Ultrasound of the Month: Severe Hydronephrosis in a patient with Horseshoe Kidney
/THE CASE
An elderly female patient presents to the emergency department (ED) with 1 week of generalized weakness leading to numerous falls. She denies any fevers, abdominal pain, nausea, or vomiting. She has a history of previous episodes of nephrolithiasis, frequent urinary tract infections, and states she takes gabapentin for overactive bladder. She denies any current dysuria or hematuria. Her vital signs are within normal limits. Her cardiopulmonary exam is unremarkable and palpation of the back and abdomen reveals no significant tenderness. She is neurologically intact, with no gait abnormalities, intact cranial nerves and 5/5 strength in all extremities. She has no rashes or ecchymosis and otherwise her skin exam is unremarkable.
Given her age, history of nephrolithiasis, and frequent urinary tract infections, a bedside renal ultrasound was performed to assess for hydronephrosis due to concern her generalized weakness may be secondary to an infected stone or obstructive uropathy. Images are seen below.
Video 1: Left kidney seen in longitudinal view. Spleen coming into view on the left of the image and multiple hypoechoic structures seen on the right of the image. Difficult to appreciate renal parenchyma and inferior pole
Video 2: Left kidney seen in transverse view. Spleen coming into view on the right of the image. Multiple hypoechoic structures seen midline in the image. Difficult to appreciate renal parenchyma and inferior pole
Video 3: Right kidney seen in longitudinal view. Liver coming into view on the left of the image. Multiple hypoechoic structures (one smaller and the other significantly larger) seen immediately to the right of the liver edge with disruption of the renal parenchyma. Hypoechoic structures appear to be interconnected. No visualization of inferior pole of kidney.
Video 4: Right kidney seen in transverse view. Liver coming into view on the left of the image. Multiple hypoechoic structures seen, most interconnected, to the right of the liver. No visualization of inferior pole of kidney.
Overall impression: Numerous, bilateral, anechoic structures with disruption of the renal parenchyma and difficult to identify inferior poles bilaterally.
The patient’s presentation was concerning for hydronephrosis secondary to an obstructive process, such as a kidney stone. However, the bilateral nature of the anechoic structures raised some confusion. Other diagnoses considered included polycystic kidney disease, renal abscess, hematoma, infected stone, and malignancy. Her workup showed a urinalysis notable for large leukocytes and >100 WBC on microanalysis. Additionally, she was found to have an AKI on CKD with a creatinine of 4.10 (baseline Cr 1.3) and hyponatremia to 125.
The patient had never been seen in our medical system previously. Outside hospital records were obtained and upon review, it was noted that the patient had recently been diagnosed with a horseshoe kidney on CT imaging. On further discussion with the patient, she states she was told she had an abnormality with her kidneys but did not recall the specifics. This information explains why the inferior poles of her kidneys were difficult to identify bilaterally.
HORSESHOE KIDNEY PATHOPHYSIOLOGY
Image 1. Axial CT of a horseshoe kidney Hufeisenniere CT axial by Hellerhoff. Licensed under a Creative Commons Attribution-Share Alike 3.0 Unported license.
The horseshoe kidney is the most common renal fusion anomaly and is characterized by abnormalities with position, rotation, and vascular supply. A horseshoe kidney is identified by having functioning renal tissue on both sides of the vertebral column connected by an isthmus. The isthmus is made up of renal parenchyma in about 80% of cases, with the remainder made up of fibrous tissue. This isthmus is located either midline or laterally, with the lateral position resulting in an asymmetric horseshoe, 70% of which are left dominant [1].
Normally, the kidneys are located in the retroperitoneum, between T12 and L3, with the left kidney being located slightly higher than the right. The upper poles will typically be located medially and posteriorly when compared to the inferior poles. Comparatively, the horseshoe kidney’s ascent during development is limited by the inferior mesenteric artery, located around L3. The horseshoe kidney may also be found lower in the pelvis as seen in image 1 [2]. The fusion, which typically occurs in the inferior poles, causes these poles to be located medially, the reverse of the normal renal axis [3].
While about one third of patients with a horseshoe kidney are asymptomatic and diagnosis is made incidentally, the intrinsic anatomical defects associated with a horseshoe kidney predisposes patients to urologic complications such as ureteric obstruction and impaired urinary output [3].
IMAGING WITH ULTRASOUND
In patients with a horseshoe kidney, there are several differences that make point-of-care ultrasound (POCUS) evaluation difficult. The kidney will have a different axis as well as level of lie, so care should be taken to scan broadly if the renal cortex is not immediately apparent. The right side of the kidney can be evaluated with an anterolateral approach, using the liver as a window, with the patient either supine or in left lateral decubitus. The left side of the kidney should be evaluated with a posterolateral approach using the spleen as a window, with patient either supine or right lateral decubitus. Operators should take care to obtain clear images of the upper renal poles with the understanding that inferior pole evaluation will be limited from the isthmus. A midline approach, with the transducer over the umbilicus, is useful for identifying the isthmus itself [4]. It typically lies anterior to the inferior vena cava and aorta in most cases, although there are reports of the isthmus running posterior or even between the great vessels [2]. Ultrasonography is limited in some cases, particularly in patients who are obese or when the isthmus has a fibrous composition [5].
If a horseshoe kidney is not expected, the abnormal rotation and inferiorly located kidney can result in poor visualization of the inferior pole and underestimation of their length. Additionally, renal tissue located anterior to the aorta may be mistaken for retroperitoneal tissue, such as lymphoma or metastatic nodal enlargement [3].
POCUS FOR EVALUATION OF RENAL CYSTS AND HYDRONEPHROSIS
Determining whether the patient with numerous anechoic structures on renal ultrasound has hydronephrosis versus numerous renal cysts can be difficult, made trickier in patients with abnormal anatomy (such as a horseshoe kidney). In brief, hydronephrosis will appear as interconnected, branching anechoic areas, often with irregular contours indicative of renal collecting duct system. In contrast, parapelvic cysts will be thin walled, spherical and not connected to the ureter distally. However, given their anechoic nature and location near the renal collecting system, they can still be easily confused for hydronephrosis and vice versa [6]. Beyond parapelvic cysts, any large cyst can be confused for hydronephrosis if not viewed in both planes. Evaluation of the anechoic structure itself, as well as the relative location and communication with the renal pelvis is crucial. If there remains a question, further evaluation with CT imaging is indicated.
CASE RESOLUTION
The patient was started on intravenous fluids for her AKI and ceftriaxone for antibiotic coverage of her UTI. She had a foley placed and was admitted to the hospital. CT scans confirmed severe non-obstructive hydronephrosis and after several days of IVF and antibiotics, she had improvement of her weakness and creatinine. She was discharged with plans for follow-up with urology for foley removal.
AUTHORED BY OLIVIA GOBBLE, MD
Dr. Gobble (@oliviagobble) is a PGY-2 in Emergency Medicine at the University of Cincinnati.
PEER REVIEW BY Patrick Minges, MD
Dr. Minges (@mingespg) is an Ultrasound-trained and Clinical Faculty in Emergency Medicine at the University of Cincinnati.
EDITING AND LAYOUT BY MARTINA DIAZ, MD
Dr. Diaz (@martina_diazb) is a PGY-3 in Emergency Medicine at the University of Cincinnati and the current Resident Editor of Ultrasound of the Month.
REFERENCES
Kirkpatrick JJ, Leslie SW. Horseshoe Kidney. 2022 Nov 28. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–. PMID: 28613757.
Shah HU, Ojili V. Multimodality imaging spectrum of complications of horseshoe kidney. Indian J Radiol Imaging. 2017;27(2):133-140. doi:10.4103/ijri.IJRI_298_16
Niknejad M. Horseshoe Kidney: Radiology Reference Article. Radiopaedia Blog RSS. https://radiopaedia.org/articles/horseshoe-kidney?lang=us. Published October 7, 2022. Accessed December 13, 2022.
Clinical imaging. UT Southwestern Medical Center. https://www.utsouthwestern.edu/education/medical-school/departments/radiology/protocols/. Accessed December 13, 2022.
Sethi SK, Raina R, Koratala A, Rad AH, Vadhera A, Badeli H. Point-of-care ultrasound in pediatric nephrology. Pediatr Nephrol. 2022 Sep 26:1–19. doi: 10.1007/s00467-022-05729-5. Epub ahead of print. PMID: 36161524; PMCID: PMC9510186.
Koratala A. Parapelvic cyst mimicking hydronephrosis. NephroPOCUS. https://nephropocus.com/2019/06/05/parapelvic-cyst-mimicking-hydronephrosis/. Published July 14, 2022. Accessed December 13, 2022.
