Transient ischemic attack

Transient ischaemic attack (TIA), in the most recent definition, corresponds to a transient episode of neurological dysfunction caused by focal brain, spinal cord, or retinal ischaemia, without acute infarction.

Terminology 

In the past TIA was arbitrarily distinguished from stroke by the duration of neurological symptoms of less than 24 hours¹. It However, this definition allowed for risk of permanent infarction to be classified as a TIA, and thus the aforementioned tissue-based definition was favoured ¹. It is well established nowadays that in most TIAs symptoms resolve in less than one hour but occasionally prolonged episodes may occur ^1-2,2. 

The clinical features are variable and dependent on the mechanism (see below):

• Low-flow TIA: classic transient focal neurological symptoms, either anterior or posterior circulation symptoms, last only minutes, and are often recurrent ³
• Embolic TIA: similar presentation to a thromboembolic ischaemic stroke, localised to a specific arterial territory rather than a general circulation, tend to last hours, and do not tend to be recurrent ³
• Lacunar or small penetrating vessel TIA: similar to low-flow TIAs but the transient neurological symptoms are those of lacunar stroke syndromes, also known as the capsular warning syndrome ⁴

Pathology

There are three pathophysiological mechanisms:

• Low-flow TIA: caused by large artery stenosis (e.g. internal carotid artery stenosis) ^1,3
• Embolic TIA: caused by the same aetiologies of thromboembolic ischaemic stroke^1,3
  • There is controversy as to whether these should actually be classified as strokes as per the tissue-based definition
• Lacunar or small penetrating vessel TIA: caused by either stenosis of intracranial arteries (e.g. middle cerebral artery stenosis) or lipohyalnosis of penetrating arteries ^1,4

Radiographic features

There is uncertainty regarding the radiographical features of TIAs given that there is currently no perfect radiographic corollary to detecting minute regions of infarction ¹. While CT brain is often unremarkable, there is controversy regarding the significance of regions demonstrating high signal on DWI ⁵.

It has been proposed that if a region does demonstrate high signal on DWI and if there is no early DWI reversal, then that is a region of infarction and hence should be a stroke and not a TIA ⁵. However, the confusion lies in a lesion that does in fact demonstrate early DWI reversal, because it has been found that these such lesions may still have infarction, and thus the patient may in fact have a stroke instead of a TIA ^6,7. 

Treatment and prognosis