Page 56 - Libro 2

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PART 2 — CEREBROVASCULAR
Evaluation of the extracranial cerebrovasculature was the first clinical application of the duplex ultra- sound device that was developed at the University of Washington in the late 1970s.1,2 Although B-mode imaging and Doppler flow detection had been used separately to characterize vascular disorders, the duplex concept combined real-time B-mode imag- ing and pulsed Doppler flow detection in a single instrument to obtain both anatomic and physiologic information on the status of blood vessels. In addi- tion to the B-mode imaging and pulsed Doppler sys- tems, the first duplex scanning instrument contained a spectrum analyzer for generating Doppler spectral waveforms. The position of the Doppler beam and the pulsed Doppler sample volume was indicated by a line and a cursor superimposed on the B-mode image. Ultrasound technology has advanced signifi- cantly since the introduction of the duplex scanner, with improved B-mode image resolution, a wider selection of transducers, and alternative approaches to displaying flow information such as color Doppler and power Doppler. Unlike catheter contrast arteriog- raphy, which can be interpreted in terms of a specific percentage of diameter reduction, duplex scanning classifies arterial lesions into categories that include ranges of stenosis severity.
The primary goal of noninvasive testing for ex- tracranial cerebrovascular disease is to identify pa- tients who are at risk for stroke due to atherosclerotic plaque and to facilitate treatment by either carotid endarterectomy or stenting. A secondary goal is to document progressive disease in patients already known to be at risk for recurrent stenosis after in- tervention. Duplex scanning can also detect a vari- ety of nonatherosclerotic conditions that involve the extracranial carotid and vertebral arteries, such as dissection, fibromuscular dysplasia, trauma, arteritis, radiation effects, and aneurysms.
SONOGRAPHIC EXAMINATION TECHNIQUES
The major indications for a duplex scan of the ca- rotid and vertebral arteries include an asymptomatic neck bruit; hemispheric cerebral or ocular transient ischemic attacks (TIAs); a history of stroke; screen- ing prior to major cardiac, peripheral vascular, or other surgery; and a follow-up after carotid endar- terectomy or stenting. Atherosclerotic lesions of the extracranial carotid arteries can be present without neurologic symptoms and some may produce a neck bruit. Experience has shown that only about one-third of bruits are related to high-grade (􏰀50% diameter reducing) internal carotid stenoses.3,4 Symptoms of cerebrovascular disease can be produced by emboli
from atherosclerotic plaques, reduction of flow due to high-grade stenoses, and arterial thrombosis. An important mechanism for both transient and perma- nent neurologic deficits appears to be small emboli consisting of platelet aggregates or atheromatous de- bris arising from ulcerated plaques in the extracra- nial carotid system. Hemorrhage or necrosis within a plaque may lead to ulceration and the appearance of symptoms. Although high-grade stenoses can reduce flow through the involved internal carotid artery, this is rarely a primary cause of symptoms because of the collateral circulation available through the circle of Willis.
Symptoms typically associated with extracranial carotid artery lesions include TIAs, amaurosis fu- gax, reversible ischemic neurologic deficits (RINDs), and strokes. A TIA is sometimes referred to as a “mini stroke” and is characterized by focal weakness (paralysis) or numbness (paresthesia) involving some combination of the face, arm, and leg on one side of the body. Difficulty speaking (aphasia) may also oc- cur. These symptoms occur on the side of the body opposite to the affected carotid artery and cerebral hemisphere. Symptoms of a TIA typically last from several minutes to a few hours, but not longer than 24 hours. Amaurosis fugax is a TIA of the eye that produces transient monocular blindness on the same side as the responsible carotid artery lesion. A RIND is similar to a TIA but with symptoms lasting between about 24 and 72 hours. A stroke, also known as a cerebrovascular accident (CVA), results in fixed or permanent neurologic deficits. The symptoms of ver- tebrobasilar arterial insufficiency are less specific than those related to the carotid circulation and include dizziness, diplopia, and ataxia. In general, patients with transient neurologic symptoms in the distribu- tion of an internal carotid artery (TIAs, RINDs, am- aurosis fugax) are considered to be at risk for stroke.5 For patients with TIAs, the overall stroke risk is about 6% per year, with a 12% risk of stroke during the first year after onset of symptoms. Patients who survive their initial stroke have a continuing stroke risk in the range of 6% to 11% per year.6
PATIENT PREPARATION
In preparation for the carotid artery duplex evaluation, the patient should remove jewelry and tight clothing from the neck area, allowing unobstructed access to the cervical carotid artery segments. Interview the pa- tient to obtain the pertinent past medical history and current signs or symptoms that prompted the request for a carotid duplex evaluation (Table 4-1). A brief physical examination can be performed, which in- cludes palpation of pulses for strength and symmetry (carotid, axillary, brachial, radial) and auscultation