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Nephrotic syndrome results from severe glomerular damage causing proteinuria (>3.5 g/day), hypoalbuminemia, edema, and hyperlipidemia. The loss of plasma proteins compromises oncotic pressure, triggers hepatic compensatory synthesis, and increases thrombotic risk.
Study the four cardinal features and their mechanisms. Understand selective (albumin only) versus non-selective (all sizes) proteinuria as prognostic markers. Review common causes: minimal change disease, membranoproliferative GN, diabetic glomerulosclerosis.
Nephrotic range proteinuria is not synonymous with nephrotic syndrome—hypoalbuminemia and edema must be present. Lipiduria is pathognomonic, not a cause of nephrotic syndrome.
Your study of the glomerular filtration barrier established that the glomerulus is not a passive sieve — it selectively restricts passage of large and negatively charged molecules, keeping albumin and other plasma proteins in the circulation through both size exclusion and charge repulsion. Nephrotic syndrome is what happens when that selective barrier fails catastrophically: proteins flood into the filtrate, and the downstream consequences cascade through every organ system.
The defining feature is massive proteinuria, conventionally greater than 3.5 grams per day in adults (normal is less than 150 mg/day). The glomerular filtration barrier normally prevents albumin loss through two mechanisms: GBM pore size and charge — both albumin and the GBM are negatively charged, so albumin is repelled. In nephrotic syndrome, podocyte damage is central. The foot process architecture collapses ("foot process effacement"), the filtration slits widen, and the charge barrier is disrupted. In minimal change disease, the most common cause in children, the damage is subtle by light microscopy but selectively destroys the charge barrier, so primarily albumin leaks through. In more severe glomerulopathies, both the size and charge barriers fail, allowing larger proteins — immunoglobulins, clotting factors — to escape as well.
Hypoalbuminemia follows because even maximal hepatic synthesis cannot replace proteins at the rate the damaged kidney discards them. Albumin is the principal determinant of plasma oncotic pressure — the osmotic force holding fluid in the capillary compartment. When albumin falls, oncotic pressure drops, and fluid shifts from intravascular to interstitial space. This produces the pitting edema characteristic of nephrotic syndrome, beginning in dependent locations — ankles in ambulatory adults, periorbital tissue in children who spend time lying flat. The falling intravascular volume simultaneously activates the renin-angiotensin-aldosterone system and ADH release, causing sodium and water retention that worsens the edema even further.
The liver's compensatory response to hypoalbuminemia is non-selective: it ramps up synthesis of all proteins, including lipoproteins. Since the kidney also loses the lipases needed for lipoprotein clearance, LDL and VLDL accumulate in plasma — the hyperlipidemia of nephrotic syndrome. Lipoproteins appear in the urine as lipiduria, visible on microscopy as oval fat bodies and "Maltese cross" birefringent droplets under polarized light, which is pathognomonic for the syndrome.
The thrombotic risk is among the most dangerous systemic complications. The kidney loses not only albumin but also small anticoagulant proteins — antithrombin III, protein C, and protein S — all small enough to pass through the damaged barrier. Simultaneously, the liver's compensatory synthesis overproduces pro-coagulant proteins: fibrinogen and factors V and VIII. The result is a hypercoagulable state predisposing to DVT, pulmonary embolism, and most dramatically, renal vein thrombosis — which further worsens nephropathy. This explains why nephrotic patients require careful thrombosis risk assessment and why the physical exam should always assess for asymmetric leg swelling even in a patient who appears edematous systemically.