Fibromuscular Dysplasia Does Not Typically Cause True Tortuosity of Renal Arteries
Fibromuscular dysplasia (FMD) produces a characteristic "string of beads" appearance on angiography, not generalized arterial tortuosity. This beaded pattern represents alternating areas of stenosis and dilation along the vessel, which is distinctly different from the smooth, serpentine curves seen with true arterial tortuosity 1.
Understanding the Angiographic Appearance
The most common form of FMD—medial fibroplasia, accounting for 80-85% of cases—creates a pathognomonic "string of beads" pattern where:
- The "beads" (dilated segments) are actually larger in diameter than the normal artery 1
- This results from alternating areas of thinned media and thickened fibromuscular ridges containing collagen 1
- The lesions characteristically involve the middle and distal two-thirds of the main renal artery, not the proximal portion 1
Distinguishing FMD Patterns from Tortuosity
Different subtypes of FMD have distinct appearances:
- Medial fibroplasia (80%): Classic "string of beads" with beads larger than the vessel diameter
- Perimedial fibroplasia (10-15%): Beading where beads are smaller than the artery diameter 1
- Medial hyperplasia (1-2%): Concentric smooth stenosis without beading 1
- Focal FMD: Can present with focal narrowing and adjacent aneurysmal dilation 2, 3
True arterial tortuosity—smooth, serpentine, winding vessels—is not a characteristic feature of FMD. While recent case reports describe "tortuosity" in association with FMD 2, 4, this typically refers to focal areas of vessel irregularity adjacent to stenotic or aneurysmal segments, not diffuse vessel tortuosity.
Clinical Pitfalls to Avoid
When evaluating imaging for suspected FMD:
- Don't confuse the beaded appearance with tortuosity—they are fundamentally different vascular patterns
- Look specifically at the mid-to-distal renal artery segments, as proximal involvement suggests atherosclerosis rather than FMD 1, 5
- In young women with hypertension, a segmental low-density area on contrast-enhanced CT may be an important clue to branch artery FMD 2
- The "string of beads" pattern may involve multiple vascular beds beyond the renal arteries, including carotid, vertebral, and extremity vessels 4, 6
Recommended Imaging Approach
For suspected FMD based on clinical presentation (young woman with hypertension, resistant hypertension, or hypertension with abdominal bruit):
- CT angiography or MR angiography are the primary screening modalities 1, 5
- Catheter angiography remains the gold standard when noninvasive tests are inconclusive 1
- Duplex ultrasonography can be used as an initial screening test 1
The key diagnostic feature remains the characteristic beaded appearance, not tortuosity, which should prompt consideration of alternative diagnoses if present as the dominant finding.