Fibromuscular dysplasia of the right renal artery
[Dec 2017]

Renovascular hypertension (RVHT) is caused by renal artery stenosis (RAS) which is typically caused by atherosclerotic stenosis (95 %) and much less often caused by fibromuscular dysplasia (FMD) (< 5 %). FMD is more common in women and younger patients (average age at time of diagnosis: 35 years). The most common subtype is the media fibrosis of the distal two-thirds of the renal artery as shown in our patient. Screening by ultrasound is recommended if a secondary cause is suspected.

Conventional B-mode ultrasound and Doppler ultrasound techniques are established as the initial imaging methods for screening, diagnosis and follow-up post- treatment of renal artery stenosis. The evaluation of the kidney size and of other morphological signs of parenchymal kidney disease and the evaluation of the renal and intrarenal vessels (intrarenal segmental, interlobar and arcuate arteries and veins) and the examination of the renal spectral waveform including quantification of flow parameters including vascular resistance are of importance and well established [(1-4)]. Doppler should be used to analyse the main renal artery if possible and the intrarenal arteries. The visualisation of the intrarenal arteries is technically often easier compared to the examination of the main renal arteries [(5)]. The arteries are typically examined within the sinus (segmental arteries), the corticomedullary junction (interlobar arteries) and at the base of the medullary pyramids (arcuate arteries). From central to distal, a physiological and continuous decrease of velocities can be observed.

The peak systolic velocity (PSV) in the renal artery is physiologically below < 1.5 m/s and typically around 1 m/s. In contrast, values > 1.8 m/s are indicative of significant stenosis. The normal RI values within the kidney are in the range of 0.55 - 0.65 and typically below 0.7. The normal values are increasing with age (up to 20 %) (1, 5, 6).

If the intrarenal RI is above 0.80, endovascular recanalisation is usually not successful owing to irreversible changes of the renal vasculature. Manifestation of chronic renal failure and loss of kidney size are negative predictors fpr the outcome after dilatation.

Percutaneous transluminal angioplasty (PTA) has proven to be effective for the treatment of fibromuscular dysplasia with significantly better results when compared to patients with atherosclerotic stenosis. The disappearance of the parvus-tardus spectrum is observed after successful PTA.

Doppler ultrasound techniques are recommended as the initial imaging method to diagnose RAS. Doppler ultrasound also allows treatment evaluation, follow-up after PTA and endovascular stenting.

Figure 1: Renal size. 37-year old female patient with fibromuscular dysplasia and high-degree stenosis of the renal artery and severe arterial hypertension. The typical asymmetric size of the kidneys was demonstrated (a, right: 9.6 cm; b, left: 10,6 cm. .
Figure 2: The retrocavally located right renal artery could be visualized posterior to the inferior vena cava (IVC) with a bruit in (a) and turbulent flow above 4 m/s in (b).
Figure 3: (a) The typical intrarenal interlobar poststenotic parvus-tardus spectrum is shown (RI 0.42 – 0.46) in comparison to the contralateral side (RI 0.62 - 65).
(b) The intrarenal interlobar spectrum one day after the successful percutaneous transluminal angioplasty showed normalisation of the resistive index (0.53, not shown).
Figure 4:(a) A normal and low-resistance Doppler spectrum was demonstrated one day after dilatation of the right renal artery (in between markers) without a bruit and turbulence.
(b) There was a PSV/EDV of 103/43 cm/s (RI = 0.59) (AO: aorta). .
Figure 5: 37 year old female with fibromuscular dysplasia of the right renal artery. Digital subtraction angiography (DSA) showing the classical "string of beads" appearance of the right renal artery with stenosis in the mid segment in (a). DSA after ballon angioplasty without relevant stenosis in (b).

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4. Dietrich CF, Siehr K: Arterielle Hypertonie. In: Dietrich CF, ed. Medizin im Vortrag. Balingen: Spitta Verlag, 2004; 1-19.

5. Brkljacic B, Castellani S, Deane C, Dietrich CF: Doppler ultrasound of renal vessels. In: Dietrich CF, ed. EFSUMB Course Book on Ultrasound. London, 2012; 716-736.

6. Brkljacic B, Castellani S, Deane C, Dietrich CF: Doppler ultrasound of the aorta, inferior vena cava and visceral arteries. In: Dietrich CF, ed. EFSUMB Course Book on Ultrasound. London, 2012; 693-714.