Clinical History
A 24-years-old, first-time pregnant diabetic female patient at 32 weeks gestational age was referred to the Radiology Department for lumbar pain. MRI was performed to investigate severe bilateral hydronephrosis detected during gynaecological ultrasound.
Fetal biometry and heartbeat, laboratory assays (including C-Reactive Protein, serum creatinine and urinalysis) were within normal limits.
Imaging Findings
Unenhanced MR examination was performed with preliminary acquisition of axial and fat-suppressed coronal T2-weighted images (Figures 1a-c), showing normal size and parenchymal thickness of both kidneys, mild perinephric fat stranding and fluid effusion around the lower poles.
Static-fluid MR-urography acquisition included breath-hold thick-slab single-shot turbo spin echo and volumetric respiratory-triggered sequences, the latter postprocessed using maximum-intensity-projection (MIP) algorithm to obtain pyelographic images. MRU findings (Figures 1d-f) confirmed bilateral hydronephrosis, more severe on the left side, with dilated lumbar ureters (caliber 16-18 mm) smoothly tapering at the pelvic brim. Hydronephrosis was caused by ureteral compression between the gravid uterus, psoas muscles and iliac vessels (Figures 1g-h). The pelvic ureters were collapsed and no endoluminal filling defects consistent with lithiasis were appreciable on thin-slice source MR-urography images.
Hydroureteronephrosis from extrinsic fetal compression was diagnosed, and the gynaecologist choose to plan early caesarean section at 35 weeks gestational age.
Discussion
During pregnancy, abdominal or lumbar pain is a common clinical problem and the first non-obstetric cause of hospitalisation. Among a wide spectrum of causes, urologic conditions mostly include hydronephrosis, lithiasis and infection [1]. Collecting system dilatation is commonly observed with increasing frequency as pregnancy advances, as a result of combined hormone-induced ureteral relaxation and extrinsic compression by the gravid uterus at the level of the lumboiliac junction. Sometimes painful but usually asymptomatic, “physiologic” hydronephrosis involves the upper cavities and lumbar ureters up to the iliac arteries, predominantly on the right side in 80-90% of cases [1-4]. Usually treated conservatively, “physiologic” dilatation should be differentiated from pathological obstruction caused by calculi (rarely by clots, abscesses or tumours) that may require ureteral stent placement or nephrostomy [1, 4, 5].
Invariably the initial diagnostic approach, ultrasound readily demonstrates hydronephrosis with perinephric fluid, absent ureteral jet and elevated (>0.7) intrarenal Doppler resistive index suggesting acute obstruction. Highly operator-dependent, sonography allows limited assessment of the ureters because of maternal body habitus, overlying bowel gas and presence of the fetus, therefore, equivocal clinical or ultrasound findings and predominantly left-sided dilatation require further investigation [2-4, 6]. In the past, intravenous urography was used to investigate suspected ureteral obstruction, with severe drawbacks and diagnostic limitations [1, 2, 7].
Today unenhanced MR-urography (MRU) represents the ideal modality to noninvasively assess severity and extent of urinary tract dilation without irradiation and intravenous contrast, even with quiescent renal function. Performed using heavily T2-weighted pulse sequences, MRU shows the static or slowly flowing urine as hyperintense because of its long relaxation time, against a very low signal background [2, 5, 7, 8].
As this case exemplifies, MRU is valuable to investigate painful hydronephrosis in pregnancy, providing accurate localisation of the obstruction level. Even when severe, extrinsic “physiologic” dilatation includes gradual, smooth ureteral tapering at the pelvic brim with distal collapse without identifiable filling defects. Ureteral tapering above or below the uterus, an abrupt calibre change or distended pelvic ureter suggest an obstructive cause, most usually a stone that is best identified in thin-section source images [2-6].
Furthermore, perirenal and periureteral high signal on T2-weighted images corresponding to oedema, lymphatic distension or free fluid from forniceal rupture indicate help distinguish acute ureteric obstruction from chronic dilatation [3, 5, 6].
Differential Diagnosis List
Acute bilateral "physiologic" hydronephrosis in pregnancy.
Obstructing urolithiasis
Urinary tract infection
Pyelonephritis
Acute appendicitis
Final Diagnosis
Acute bilateral "physiologic" hydronephrosis in pregnancy.
