RENAL VEIN THROMBOSIS
Renal vein thrombosis (RVT) is a frequent complication among patients with nephrotic syndrome (see Plate 4-5). The overall prevalence in this population ranges from 5% to 60% in different series, and known risk factors include membranous nephropathy, heavy proteinuria (>10 g/day), and low serum albumin (<2 g/dL). Although it was once believed that RVTs caused nephrotic syndrome, the direction of causality is now known to be reversed.
RVTs may also occur in the setting of renal malignancy, renal transplant, steroid use, oral contraceptive use, renal trauma (including biopsies and other procedures), and hypovolemia.
The pathophysiology of the hypercoagulable state in nephrotic syndrome is complex and controversial. Possible causes include the urinary loss of small molecular weight anticoagulants (antithrombin III, protein S) and ﬁbrinolytics (plasminogen). In addition, however, the hepatic overproduction of proteins, which occurs in response to hypoalbuminemia, also leads to elevated concentrations of higher molecular weight procoagulants, such as factor V, factor VIII, and ﬁbrinogen.
Platelet abnormalities may also play a role. First, there may be increased platelet activation because of associated systemic inﬂammation. Second, hypoalbuminemia promotes increased levels of free (i.e., unbound) arachidonic acid, which may facilitate synthesis of proaggregants, such as thromboxane A2.
Although these changes increase the risk of thrombotic events throughout the vasculature, the renal vein is especially susceptible. One of the main reasons is thought to be ﬂuid loss at the glomerulus, which concentrates postglomerular plasma and thereby increases the risk of thrombus formation.
PRESENTATION AND DIAGNOSIS
RVT may be classiﬁed as acute or chronic, depending on the size of the clot and degree of obstruction. A chronic RVT is typically a small clot causing incomplete obstruction. It is often asymptomatic, although in some cases the progressive development of collateral circulation may result in a noticeable varicocele or dilated epigastric vein. In most cases, however, a chronic RVT is not discovered unless the patient has a more acute (i.e., pulmonary) thrombus.
An acute RVT, in contrast, is a large clot causing either complete or near-complete obstruction of renal venous outﬂow. It presents with symptoms of renal infarction, including nausea/vomiting, ﬂank pain, and gross or microscopic hematuria. In very rare cases, the tense, enlarged kidney may be palpable. If there are bilateral thrombi, acute kidney injury may occur, with a rapid increase in serum creatinine concentration. Acute RVT is an uncommon presentation among patients with nephrotic syndrome and is more characteristic of cases related to hypovolemia or renal trauma. In a patient whose body habitus permits accurate depiction of the renal veins, ultrasound may be used for initial evaluation. A clot may be seen in the vessel lumen, and color Doppler examination may reveal either increased turbulence and ﬂow velocities in partial obstruction, or a lack of ﬂow altogether in complete obstruction. Especially in acute cases, the kidney may appear enlarged and less echogenic than usual because of diffuse edema. The diagnostic utility of ultrasound, however, also depends on the angle of the vein and operator experience.
Spiral computed tomography (CT) with contrast enhancement and magnetic resonance venography (MRV) are typically performed for further evaluation and are highly sensitive modalities. These tests have largely replaced invasive renal venography, which was previously the gold standard. They reveal the thrombus to be a relatively lucent intraluminal defect surrounded by or occluding the ﬂow of contrast-enhanced blood. The vein may be enlarged and, if the thrombus is chronic, renocaval varices may appear. If there is acute ischemia, the kidney appears swollen, with diminished and inhomogeneous enhancement. A tumor, if present, may have visible internal vessels.
Patients diagnosed with either acute or chronic renal vein thrombosis require anticoagulation treatment. Unfractionated or low molecular weight heparin is appropriate for initial treatment. Although most patients have adequate antithrombin III levels for heparin treatment, the rare patient with extremely low levels may require fresh frozen plasma. Patients should undergo subsequent transition to warfarin, with a target international normalized ratio (INR) of 2.0 to 3.0, and continue on this therapy for as long as the nephrotic syndrome remains. If there are contraindications to anticoagulation, an inferior vena cava ﬁlter may be placed. In addition, in the rare case of acute RVT, percutaneous thrombectomy or thrombolysis may be considered. Open surgery should be reserved for those patients with renal failure and bilateral acute thrombos s who cannot be treated with percutaneous techniques.