Unilateral Renal Enlargement

Case Presentation

A 73-year-old woman presented with a 3-day history of nausea, vomiting, and abdominal pain, noting gross blood in her urine since morning. In the emergency department, she was tachycardic, with all other vital signs normal. Routine laboratory work revealed an elevated BUN and creatinine, and a urinalysis showed gross hematuria. Her medical and surgical history were noncontributory. Plain films of the abdomen and chest did not demonstrate a significant abnormality. A computed tomography (CT) image of the abdomen and pelvis without contrast was performed to exclude nephrolithiasis, with selected images shown (Figure 1 A-D).

Key Imaging Finding

Unilateral renal enlargement

Differential Diagnosis

Renal arterial infarction

Renal vein thrombosis

Anatomic compression of the renal vein

Congenital renal abnormalities

Compensatory hypertrophy

Acute bacterial nephritis

Obstructive uropathy

Discussion

Unilateral smooth renal enlargement is not uncommon in adults. The differential diagnosis encompasses a wide range of pathologies, which are generally divided into prerenal, intrarenal, and postrenal etiologies. Prerenal causes include arterial infarction, renal vein thrombosis, and extrinsic compressive phenomenon. Intrarenal causes include compensatory hypertrophy and acute bacterial nephritis, as well as congenital entities such as a duplicated pelvicalyceal system and crossed fused ectopia. Obstructive uropathy is the most common postrenal cause.

Prerenal

Renal arterial infarction. Acute renal arterial infarction is uncommon. Patients typically present with acute abdominal and flank pain, fever, hematuria, proteinuria, and potentially elevated LDH and leukocytosis.^1,2 Typically, focal wedge-shaped hypoattenuating areas of infarction will be seen on contrast-enhanced CT. In severe cases, a diffusely hypoattenuated appearance is noted, with renal enlargement secondary to edema.¹ Potential causes include aortic and renal vascular pathology, including aneurysm, dissection, severe atherosclerosis, vasculitis, and fibromuscular dysplasia.² Thromboembolism secondary to atrial fibrillation, endocarditis, hypercoagulability, trauma, and iatrogenic causes are additional possible etiologies.²

Renal vein thrombosis. The causes of acute renal vein thrombosis are broad, but are most commonly related to an underlying hypercoagulable state, such as systemic lupus erythematosus (SLE), nephrotic syndrome, antithrombin III, protein S or protein C deficiency, and dehydration.¹ Trauma and neoplastic infiltration may also result in renal vein thrombosis. Unilateral enlargement secondary to edema and a striated nephrogram with delayed contrast excretion on the affected side will typically be seen on contrast-enhanced CT.² The visualization of a structural renal vein obstruction or the underlying pathology, such as tumor infiltration, also can be demonstrated with contrast-enhanced CT or MRI.¹ Bland, nontumor thrombus does not demonstrate enhancement or vascular flow, whereas tumor thrombus often will demonstrate enhancement and/or vascular flow.

Anatomic renal vein compression. The “nutcracker phenomenon” refers to extrinsic anatomic compression of the left renal vein between the superior mesenteric artery (SMA) and aorta, secondary to a steep angle of the SMA relative to the aorta and a reduced distance between the two.³ Presenting symptoms include flank pain, intermittent hematuria, and proteinuria. There is some controversy regarding CT and MR diagnosis of renal vein compression, as different degrees of aortomesenteric angle shortening and reduced aortomesenteric distance yield differing sensitivities and specificities.⁴ A qualitative reduction will often be seen in symptomatic cases. Secondary imaging findings include varices of the renal pelvis and ureter, dilation of the gonadal vein, and pelvic venous congestion.³ Duplex ultrasound can further support diagnosis, as the peak systolic velocity at the point of renal vein compression will often be elevated when compared to the peak systolic velocity of the hilar renal vein.^3,4 Other causes of extrinsic renal vein compression include but are not limited to abdominal aortic aneurysm, adrenal mass, and lymphadenopathy.

Intrarenal

Compensatory hypertrophy. Compensatory hypertrophy is a fairly common cause of unilateral renal enlargement. It may result from congenital renal anomalies, such as unilateral renal agenesis, or acquired causes, such as unilateral renal atrophy or prior nephrectomy.⁵ Unilateral renal agenesis is usually asymptomatic at birth.⁶ Physiologically, compensatory hypertrophy is a response to loss of renal mass and has been shown in animal studies to result from hypertrophic (not hyperplastic) growth of the proximal tubule, resulting in increased renal cortical size.⁵

Acute bacterial nephritis. Pyelonephritis is typically the sequela of an ascending urinary tract infection, although it can also result from hematogenous seeding. Imaging is not warranted for uncomplicated cases, but if used, contrast-enhanced CT is the preferred, as it can provide both anatomic and physiologic information. Global renal enlargement, perinephric fat stranding, and thickening of Gerota’s fascia may be seen with or without the use of IV contrast, whereas the wedge-shaped areas of hypoattenuation, commonly referred to as a “striated nephrogram,” can be seen only after contrast administration. These areas represent regions of decreased blood flow from interstitial edema, vasospasm, and tubular obstruction due to inflammatory debris. On excretory phase imaging, the affected kidney will demonstrate persistent enhancement from slow flow through the affected tubules and prolonged accumulation of contrast.⁷

Congenital renal anomalies. A unilaterally enlarged kidney may occasionally result from a wide range of underlying congenital anomalies. A duplicated pelvicalyceal system is the most common congenital renal anomaly. It is secondary to incomplete fusion of the upper and lower pole moieties, and is sometimes complicated by reflux, obstruction, and ureterocele formation. In the vast majority of cases, the upper pole obstructs and the lower pole refluxes. An additional but less common congenital cause of unilateral renal enlargement is crossed-fused renal ectopia, where abnormal embryological assent of the kidneys results in fusion and abnormal location on the same side of the abdomen.⁶ Typically, the distal ureters maintain normal anatomic insertion into the bladder on either side.

Postrenal

Obstructive uropathy. Obstructive uropathy is a well-known cause of unilateral enlarged kidney. With complete obstruction there is a backflow of urine into the proximal collecting system causing increased pressure, dilatation and, ultimately, edematous renal parenchyma. Obstruction manifests on dynamic contrast-enhanced CT with a delayed and prolonged nephrographic phase. Excretory phase imaging demonstrates delayed contrast excretion into a dilated renal collecting system. The causes of ureteral obstruction are numerous and can be further subcategorized into intrinsic and extrinsic etiologies. Intrinsic causes result from obstruction within the lumen of the ureter, most commonly related to urolithiasis. Less common etiologies of intrinsic obstruction include sloughed papilla, blood clots, and urothelial neoplasms. Extrinsic obstruction results from extraluminal compression of the ureter, often seen with pelvic malignancies and retroperitoneal fibrosis.

Diagnosis

Anatomic renal vein compression (“nutcracker syndrome”)

Summary

Unilateral smooth renal enlargement is relatively common in abdominal imaging, and it is important to be aware of the multiple causes to form a useful working differential diagnosis. These etiologies are categorized into prerenal, intrarenal, and postrenal, with ureteral obstruction from urolithiasis as the most common underlying cause. CT is one of the imaging mainstays (along with ultrasound), with MRI reserved for problem-solving in more difficult cases. Establishing the correct diagnosis has important implications for appropriate clinical management and patient outcome.

References

1. Kawashima A, Sandler CM, Ernst RD, Tamm EP, Goldman SM, Fishman EK. CT evaluation of renovascular disease. Radiographics. 2000;20(5):1321-1340.
2. Antopolsky M, Simanovsky N, Stalnikowicz R, Salameh S, Hiller N. Renal infarction in the ED: 10-year experience and review of the literature. Am J Emerg Med 2012;30(7):1055-1060.
3. Lamba R, Tanner DT, Sekhon S, McGahan JP, Corwin MT, Lall CG. Multidetector CT of vascular compression syndromes in the abdomen and pelvis. Radiographics. 2014;34(1):93-115.
4. Kurklinsky AK, Rooke TW. Nutcracker phenomenon and nutcracker syndrome. Mayo Clin Proc 2010;85(6):552–559.
5. Liu B, Preisig P. Compensatory renal hypertrophy is mediated by a cell cycle-dependent mechanism. Kidney Int 2002;62(5):1650-1658.
6. Rosenblum ND. Malformation of the kidney. In: Taal M et al, eds. Brenner and Rector’s the Kidney, Ninth Edition. Philadelphia, PA: Elseiver-Saunders; 2012:2558-2571.
7. William C, Wagner B, Travis M. Pyelonephritis: radiologic-pathologic review. RadioGraphics 2008;28:255-276.