Renal replacement lipomatosis of a single moiety in a horseshoe kidney without calculi or infection: A rare CT diagnosis

Abstract

Renal sinus lipomatosis is a benign proliferation of fatty tissue within the renal sinus, typically preserving the renal parenchyma, and is usually detected incidentally. Renal replacement lipomatosis (RRL) represents the advanced end of the spectrum of renal sinus lipomatosis. It is characterized by extensive fatty proliferation associated with marked renal parenchymal atrophy. Horseshoe kidney is the most common renal fusion anomaly; however, its association with unilateral RRL is rarely reported. We report a case of a 28-year-old male who presented with right flank pain and was found to have a horseshoe kidney with near-complete RRL of the right moiety on CT urography. He had no history of renal surgery, calculus disease, or infection. This case highlights the importance of recognizing RRL in a horseshoe kidney to avoid misdiagnosis and underscores the diagnostic value of CT urography in establishing the diagnosis.

Introduction

Horseshoe kidney is a congenital renal fusion anomaly occurring in approximately 0.25% of the population [1]. It is characterized by fusion of the lower poles of the kidneys across the midline with either renal parenchyma or a fibrous isthmus. It is frequently associated with complications such as ureteropelvic junction obstruction, urolithiasis, and infection. Although RRL in a horseshoe kidney has been reported in association with xanthogranulomatous pyelonephritis [2], isolated unilateral renal replacement lipomatosis (RRL) in the absence of infection, calculus disease, or other predisposing factors appears to be exceptionally rare.

Renal sinus lipomatosis is a benign fatty proliferation within the renal sinus. It is often an incidental finding with preservation of the renal parenchyma [3]. RRL is characterized by marked renal parenchymal atrophy and replacement with mature adipose tissue, most commonly associated with chronic inflammatory or obstructive processes [[3], [4], [5]]. Most reported cases are associated with calculi [2], and its clinical significance lies in its potential to mimic lipomatous tumors of the kidney or perirenal region. We present a rare case of RRL of the right renal moiety in a horseshoe kidney, diagnosed using CT urography.

Case presentation

A 28-year-old male presented with gradually progressive right flank pain for 1 month. He had no urinary frequency, fever, dysuria, gross hematuria, or weight loss. He had no history of renal surgery, urinary tract infection, nephrolithiasis, or trauma. Physical examination was unremarkable; urinalysis was normal, and no microscopic hematuria was detected. Serum creatinine was normal (0.9 mg/dL). Urine culture and additional functional renal studies were not performed. There was no previous clinical data or imaging study available to exclude previous infection or stone formation. However, the absence of clinical history, symptoms, and negative laboratory findings at presentation makes active or recent infection less likely.

CT urography was performed for further evaluation. The CT protocol included a non-contrast phase with 3 mm slice thickness, followed by intravenous administration of 1.5 mL/kg of iohexol (Omnipaque). Multiphasic imaging was performed during the arterial phase (30 seconds), nephrographic phase (100 seconds), and delayed excretory phase (10 minutes). Corticomedullary phase of axial CT scan image demonstrated a horseshoe kidney with fused lower poles and enhancing parenchyma across the midline (Fig. 1). The right renal moiety showed marked parenchymal atrophy with near-complete replacement by fatty tissue involving the renal sinus (attenuation values ranging from −100 to −105 HU) (Fig. 2). Arterial phase images demonstrated a thin rim of residual enhancing parenchyma in the right moiety. Both renal arteries were visualized, with relative atrophy of the right renal artery (Figs. 3A and B). No renal calculi or enhancing solid renal masses were identified on imaging. No hydronephrosis, ureteropelvic junction obstruction, or early venous opacification was identified. Excretory phase images demonstrated absent excretory function of the right ureter (Figs. 4A and B). The left renal moiety showed preserved parenchymal thickness, normal contrast enhancement, and normal excretory function. No abnormal lymphadenopathy or retroperitoneal mass was identified. The patient was under conservative management.

Fig. 1.

Axial corticomedullary phase CT image showing a horseshoe kidney with fusion of the lower poles forming an enhancing midline isthmus (arrow).

Fig. 2.

Axial pre-contrast CT image demonstrating near-complete fatty replacement of the right renal moiety (arrowhead) with marked parenchymal atrophy (fat attenuation −100 to −105 HU) in a horseshoe kidney.

Fig. 3.

Axial nephrographic phase contrast-enhanced CT images. (A) Predominant fat-density replacement of the right renal moiety with a thin rim of residual enhancing renal parenchyma (arrow). (B) Axial image at the level of the renal arteries demonstrating relative atrophy of the right renal artery (arrowhead) compared with the left (arrow).

Fig. 4.

Axial contrast-enhanced CT images demonstrating differential excretory function. (A) Nephrographic phase image showing early contrast excretion in the left distal ureter (arrow), while the right ureter remains unopacified (arrowhead). (B) Delayed excretory phase image demonstrating opacification of the left ureter and urinary bladder, with persistent non-opacification of the right distal ureter (arrowhead), consistent with absent excretory function of the right renal moiety.

Based on these characteristic imaging findings, a diagnosis of RRL of the right renal moiety in a horseshoe kidney was established.

Discussion

RRL represents the advanced end of the spectrum of renal sinus lipomatosis and is characterized by diffuse fatty infiltration associated with severe parenchymal atrophy [2,5,6]. It is typically associated with long-standing obstruction, infection, or calculus disease [2]. In our case, imaging demonstrated near-complete fatty replacement of the right renal moiety with marked parenchymal atrophy (Fig. 2) and absence of excretory function (Figs. 4A and B), without evidence of calculi, infection, or prior surgical intervention.

Reports of RRL have been largely described in association with xanthogranulomatous pyelonephritis or nephrolithiasis [2,4,6]. To our knowledge, there is only 1 reported case of RRL in a horseshoe kidney, and that case was associated with xanthogranulomatous pyelonephritis [2]. We could not find any reported case of RRL in a horseshoe kidney occurring without identifiable predisposing factors emphasizing the novelty of the present case.

Congenital anatomical abnormalities of the kidney, such as those seen in a horseshoe kidney, have been hypothesized to predispose to altered urinary drainage. These anatomical abnormalities may contribute to chronic subclinical ischemic or inflammatory changes within the renal parenchyma. However, in the absence of documented obstruction, infection, or calculi, this pathophysiologic link remains speculative. Similarly, the mechanism underlying RRL in our patient is not well understood, and current evidence is limited to observational reports [1,2]. The preserved morphology and function of the contralateral renal moiety in this case further support a localized rather than systemic process (Figs. 1 and 4).

The imaging differential diagnoses include xanthogranulomatous pyelonephritis, angiomyolipoma, and retroperitoneal liposarcoma. Xanthogranulomatous pyelonephritis typically presents with renal enlargement, staghorn calculi, infection, and inflammatory changes in the perirenal fat. It is often accompanied by systemic symptoms such as fever and malaise [7,8]. It is frequently called the “the great imitator” as its clinical and radiologic findings can mimic other renal conditions, including tumors. Radiologically, it is characterized by the “bear paw” sign, a distinctive imaging finding, which represents dilated renal calyces and pelvis filled with pus. Unlike RRL, it usually demonstrates residual parenchyma surrounding the dilated calyces and is commonly associated with chronic infection and calculi. These clinical and imaging features were absent in our patient, and no obstructing calculus or inflammatory mass was identified on CT (Figs. 2 and 3).

Angiomyolipomas are uncommon benign renal tumors, usually discovered incidentally on diagnostic imaging. They occur more commonly in females than males. Approximately 80% are sporadic and typically occur in the fifth decade, whereas 20% are associated with tuberous sclerosis and occur in a younger age group, usually in the third decade [9]. They are members of the perivascular epithelioid cells tumor group (PEComas), composed of variable amounts of blood vessels, smooth muscles and adipose tissues. Classic angiomyolipoma is characterized by a well-defined focal renal mass containing macroscopic fat with enhancing soft-tissue components, and typically preserved renal architecture. Larger intralesional vessels are frequently identified. Larger lesions may also demonstrate hemorrhage [9]. In contrast, RRL manifests as diffuse fatty replacement with severe parenchymal atrophy and loss of function, rather than a focal mass lesion, as demonstrated in this case (Fig. 2, Fig. 3, Fig. 4).

Retroperitoneal liposarcoma is a mesenchymal tumor which typically occurs in the 5th to 70th decades of life with slight male predilection. Its imaging features demonstrate varying amounts of fat and soft tissue. Well-differentiated liposarcomas are predominantly fatty but may contain thick septa or enhancing nodular soft-tissue elements. Owing to the growth or compression of the tumor, it often results in the displacement of the kidney, bowels, pancreas and retroperitoneal vessels [10,11]. Contrast-enhanced imaging typically demonstrates predominantly fatty mass with thick septal and nodular enhancement [11].

CT urography allows comprehensive evaluation of renal morphology, vascularity, and excretory function, and is crucial for differentiating these entities and establishing a confident diagnosis [9,12] (Fig. 1, Fig. 2, Fig. 3, Fig. 4).

Conclusion

Unilateral RRL affecting a single moiety of a horseshoe kidney is exceptionally rare, particularly in the absence of renal calculi or infection. Awareness of its characteristic imaging features is essential to avoid misdiagnosis and unnecessary interventions. CT urography serves as a key diagnostic providing comprehensive anatomical and functional assessment, and should be considered the imaging modality of choice in such rare cases.

Ethics approval

Ethical approval was not required for this study as it involves a single anonymized case report.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Patient consent

Written informed consent was obtained from the patient for publication of this case report and the accompanying images.