Fibromuscular dysplasia (FMD) is an arterial disease of unknown etiology typically affecting the medium and large arteries of young to middle-aged women. This article focuses on FMD of the carotid arteries. 
There are 3 types of FMD: intimal, medial, and subadventitial (perimedial) of the arterial wall. The 3 types of FMD are not easily differentiated by findings on angiography. The medial type of FMD is by far the most common; medial FMD is classically diagnosed on the basis of a "string of beads" appearance on angiography. This appearance is explained by the presence of luminal stenosis alternating with aneurysmal outpouchings. Classically, the intimal form of FMD is associated with smooth focal stenoses on angiography. [2, 3] (See the images below.)
Although conventional computed tomography (CT) scanning has no role in the diagnosis of fibromuscular dysplasia, it is essential for assessing the intracranial consequences of the disease. CT angiography (CTA) may be used as a noninvasive means of diagnosing the vascular changes of FMD. CTA may display the pathognomonic string of beads appearance of the internal carotid artery. It is recommended that CTA findings be confirmed with carotid angiography.
Current resolution (voxel sizes) of CT may limit the usefulness of this modality in the diagnosis of subtle cases. As this technique improves, its negative predictive value may increase so as to make it useful in excluding FMD from the diagnosis.
Signal/noise artifacts or reconstruction artifacts that occur with CTA occasionally may mimic the contour irregularity of FMD.
Magnetic Resonance Imaging
Magnetic resonance angiography (MRA) may display the pathognomonic string of beads appearance of the internal carotid artery.
As with conventional CT scanning, magnetic resonance imaging (MRI) has no role in the diagnosis of fibromuscular dysplasia, but it is essential for assessing the intracranial consequences of the disease.
Like CTA, MRA may be used as a noninvasive means of diagnosing the vascular changes of FMD. Although MRA does not require the use of contrast, intravenous (IV) administration of a gadolinium contrast agent often yields superior imaging results. Confirmation of the findings through the use of carotid angiography is recommended before endovascular or surgical interventions are considered.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis.
NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.
Current resolution (voxel sizes) of MRA may limit the usefulness of this modality in the diagnosis of subtle cases. As this technique improves, its negative predictive value may increase so as to make it useful in excluding FMD from the diagnosis.
MRI is sensitive to motion and metallic artifacts. MRI images may appear artifactually beaded as a result of the patient's swallowing during the examination. The presence of a ferrous metal object may cause the contour of the vessels to be distorted.
Signal/noise artifacts or reconstruction artifacts that occur with CTA or MRA occasionally may mimic the contour irregularity of FMD.
Ultrasonography (US) is useful in imaging the common carotid artery and the carotid artery, but because most FMD lesions occur at the level of the first and second cervical vertebrae, US may fail to show the more cephalad lesions.
Power Doppler ultrasonography may be useful in imaging lesions accessible within the acoustical window. FMD most often affects the cervical segment C1-C2; because this segment is poorly visualized ultrasonographically, most lesions of FMD may be missed.
Angiography is the criterion standard for diagnosing fibromuscular dysplasia.  The string-of-beads appearance is considered pathognomonic for medial fibroplasia on diagnostic angiography. The classic string-of-beads contour is characterized by a long-segment tubular stenosis or ovoid-shaped outpouchings.
When the carotid arteries are involved, the cervical segment C1-C2 most often is affected. Intracranial disease is rare; when it does occur, the beaded appearance may extend to the supraclinoid segment of the internal carotid artery or to the middle cerebral artery. A diagnosis of intracranial FMD should not be considered in the absence of cervical carotid disease. (See the images below.)
Patients with carotid FMD also may have renal artery FMD and, less commonly, FMD of the lumbar, mesenteric, celiac, hepatic, and iliac arteries. Therefore, peripheral angiography should be considered for those patients manifesting symptoms of arterial stenosis or thromboembolic disease.
Conversely, if fibromuscular dysplasia is encountered anywhere in the peripheral circulation, the carotid arteries should be evaluated.
The 3 types of carotid FMD, as classified through angiographic imaging, are discussed below. (See also the image below.)
This is the most common form. In 80-85% of patients with FMD, angiography reveals the typical string-of-beads appearance, with alternating segments of stricture and dilation. This type usually is a result of medial fibroplasia of the arterial wall. The differential diagnosis for this finding includes atherosclerotic disease, arteritis, and vasospasm.
In 6-12% of patients with arterial fibroplasia, a long tubular stenosis may be seen. This may be associated with any of the histologic types, but it is most commonly seen with the intimal form. The differential diagnosis for findings of a long tubular narrowing of the internal carotid artery includes dissection; arteritis; congenital hypoplasia; extrinsic compression by an adjacent structure or mass; vasospasm; and narrowing resulting from decreased inflow to or outflow from the carotid artery secondary to the presence of proximal or distal stenotic lesions.
This unusual form (seen in 4-6% of patients) is characterized by involvement of only 1 side of an artery. Such involvement leads to diverticularizations of the arterial wall. These lesions may be difficult to distinguish from atherosclerotic ulceration and pseudoaneurysm.
Degree of confidence
Confidence in the diagnosis of fibromuscular dysplasia is high when the above findings are seen.