Figure 1: The right common carotid artery is patent. There is a 2.5 cm segment of severe narrowing of the internal carotid artery 1.5 cm distal to the bifurcation. Beyond this level of focal narrowing, the internal carotid demonstrates a “beaded” appearance. There is severe narrowing of the right external carotid artery just distal to its origin.

Figure 2: The left common, internal, and external carotid arteries are patent. The internal carotid artery also demonstrates a beaded appearance, but there is only mild segmental narrowing of the left internal carotid artery just distal to the bifurcation. The external carotid is patent. The vertebral arteries are patent with antegrade flow.

Bilateral carotid fibromuscular dysplasia.

Fibromuscular dysplasia (FMD) is an arterial disease of unknown etiology that typically
affects the medium and large arteries. FMD commonly affects females, and can occur in any age group but is most common in the fifth decade. It typically affects the renal arteries in 85% of patients and often presents as renovascular hypertension. The internal carotid artery is the second most common location (3%) and is affected bilaterally in 65% of patients. Other arteries potentially affected by FMD include the iliac artery, lumbar, mesenteric, celiac, and hepatic arteries.

FMD affects the intima, media, or adventitia of the artery. FMD can be classified into subgroups based upon the primary site of involvement in the arterial wall: intimal hyperplasia, medial fibroplasias, fibromuscular hyperplasia, and subadventitial fibroplasias. Medial fibroplasia is the most common of the subtypes, and consists of the classic “string of beads” appearance on angiography. The etiology of FMD in unknown, but possible etiologies include ischemic damage to vessel walls due to an impaired vasa vasorum, alpha-1 antitrypsin deficiency and hormonal effects on smooth muscle.

FMD of the internal carotid artery, as demonstrated by this case, accounts for less than 1% of carotid occlusive disease. The usual location of carotid involvement is in the middle and distal third of the internal carotid artery. The lesions are more common in middle-aged women and are frequently bilateral. Patients present with nonspecific symptoms of occlusive disease, such as cerebral ischemia, TIA, amaurosis fugax, and stroke. The carotid lesions are associated with intracranial aneurysms in approximately 25-30% of cases and spontaneous carotid artery dissection in 10-20%. Approximately one third of patients with carotid FMD also have renal artery FMD and 10% have vertebral artery involvement. Complications include carotid dissection, aneurysms, premature atherosclerosis, carotid cavernous fistula and AVM, thromboembolism, and hypertension.

Radiographically, carotid FMD most commonly involves the internal carotid artery (ICA) near the first and second cervical vertebrae, sparing the proximal 1-2 centimeters. Angiography remains the gold standard for diagnosing FMD. However, MRA can be used as a non-invasive means of diagnosing the vascular changes of FMD and intravenous administration of gadolinium contrast agents often yields superior imaging results.

FMD has various angiographic presentations:

• Type 1 is the classic “string-of-beads” appearance and is the most common radiographic pattern (89% of cases). This appearance is pathognomonic for medial fibroplasias and anatomically represents alternating luminal stenoses and aneurysmal out-pouchings. The differential diagnosis for this finding includes atherosclerotic disease, arteritis, and vasospasm.

• Type 2 demonstrates a tubular stenosis and is seen in 7% of cases. This type is associated with any histologic type but is most commonly seen with the intimal form. The differential diagnosis for long tubular narrowing of the internal carotid artery includes dissection, arteritis, congenital hypoplasia, extrinsic compression by an adjacent structure or mass, vasospasm, and narrowing secondary to decreased inflow/outflow from the carotid artery due to proximal or distal stenotic lesions.

• Type 3 reveals a “semi-circumferential” lesion with out-pouchings. This type is seen in 4% of cases and this pattern is indistinguishable from atherosclerotic ulceration and pseudoaneurysm.

References:

1. Brant WE and Helms CA. Fundamentals of Diagnostic Radiology, 2nd Edition. Philadelphia: Lippincott Williams & Wilkins, 1999. pp. 640, 645-6.
2. Kochan, Jeffrey P. Fibromuscular Dysplasia (Carotid Artery). eMedicine, 2002.