RVT is defined as thrombus formation in the main and/or branch renal veins. This may result in full or partial blockage of renal veins and, subsequently, to a series of pathological changes and clinical manifestations –. RVT is the most frequent vascular abnormality in newborns. In most infants, RVT is bilateral and is accompanied by dehydration after diarrhea or vomiting . RVT is rarely observed in healthy adults; in most affected adults, it is unilateral and may be accompanied in 15–20 % of patients by nephrotic syndrome. RVT is associated with abdominal surgery, including laparoscopic cholecystectomy, trauma, tumor invasion of the renal vein or invasion by primary retroperitoneal diseases.
The pathophysiology of venous thrombosis has been reported to involve a combination of three interrelated factors: endothelial damage, stasis, and hypercoagulability. Although a single abnormality may precipitate thrombosis, most venous thromboses are triggered by at least two of these factors. The causes and mechanisms of RVT are no different from venous thromboses elsewhere in the body (Table 1).
Table 1. Causes of renal vein thrombosis
RVT often commences in small intrarenal veins and subsequently extends via larger interlobar veins to the main renal veins and even to the inferior vena cava, where it may cause pulmonary embolism . The clinical presentation of RVT in adults depends on the rate, extent, and completeness of thrombus formation. Patients may be asymptomatic, have minor nonspecific symptoms such as nausea and weakness, or have more major nonspecific symptoms such as upper abdominal pain, flank pain, and hematuria . RVT is likely underreported, as some patients may go undiagnosed due to a lack of clinical manifestations. Establishing the diagnosis is essential because of the possible sequelae, including pulmonary embolism and progressive renal impairment related to vascular compromise, and the risks of potentially harmful treatment (anticoagulation or thrombolysis). In young patients, flank pain and hematuria are usually regarded as symptoms of renal and ureteric calculi; and similar clinical presentation due to other causes is often overlooked in the emergency room. Pulmonary thrombosis may occur in as many as 50 % of patients with RVT, and RVT complicated by pulmonary embolism can have similar symptoms, suggesting a high index of differential diagnosis not to miss the diagnosis of RVT .
In the absence of specific diagnostic laboratory tests and the paucity of clinical manifestations, imaging remains the cornerstone of diagnosing RVT. The gold standard method for diagnosing RVT is selective renal venography, but this is not often performed because of the invasiveness of this procedure, including exposure to high levels of radiation, injection of iodinated contrast, and the potential risk of venous injury causing de novo RVT .
In diagnosing RVT, ultrasound imaging and Doppler ultrasonography are not recommended because their results are inconsistent and operator-dependent. Ultrasound scans may show an enlarged kidney, and a hyper-echogenic kidney is observed in approximately 90 % of patients during the early phase of acute RVT . Color Doppler ultrasound is ineffective in detecting segmental venous thrombosis, but is superior to conventional ultrasound and abdominal plain film in detecting flow in the renal artery and vein. Although color Doppler ultrasound is highly sensitive when performed by an experienced operator, but remains highly operator-dependent . Rarely, the calcified vessel walls of the renal venous branches coursing through the sinus may be mistaken for a renal calculus on ultrasonography. In the patient described here, there was no evidence of turbulent flow within the calcified RVT. Thus, RVT was not considered in the initial evaluation, although subsequent color Doppler yielded results suggestive of RVT.
CT is currently the imaging method of choice for diagnosing RVT, as it is non-invasive, is somewhat less expensive than other methods, can be performed quickly, and has a high diagnostic accuracy. CT scans have shown high sensitivity (92 %) and specificity (100 %) in diagnosing these lesions and is therefore recommended as an initial diagnostic tool . Our findings showed that a renal calcified RVT may mimic a ureter calculus on noncontrast CT scans, with subsequent CT angiography used in the definitive diagnosis of a calcified RVT. CT angiography has shown nearly 100 % sensitivity in diagnosing RVT . The diagnostic accuracy of CT angiography is similar to that of renal venography, with CT angiography having additional benefits, being a rapid, cost-effective, non-invasive method for evaluating the renal vasculature and for detecting renal tumors and other renal pathologies simultaneously. The disadvantages of CT include exposure to radiation and use of nephrotoxic iodinated contrast media, a potential risk factor in patients with impaired renal function .
The treatment modalities for patients with RVT have changed over the past decades, from surgical to predominantly medical management, consisting of initial intravenous and subsequent oral anticoagulation . Asymptomatic patients with unilateral RVT may not require any specific treatment . Rather, active surveillance, along with supportive measures including salt and protein restriction, may partially reverse the hypercoagulability, as in this patient. However, if a patient’s condition deteriorates due to either the progression of thrombosis or embolism, active intervention should be considered.
RVT may be diagnosed incorrectly as renal colic or renal cell carcinoma on abdominal ultrasonography –. Results in our patients showed that a calcified RVT may mimic a urinary calculus on conventional ultrasonography, abdominal plain film and noncontrast CT. Renal stones may also resemble paragonimus calcified ova , renal artery aneurysms  and acute renal infarctions . Thus, awareness of the conditions that could mimic those observed during the generation of a urinary calculus is important, particularly if a percutaneous procedure is considered. Ultrasonography alone is not sufficient to rule out RVT in these patients, suggesting the need for CT angiography in evaluating our patients.