This baby who is just a few hours old has presented with a very concerning finding: red-colored urine. Dipstick testing has shown this to be most likely gross hematuria. His history is unremarkable, but examination reveals a ballotable mass in the right flank: quite possibly the enlarged right kidney. Key differential diagnoses to consider include renal vein thrombosis (RVT), polycystic kidney disease, and other structural malformations of the kidney; a neonatal renal tumor is a rare possibility. Abdominal ultrasound is a good next step; this confirms enlargement of the right kidney. The loss of corticomedullary demarcation and echogenic streaks in the lower pole provide a further clue: these are features typically associated with RVT. Color Doppler imaging confirms this, while also showing that the thrombus is yet to extend to the inferior vena cava (IVC). Several supportive investigations are necessary. In particular, his renal functions should be obtained as a baseline; these are currently normal, as would be expected with unilateral RVT. Furthermore, as both sepsis and congenital cardiac disease are risk factors for RVT, he should be screened in this respect. There is little additional benefit in contrast CT imaging currently; this is unlikely to reveal any new information, while also exposing him to unnecessary radiation. Excellent supportive management is a must; this includes careful hydration and close monitoring of blood pressure. Note that in unilateral RVT without IVC involvement, anticoagulation has not been shown to be of benefit. However, he should be regularly monitored for thrombus extension, and low molecular weight heparin (LMWH) therapy initiated if the IVC becomes involved. Systemic thrombolysis is mainly indicated in bilateral RVT; surgical intervention is generally not indicated in the acute phase.
Renal vein thrombosis (RVT) accounts for around 10% of episodes of venous thromboembolism (VTE) in neonates. While rare, it can cause significant morbidity and even mortality. To appreciate the underlying pathophysiology, one must appreciate that the neonatal hemostatic system is both quantitatively and qualitatively different from that of older children and adults. In particular, levels of both procoagulant and anticoagulant factors are lower; while healthy newborns exist in hemostatic balance, this is delicate and easily tipped in either direction. Both maternal and neonatal risk factors for VTE exist. The former includes infections, premature rupture of the membranes, preeclampsia, emergency Cesarean section, inherited thrombophilias, antiphospholipid antibody syndrome, diabetes, hypertension, dyslipidemia, and metabolic syndrome. Neonatal risk factors for VTE include central venous catheter placement, congenital heart disease, perinatal asphyxia, sepsis, and dehydration. The risk associated with inherited thrombophilias is unclear, with current evidence suggesting that other clinical factors are more important. In RVT, thrombus formation typically begins in small caliber vessels (e.g., arcuate or interlobular veins), with subsequent extension to the renal vein, and then the inferior vena cava (IVC). Around 25% of cases are bilateral. The presentation depends on the site and extent of thrombus. The classic triad is a palpable abdominal mass, hematuria, and renal failure, but is seen in only 13% of patients. Thrombosis of the IVC may give rise to edema, hypothermia and lower limb cyanosis. Note that RVT can also be insidious and asymptomatic. Color Doppler ultrasound is the most commonly used diagnostic test; typical findings include enlargement of the affected kidney, a loss of corticomedullary differentiation, hyperechoic streaks, reversed end-diastolic renal arterial flow, and reduced or absent renal venous flow. Doppler ultrasound can also detect thrombus extension to the inferior vena cava. Contrast computed tomography (CT) and magnetic resonance imaging (MRI) may be considered where ultrasound is equivocal or negative, but there is strong clinical suspicion of the disease. Venography is considered the reference standard for the diagnosis of VTE, but is invasive, technically demanding, and associated with radiation exposure. Basic laboratory studies are not diagnostic. Urinalysis can buttress the clinical suspicion by demonstrating hematuria and possibly, proteinuria. Complete blood counts may reveal thrombocytopenia. Renal functions should be assessed as a baseline. Depending on the clinical findings, screening for sepsis and congenital cardiac disease may need to be considered. The role of inherited thrombophilia screening is still under debate, as this does not appear to influence the nature or duration of treatment. Close supportive management is essential, including careful attention to hydration and nutrition, and monitoring of renal functions, electrolytes, and blood pressure. The role of anticoagulation is controversial due to a lack of randomized controlled trials. In unilateral RVT limited to the renal vein, anticoagulation appears to confer no benefit compared to supportive management alone, while also exposing the neonate to the risk of bleeding complications. However, if there is extension to the IVC, anticoagulation for 6 weeks to 3 months is recommended. Furthermore, if bilateral disease is present, systemic thrombolysis should be initiated, followed by anticoagulation for the same duration as above. Long-term follow-up is essential; potential complications include hypertension, renal vein calcification, shrunken hypoplastic kidneys, and tubular impairment leading to progressive renal failure.