1 From the Department of Radiology, Duke University Medical Center, Durham, NC. Received January 21, 2000; revision requested March 3; revision received April 7; accepted April 20.
Index terms: Arteriosclerosis, 1727.721 • Carotid arteries, angiography, 1727.1247 • Carotid arteries, stenosis or obstruction, 1727.721 • Signs in Imaging
The angiographic string sign appears on conventional arteriograms
as a thin, diminished, antegrade trickle of contrast material
in the internal carotid artery (ICA) (
Figure).
| fig.ommitted |
Figure 1a. Digital subtraction angiograms, lateral views, show the angiographic string sign in a 64-year-old man with a 3-day history of transient ischemic attacks. Images were obtained after injection of contrast material into the left common carotid artery. (a) Early arterial phase image demonstrates a 99% stenosis (open arrow) of the proximal ICA. A trickle of contrast material (solid arrows) is seen in the ICA distal to the stenosis. (b) Late arterial phase image shows contrast material in the cervical ICA (arrows) with dependent layering due to extremely slow antegrade flow. At surgery the ICA distal to the stenosis was normal in caliber.
| |
| fig.ommitted |
Figure 1b. Digital subtraction angiograms, lateral views, show the angiographic string sign in a 64-year-old man with a 3-day history of transient ischemic attacks. Images were obtained after injection of contrast material into the left common carotid artery. (a) Early arterial phase image demonstrates a 99% stenosis (open arrow) of the proximal ICA. A trickle of contrast material (solid arrows) is seen in the ICA distal to the stenosis. (b) Late arterial phase image shows contrast material in the cervical ICA (arrows) with dependent layering due to extremely slow antegrade flow. At surgery the ICA distal to the stenosis was normal in caliber.
| |
Preocclusive atherosclerosis is the most common cause of an
angiographic string sign, and it most often occurs at the arterial
bifurcation, where plaque production and turbulent flow are
most pronounced. As the atherosclerotic plaque circumferentially
enlarges, it eventually causes a critical hemodynamically significant
stenosis. Perfusion pressures distal to the stenosis are reduced,
which leads to subsequent collapse of the distal ICA vessel
lumen and production of the string sign. This collapse is often
a result of decreased flow and reduced arterial pressure rather
than atherosclerotic disease (
1,
2). Despite the misleading arteriographic
appearance of the distal collapsed ICA, the artery is typically
not diseased, and there is usually opportunity for successful
revascularization once the proximal lesion is repaired with
carotid endarterectomy.
A number of different clinicopathologic entities can produce a carotid string sign (1,3,4), including preocclusive atherosclerosis at the carotid bifurcation, dissection of the ICA, carotid artery disease resulting from radiation, subacute partial thrombosis of the ICA, chronic subtotal thrombosis of the ICA, and partial recanalization of an occluded artery. All of these entities have the potential to produce narrowing of a long segment of the ICA.
The initial workup of patients with transient ischemic attacks
and stroke includes widely used noninvasive imaging modalities
to screen patients for extracranial atherosclerosis. Carotid
duplex ultrasonography with color Doppler flow imaging, magnetic
resonance angiography, and computed tomographic angiography
have all been proposed as primary diagnostic modalities for
study of patients suspected of having craniocervical atherosclerotic
vascular disease (
5). When results of a noninvasive study show
an occlusion or near occlusion, confirmatory imaging is sometimes
requested.
Patients are referred for conventional arteriography primarily to determine if a lesion is occlusive or preocclusive (6). Conventional angiography can reliably be used to assess the degree of stenosis, identify tandem lesions, and evaluate existing and potential pathways for collateral circulation. Results of conventional angiography can also show coexisting abnormalities (eg, abnormalities of the aortic arch and great vessels and problems with intracranial and collateral flow) that are difficult to see with noninvasive imaging.
Making the distinction between occlusion and near occlusion with conventional arteriography is critical for patient care, and strict adherence to a meticulous protocol is mandatory (7). The carotid bifurcation, as well as the carotid siphon and the intracranial circulation, should be profiled in the lateral and anteroposterior views. Oblique projections are usually necessary to enable separation of overlapping branches of the external carotid artery, since contrast material in an ascending pharyngeal artery or a descending branch of the occipital artery that closely follows the ICA could easily be construed as a string sign (7). Distinction between a string sign and a vessel branching off of the external carotid artery is also made by appreciating that external carotid artery branches do not enter the petrous carotid canal. It is sometimes necessary to use a long imaging series with digital subtraction to help distinguish an occlusive lesion from a preocclusive lesion. With a preocclusive lesion, a fine string of contrast material is seen slowly progressing along the expected course of the ICA and eventually moving into the intracranial circulation.
Patients with occlusive lesions are typically treated pharmacologically, whereas patients with preocclusive lesions may benefit from surgical treatment (8,9). Results from the North American Symptomatic Endarterectomy Trial, or NASCET, indicate that surgical intervention is highly beneficial for symptomatic patients with high-grade (70%–99%) ICA stenosis (10). In this group, absolute risk reduction with carotid endarterectomy was 17% lower than absolute risk reduction after the best medical treatment for ipsilateral stroke (10). Patients who are not candidates for surgery may benefit from percutaneous transluminal angioplasty and carotid stent placement, which may reduce stenosis (11).
In conclusion, the string sign is a conventional arteriographic finding of a long, tapered narrowing of the ICA. Differentiation between a string sign and a complete carotid occlusion is an important distinction that has implications for patient care.
- Ehrenfeld WK, Wylie EJ. Spontaneous dissection of the internal carotid artery. Arch Surg 1976; 111:1294-1301.
- Sekhar LN, Heros RC, Lotz PR, Rosenbaum AE. Atheromatous pseudoocclusion of the internal carotid artery. J Neurosurg 1980; 52:782-789.
- Lippman HH, Sundt TM, Holman CB. The poststenotic carotid slim sign: spurious internal carotid hypoplasia. Mayo Clin Proc 1970; 45:762-767.
- Mehigan JT, Olcott C. The carotid "string" sign: differential diagnosis and management. Am J Surg 1980; 140:137-143.
- Osborne AG. Atherosclerosis and carotid stenosis In: Diagnostic cerebral angiography. 2nd ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 1999; 359-379.
- Fredericks RK, Thomas TD, Lefkowitz DS, Troost BT. Implications of the angiographic string sign in carotid atherosclerosis. Stroke 1990; 21:476-479.
- Gabrielson TO, Seeger JF, Knake JE, Burke DP, Stilwill EW. The nearly occluded internal carotid artery: a diagnostic trap. Radiology 1981; 138:611-618.
- Little NS, Meyer FB. Indications for carotid endarterectomy. Clin Neurosurg 1997; 44:91-105.
- Winslow CM, Solomon DH, Chassin MR, Kosecoff J, Merrick NJ, Brook RH. The appropriateness of carotid endarterectomy. N Engl J Med 1988; 318:721-727.
- North American Symptomatic Carotid Endarterectomy Trial Collaborators. Beneficial effect of carotid endarterectomy in symptomatic patients with high-grade carotid stenosis. N Engl J Med 1991; 325:445-453.
- Urwin RW, Higashida RT, Halbach VV, et al. Endovascular therapy for the carotid artery. Neuroimaging Clin N Am 1996; 4:957-973.
作者：
John N. Pappas MD 2007-5-12