finding that one quarter to one half of the strokes that occur within 3 months occur within the first 2 days. For example, studies in northern California and Oxfordshire found the risk of stroke in the first 24 hours after TIA to be {approx}4%, which is about twice the risk of myocardial infarction or death in patients presenting with acute coronary syndromes ({approx}2% at 24 hours).87 These findings underscore the need for prompt evaluation and treatment of patients with symptoms of ischemia.

Ischemic stroke appears to carry a lower short-term risk of subsequent ischemic stroke than TIA, with reported 3-month risks generally ranging from 4% to 8%. The degree of early recovery may be predictive of greater risk, possibly by indicating that tissue is still at risk.

Risk of cardiac events also is elevated after TIA. In 1 large study, 2.6% of TIA patients were hospitalized for major cardiovascular events (myocardial infarction, unstable angina, or ventricular arrhythmia) within 90 days. Over the course of ≥5 years, a nearly equal number of patients with TIA will have myocardial infarction or sudden cardiac death as will have a cerebral infarction. A prior AHA scientific statement provides detailed guidance on the coronary risk evaluation in patients with TIA.