Nephrotic Syndrome

The glomerulus filters plasma through three stacked layers: a fenestrated endothelium, the glomerular basement membrane, and an outer coat of podocytes whose interlocking foot processes are sealed to one another by slit diaphragms. The barrier is selective by size — proteins larger than 69 kD are normally excluded — and by charge, thanks to negatively charged podocalyxin and heparan sulfate proteoglycans that repel negatively charged plasma proteins. Albumin sits right at that size cutoff, which is why it is the first and biggest thing to escape when the barrier fails: albumin accounts for 85% of the protein in nephrotic proteinuria.

Whatever the trigger — immune, genetic, metabolic, or a drug — the common final pathway is podocyte injury, and the microscopic signature is effacement of the podocyte foot processes: the delicate interdigitating feet flatten out and the slit-diaphragm seal is lost. Protein then pours through in gram quantities. The injury is not always recoverable — "destruction of podocytes above a critical mass" leads to irreversible glomerular damage — and in focal segmental glomerulosclerosis the chain runs further: surviving podocytes hypertrophy, effacement and protein loss follow, mesangial and endothelial proliferation set in, capillaries collapse, and the tuft scars segmentally.

The definition is quantitative. Nephrotic-range proteinuria is 3 g or more of protein per 24 hours, or 2 g of protein per gram of urinary creatinine, with adult figures quoted as 3 to 3.5 g per 24 hours. Pair that with hypoalbuminemia — serum albumin below 30 g/L (3.0 g/dL), against a normal range of 3.5 to 4.5 g/dL — and the syndrome is established. One contrast is worth memorizing now: in nephrotic syndrome hematuria and hypertension are less frequent, because they belong to the inflamed nephritic glomerulus. The nephrotic glomerulus leaks protein; the nephritic glomerulus leaks blood.

The classic explanation is the underfill hypothesis, and it is a clean chain: increased glomerular permeability causes albuminuria, albuminuria causes hypoalbuminemia, hypoalbuminemia drops the plasma colloid osmotic pressure, and the loss of that inward pull lets water filter out of the capillaries into the interstitium. The patient swells while the intravascular compartment runs dry.

It is not the whole story, and the correction matters at the bedside. The overfill hypothesis describes an intrinsic defect in the renal tubules that cuts sodium excretion — and crucially, sodium retention begins before the serum albumin starts to fall. In many nephrotic patients "intravascular volume is normal or even raised." Neither mechanism is universal. That is precisely why one nephrotic patient is volume-depleted and another is volume-overloaded, and why diuresis in this disorder has to be watched rather than assumed. Push a loop diuretic hard on an underfilled patient and you do not just fail to help — you precipitate acute kidney injury, which in nephrotic syndrome is far more often caused by hypovolemia or sepsis than by the glomerular lesion itself.

The edema has a characteristic geography. In children, the first sign is usually swelling of the face — the loosest tissue, dependent overnight, so the parents notice puffy eyes on waking and blame allergies. Adults swell dependently instead: legs, ankles, feet. Over time it generalizes into anasarca, with rising weight, ascites, and pleural effusions. Even the gut wall swells, and "edema of the gut could cause defective absorption resulting in malnutrition" — so the syndrome starves the patient by two routes at once.

Albumin is the protein everyone remembers, but the glomerulus is not selective about which proteins it loses, and the three it loses alongside albumin write the complication list.

Clotting inhibitors. The leaky filter spills antithrombin III and free protein S into the urine while the liver simultaneously raises fibrinogen, factor V, factor VII, and alpha-2 macroglobulin. The platelets join in with thrombocytosis, increased adhesiveness, and faster collagen-induced aggregation, and hypoalbuminemia itself enhances thromboxane A2 production. The result is a genuinely prothrombotic state: the 10-year cumulative incidence of venous thromboembolism runs as high as 10%, and renal vein thrombosis has been reported in 25% to 30% of nephrotic syndrome cases in a systematic review of prophylactic anticoagulation in this disorder. Serum albumin is the dial — patients with albumin at or below 2.8 g/dL carry a significantly higher risk, and each 1 g/dL further drop roughly doubles it.

Immunoglobulins and complement. Urinary loss of IgG and complement factors leaves the patient unable to opsonize encapsulated organisms — they cannot coat the bacteria for phagocytosis. That is the mechanism behind bacterial peritonitis, pneumonia, and sepsis, and it is why fever in a nephrotic child is never a minor event.

Lipid handling. This is where the classic teaching needs correcting. The story you may have learned is that low oncotic pressure drives the liver to overproduce lipoproteins. The dedicated mechanism review puts the emphasis the other way: the lipid changes are "primarily a result of their impaired clearance and, to a lesser extent, their altered biosynthesis." Two named defects do the damage. Increased PCSK9 degrades the LDL receptor, so the liver stops clearing LDL — and both cholesterol and PCSK9 return to normal when the disease remits. Upregulated ANGPTL4 inactivates lipoprotein lipase by splitting active dimers into inactive monomers, so triglyceride-rich particles are never unloaded. HDL is damaged too: the maturation of HDL3 into HDL2 is impaired, reverse cholesterol transport fails, and the HDL-to-total-cholesterol ratio falls. The clinical payload is cardiovascular: adults with nephrotic syndrome carry a relative risk of 5.5 for myocardial infarction and 2.8 for coronary death against the general population.

"Nephrotic syndrome" names a pattern, not a disease, and the underlying lesion determines whether steroids will work and whether the kidney survives. In children, minimal change disease dominates — 77 to 85% of childhood cases, and 70 to 90% of children over one year old who present with nephrotic syndrome. It is called minimal change for a reason that is also a diagnostic trap: light microscopy is normal and immunofluorescence is negative, and only electron microscopy reveals the foot-process effacement. Its prognosis is excellent, with most patients remitting on corticosteroids and morbidity coming more from the medications than from the disease.

In adults the picture fragments. Diabetic nephropathy is the single most common cause overall. Among the primary glomerular diseases, focal segmental glomerulosclerosis predominates in Black adults and membranous nephropathy in White adults — and the race split is not incidental: roughly 75% of Black patients with FSGS carry a high-risk APOL1 variant, with an annual incidence of 24 per million in Black individuals against 5 per million in White individuals. Minimal change disease has not vanished in adults either — it represents 10 to 15% of adult presentations. What does rise with age is nephrotic syndrome overall, not minimal change disease specifically.

The prognostic spread is wide enough to change the conversation with the patient. Minimal change disease remits in most patients and rarely reaches kidney failure. Membranous nephropathy sits in the middle — around 30% remit spontaneously, but with persistent nephrotic syndrome 40 to 50% reach end-stage renal disease over ten years. FSGS is the worst: about 50% of patients with persistent nephrotic-range proteinuria progress to end-stage renal disease within 3 to 8 years, and steroid remission rates run only around 30%, against 85 to 90% steroid responsiveness in childhood minimal change disease. Notably, an FSGS patient without nephrotic-range proteinuria has a 10-year survival exceeding 90% even without immunosuppression — the proteinuria, not the biopsy label alone, carries the prognosis.