Definition

Traditional definition: time-based

Transient ischaemic attack (TIA) is traditionally defined as an acute loss of focal cerebral or ocular function with symptoms lasting less than 24 hours, and which is thought to be due to inadequate cerebral or ocular blood supply as a result of low blood flow, thrombosis or embolism [1].

However, this definition poses practical difficulties in acute clinical practice.

The 24 hour cut-off for definition is arbitrary. Most TIAs last less than 1 hour, with a median duration of 14 minutes in patients presenting with carotid artery territory symptoms, and 8 minutes in those with vertebrobasilar ischaemia. Only 2% of patients whose symptoms do not rapidly improve within 3 hours had complete resolution at 24 hours [6].

Furthermore, infarction can be present despite complete resolution of symptoms. MRI studies of patients with TIA have shown that up to 50% of those who have fully recovered within 24 hours have abnormalities seen on diffusion-weighted imaging (DWI) of the brain [7]. In one study, symptoms lasting more than or equal to one hour were independently correlated with abnormal findings on DWI MRI [8].

New definition: tissue-based

As a consequence of these observations, in 2002 a proposal was made for a new definition:

“A TIA is a brief episode of neurologic dysfunction caused by focal brain or retinal ischaemia, with clinical symptoms typically lasting less than 1 hour, and without evidence of acute infarction” [6].

This definition may help to differentiate TIA from minor stroke and stroke mimics. However, the change remains controversial, principally due to challenges around access to imaging.

In practice the precise definition used is not of great importance as however quickly or slowly recovery occurs and whether or not there is evidence of permanent damage on brain imaging, the investigations and medical treatment will be broadly similar [1].