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.
PATHOPHYSIOLOGY
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.
TREATMENT
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.
