The American Heart Association (AHA) has released practice guidelines on the use of diagnostic imaging in patients with transient ischemic attacks (TIAs) and acute stroke. The guidelines, published in the July 1997 issue of Stroke, were developed by the Stroke Council of the AHA.
The 17-page guidelines contain two main sections on the diagnostic imaging of patients with TIAs and acute stroke. Additional sections include discussions and recommendations on diagnostic imaging in patients with suspected subarachnoid hemorrhage, arteritis, dural sinus and venous thrombosis, arterial dissection and spinal cord stroke. The guidelines also include recommendations for diagnostic imaging in women of child-bearing age and in patients intolerant of contrast media. The guideline panel does not address ultrasonography of the neck and head because another set of practice guidelines covers the use of ultrasonography. A single reprint of the complete guidelines, called “Practice Guidelines for the Use of Imaging in Transient Ischemic Attacks and Acute Stroke,” may be obtained by calling 800-242-8721 or by writing the American Heart Association, Public Information, 7272 Greenville Ave., Dallas, TX 75231-4596. Ask for reprint No. 71-0125.
In developing the recommendations, the panel used quality of evidence ratings for diagnostic tests formulated by the American Academy of Neurology (see table). The guidelines state that imaging of the brain allows identification of the lesion (e.g., is it a stroke?), determination of whether the stroke is an ischemic infarct or a hemorrhage, and identification of the location, size and age of the lesion. Imaging of the vessels provides information on the presence, location and degree of occlusive arterial disease, on the pathology of occlusive disease (e.g., atherosclerosis, dissection) and on the presence of other vascular lesions, such as malformation, aneurysm, arterial compression and venous thrombosis.
The following is an excerpt of the recommendations in the practice guidelines. The recommendations include the quality of the evidence rating (in parentheses) as cited in the recommendations.
Recommendations in TIA
Imaging of the Brain. There is general agreement that patients with manifestations suggestive of hemispheric TIA should undergo computed tomographic (CT) scanning of the head in the initial diagnostic evaluation to exclude a rare lesion such as a subdural hematoma or a brain tumor (Class III, type C). CT may reveal an area of brain infarction appropriate to TIA symptoms in 29 to 34 percent of patients, a finding that may influence subsequent management, especially the timing of an eventual carotid endarterectomy (Class III, type C). CT of the head has only a limited role in the evaluation of patients with transient monocular blindness (Class III).
Despite a slight advantage of magnetic resonance imaging (MRI) over CT in the detection of brain infarction appropriate to hemispheric symptoms of ischemia, substitution of MRI for CT in the initial evaluation of patients with TIA is not warranted. The panel recognizes that this is a subject of considerable dissension. MRI may be considered when a CT scan fails to substantiate the clinical diagnosis or if additional diagnoses require confirmation or exclusion (Class III). There is also advantage in identifying lesions such as subdural hematoma that may be isodense with surrounding parenchyma on CT imaging as well as in arteriovenous malformations that rarely present with hemispheric TIAs (Class III, type C).
CT of the head has a limited role in the evaluation of patients with vertebrobasilar TIAs, as subdural hematoma or brain tumor is not known to present with transient symptoms resembling posterior circulation ischemia (Class III). CT can detect areas of appropriate cerebellar or, less commonly, brainstem infarction, a finding that in selected instances may alter clinical management. In addition, CT may show evidence of severe atherosclerotic disease in the vertebrobasilar system, such as dolichoectasia of the basilar artery, as a potential mechanism of TIAs (Class III).
The routine use of MRI in the evaluation of patients with vertebrobasilar TIAs is not justified (Class III) vis-à-vis general management, despite its advantages over CT in the detection of lesions potentially related to the mechanism of posterior circulation TIAs, such as atherosclerotic tortuosity, stenosis or occlusion of the basilar artery (Class III). Occult brain infarction is better identifed by MRI in the vertebrobasilar territory, a finding that may provide additional information about the source of the TIA.
Imaging of the Vessels. A noninvasive screening technique is indicated as an initial diagnostic test in most patients with TIA, particularly for the study of vessels involved in causing symptoms of carotid hemispheric or retinal ischemia. Many specialized centers use carotid duplex or Doppler ultrasonography, a technique discussed in another set of practice guidelines. Magnetic resonance angiography (MRA) provides noninvasive imaging of extracranial carotid, vertebrobasilar and major intracranial vessels but leads to overestimation of the degree of arterial stenosis so that its role in the evaluation of patients with TIA has limitations (Class II). There is general agreement, however, that high-quality MRA provides a sufficient vascular overview of the extracranial and intracranial circulations for independent evaluation of vertebrobasilar ischemia. Contrast-enhanced CT scanning of the cervical vessels with helical methodology, in particular, images the arterial wall as well as the lumen and may be helpful as a screening tool in centers where it is available (Class III).
There is general agreement that radiographic arteriography best defines surgically remediable lesions in the accessible extracranial segment of the carotid artery. Radiographic arteriography is generally recommended for a symptomatic patient when noninvasive tests indicate more than 70 percent occlusion in the appropriate carotid artery and exclusions do not apply. In some instances, complete occlusion by noninvasive tests may need confirmation or exclusion by conventional arteriography. The North American Symptomatic Carotid Endarterectomy Trial and the European Carotid Surgery Trial have proved the benefit of endarterectomy in patients with symptomatic arterial stenosis with more than 70 percent luminal reduction as measured on cerebral arteriographic images (Class I). Cerebral arteriography may also be required when a diagnosis of dissection, vasculitis, aneurysm or embolism needs confirmation or exclusion.
Recommendations in Acute Stroke
There is general agreement to strongly recommend CT of the head without contrast enhancement as the initial brain imaging procedure in patients with acute stroke (type A). A follow-up CT of the head without contrast enhancement two to seven days after stroke onset is recommended when the initial CT scan result is negative and documentation of the presence, location and extension of the ischemic infarction is needed (type B), or when clinically significant hemorrhagic transformation is suspected.
MRI of the brain is not recommended for routine evaluation of patients with acute stroke. Despite its imaging advantages, MRI of the brain is not necessary to initiate emergency treatment in the majority of patients with acute stroke; when available, MRI is an appropriate imaging alternative. The panel recognizes that special circumstances may co-occur in some patients (posterior fossa localization, suspicion of dissection, underlying lesion, age of cerebral hemorrhage, uncertain CT image) that drive the need to obtain an MRI of the head. The decision must be made on an individual basis in accordance with specific clinical situations (type B).
Imaging of cervico-cerebral vessels intended to establish a probable etiology of acute stroke is generally not necessary to intiate emergent management, and the test (ultrasound, MRA, CT angiography, conventional angiography, single-photon emission computed tomography) should not delay treatment. When indicated, these procedures should be tailored to specific requirements. Information on carotid or intracranial occlusive disease may guide decisions on cardiac work-up, anticoagulation or carotid endarterectomy for the prevention of future strokes. Ultrasound will generally suffice, but conventional radiographic angiography may be occasionally indicated, based on the findings of noninvasive screening procedures.
Recommendations in Subarachnoid Hemorrh age
Noncontrast CT of the head is strongly recommended as the initial procedure for the diagnosis of subarachnoid hemorrhage (type A). Selective catheter cerebral angiography is the recommended procedure for the diagnosis of cerebral aneurysm as the cause of subarachnoid hemorrhage (type A).
Recommendations in Arteritis and Other Arteriopathies
In patients with stroke and suspected cerebral arteritis, MRI is generally recommended (type B), because MRI is more sensitive than CT for identifying small ischemic lesions that, being common in vasculitis, may advance the etiologic diagnosis. Conventional angiography is recommended to detect beading, stenosis or aneurysm, particularly in medium and small cerebral vessels affected by vasculitis (type C).
Recommendations in Dural Sinus and Venous Thrombosis
There is general agreement in recommending MRI of the brain when thrombosis of a dural sinus, deep vein or cortical vein is suspected as the cause of cerebral stroke (type B). Sagittal and coronal T1-weighted images, T2 and gradient echocardiographic axial images and three-dimensional phase-contrast or time-of-flight MRA are suggested. When MRI and MRA are not available, contrast-enhanced CT of the brain and, in particular, conventional angiography are diagnostic options to be used independently or in combination to advance the diagnosis of deep vein, cortical vein or dural sinus thrombosis (type C).
Recommendations in Arterial Dissection
There is general agreement for recommending CT of the head in the early evaluation of young adults with acute stroke (type A). When arterial dissection is suspected, MRI/MRA of the head and neck are generally useful to screen for arterial dissection (type C). Conventional radiographic angiography may be done if the diagnosis from MRI/MRA is unclear and there is a strong suspicion of cervico-cerebral dissection. The procedure is technically simple and of low risk in young adults (type C).
Recommendations in Acute Spinal Cord Stroke
There is general agreement in recommending MRI for the diagnosis of acute spinal stroke (type C).