Examinations of Transient Ischemic Attack

Front Neurol Neurosci. Basel, Karger, 2014, vol 33, pp 115-122 (DOI: 10.1159/000351913)

______________________

Radiological Examinations of Transient Ischemic Attack

Christie E. Tung · Jean Marc Olivot · Gregory W. Albers

Stanford University Medical Center, Palo Alto, Calif., USA

______________________

Abstract

Neuroimaging is critical in the evaluation of patients with TIA. CT and MRI are the two available options for imaging. Head CT is more widely available and commonly used. Diffusion MRI is the recommended modality to image an ischemic lesion. The presence of a diffusion lesion in a patient with transient neurological symptoms is an indicator of a high risk of recurrent stroke. Perfusion imaging with perfusion MRI or CT perfusion may improve the detection of ischemic lesions. Noninvasive vessel imaging may detect a symptomatic vessel lesion associated with an increased risk of stroke.

In 2009, the American Heart Association, the American Stroke Association, and the American Academy of Neurology endorsed a new definition of TIA. A transient episode of neurological dysfunction caused by focal brain, spinal cord or retinal ischemia without acute infarction. Stroke is now defined as a symptomatic infarction of the central nervous system tissue. This tissue-based definition has several implications. First, patients with transient neurological symptoms may have acute infarction of the brain. Second, diffusion-weighted imaging (DWI), the most sensitive imaging modality to confirm the diagnosis of infarction, becomes a critical tool in the evaluation of patients presenting with transient neurological symptoms of presumed ischemic mechanism [1].

The first obstacle a clinician has to overcome is how to confirm the ischemic mechanism of transient neurological symptoms when several non-ischemic pathologies can mimic TIA. Population studies indicate that about 5% of TIA patients suffer a stroke within 2 days of symptom onset. Urgent assessment and management of TIA can drastically reduce the risk of stroke. Therefore, patients with symptoms compatible with a TIA should undergo an urgent diagnostic evaluation.

Several studies have demonstrated substantial lack of agreement regarding TIA diagnosis among emergency department (ED) physicians, neurologists and even stroke specialists, whose judgment is often considered as the gold standard [2, 3]. Substantial disagreements regarding TIA diagnosis imply that patients with transient neurologic symptoms are at high risk to receive either inadequate or unnecessary treatments. Brain imaging can help the physician to clarify TIA diagnosis.

When the diagnosis of a TIA is made, the clinician should stratify the individual risk profile of the patient. The ABCD² score refers to clinical characteristics associated with an increased risk of stroke: age, blood pressure, type and duration of symptoms and the presence of diabetes [4]. The ABCD² score has performed well in epidemiological studies, but recent data strongly suggest that some patients with a low ABCD² score may have a high risk of stroke [5]. In 2010, our group participated in 2 pooled analyses which evaluated the benefits of including brain and vessel imaging findings in conjunction with the ABCD² score to improve risk assessment in TIA patients [5, 6]. Both studies demonstrated that the incorporation of imaging results improved the predictive power of the score.

Imaging an Ischemic Lesion

Computed Tomography

The most common imaging technique used for the evaluation of acute TIA is non-contrast head CT. CT imaging is widely available and can be rapidly performed in most EDs. It can be performed in patients with MR contraindications such as claustrophobia and metal implants. Several hours after the onset of ischemia, decreased density of the brain tissue on non-contrast head CT may reveal acute infarction.

Given the short duration of ischemia in TIA, the detection of an acute ischemic lesion on a non-contrast head CT is low, and in a retrospective study an ischemic lesion was found on non-contrast CT in less than 4% of TIA patients [7]. A recent metaanalysis of 1,368 TIA patients documented 327 (24%) with evidence of prior infarction (acute and chronic) on CT [6]. Infarction on CT increased the risk of stroke to 12.8% within 7 days compared to 9% in the CT-negative group, and improved the predictive power of the ABCD² score for early recurrent stroke [6, 8]. Although the accuracy of non-contrast head CT for the diagnosis of TIA is low, indirect markers of brain ischemia (i.e. the presence of chronic infarcts) may help the physician predict the risk of stroke after a TIA.

CT perfusion (CTP) imaging has developed concurrently with MR perfusion imaging and is now available for the evaluation of acute TIA patient. CTP can be performed in sequence with other techniques, such as CT angiography. Recent advances in CT technology (128 or 256 slice scanners) offer improved brain coverage. In one study, CTP was positive in 35% (12/34) of TIA patients [9]. In another small singlecenter study evaluating CTP in TIA patients, an acute ischemic lesion was found in 22 (34%) of 65 patients with motor and/or speech TIA symptoms [10]. This study also suggested that the presence of an ischemic lesion detected on CTP was associated with an increased risk of future stroke. This result remains to be confirmed in a large prospective study.

In conclusion, CT with or without CTP is a reasonable approach for the evaluation of TIA patients when MRI is contraindicated or not available. A systematic evaluation of a multimodal CT approach for the evaluation of patients with TIA is needed.

Magnetic Resonance

MRI has several advantages over CT. It can detect acute ischemic changes sooner than CT, and it has greater interobserver and intraobserver reliability than CT. Intracranial hemorrhage can be detected on MRI as easily as on CT [11]. Multimodal MRI is more specific and sensitive than head CT for the detection of acute ischemia [11]. DWI reveals an acute infarction in 30% of patients with a discharge diagnosis of TIA [6, 8, 11–14]. Symptom characteristics (motor deficit or speech disorder) and the presence of a symptomatic vessel lesion are associated with the presence of an acute ischemic lesion on DWI [15].

Several studies have shown that DWI positivity has prognostic implications. A recent meta-analysis showed that 7% of TIA patients with a positive DWI will suffer a stroke within a week compared to only 0.5% of the DWI-negative patients [6]. Interestingly, in DWI-negative patients the ABCD² score did not influence the risk of stroke, suggesting that a proportion of these patients may not have experienced a transient ischemic event [8]. By comparison, patients with a negative head CT still have an increased risk of stroke stratified by the ABCD² score. Because of the sensitivity of DWI in detecting brain infarction in TIA patients, DWI is recommended in the emergent evaluation of every TIA patient [15].

Two types of automatically processed MR perfusion sequences are now available: dynamic susceptibility contrast (DSC) perfusion-weighted imaging (PWI) and arterial spin labeling (ASL) perfusion imaging. DSC PWI is the most widely used perfusion modality. It requires a bolus contrast injection and is contraindicated in patients with renal insufficiency [16

Only gold members can continue reading. Log In or Register to continue