Nephrotic Syndrome

• Disruption of podocyte architecture and slit diaphragm integrity: The glomerular filtration barrier normally restricts passage of proteins >60 kDa through a size-selective and charge-selective mechanism. In nephrotic syndrome, primary lesions involve podocyte foot process effacement (as in minimal change disease), either through loss of nephrin and podocin expression or through more severe structural disruption of the slit diaphragm complex. This effacement is mediated by cytoskeletal reorganization involving actin polymerization and can be triggered by circulating permeability factors (particularly in minimal change and focal segmental glomerulosclerosis [FSGS]). The charge barrier, normally maintained by negatively charged glycosaminoglycans in the basement membrane, is often compromised simultaneously, explaining the non-selective proteinuria characteristic of many nephrotic conditions. This loss of barrier function permits unrestricted passage of plasma proteins including albumin, immunoglobulins, and other macromolecules into the urinary space at rates exceeding 3.5 g/day.

• Albuminuria-driven metabolic cascades and systemic consequences: Massive urinary albumin losses lead to hypoalbuminemia (serum albumin typically <2.5 g/dL), reducing plasma colloid oncotic pressure and causing fluid transudation into interstitial spaces, manifesting as edema. Simultaneously, the liver compensates by increasing synthesis of multiple plasma proteins, including fibrinogen, complement components, and crucially, apolipoprotein B-containing lipoproteins. This hepatic upregulation is triggered by reduced plasma oncotic pressure and direct signaling through nuclear transcription factors; the result is hypertriglyceridemia and hypercholesterolemia, with particularly marked elevation in LDL cholesterol and lipoprotein(a). The loss of protective plasma proteins (including albumin-bound fatty acids and antithrombotic proteins) combines with increased hepatic synthesis to create a severe dyslipidemic state that accelerates atherosclerosis and increases cardiovascular morbidity. Additionally, loss of immunoglobulins and complement components (particularly loss of IgG and factor H-related proteins) increases susceptibility to bacterial infections, especially with encapsulated organisms.

• Activation of coagulation cascade and thrombotic microangiopathy: Urinary losses include natural anticoagulants (antithrombin III, protein C, and protein S), while simultaneously increased hepatic synthesis and reduced urinary clearance of prothrombotic factors (fibrinogen, factors V, VII, VIII, and X) create a profound prothrombotic state. This is further exacerbated by hemoconcentration from intravascular volume depletion (paradoxically, despite clinical edema, many patients have underfilled intravascular compartments). Elevated serum viscosity from hyperlipidemia and hyperfibrinogenemia additionally increases thrombotic risk. These factors combine to cause a 5-10 fold increase in venous thromboembolism, with renal vein thrombosis occurring in 10-40% of patients with membranous nephropathy and FSGS, presenting acutely with flank pain and hematuria or insidiously with progressive renal dysfunction. Thrombotic events may precipitate acute kidney injury or chronic progression.

• Progressive proteinuria-induced tubulointerstitial injury: Beyond the initial glomerular lesion, the massive protein filtration itself becomes pathogenic through tubulointerstitial mechanisms. Filtered proteins, particularly albumin and immunoglobulins, are reabsorbed by proximal tubular cells through pinocytosis and receptor-mediated endocytosis. This protein overload triggers intracellular complement activation (particularly C3 and C5a generation), reactive oxygen species production, inflammatory cytokine release (TNF-α, IL-6, MCP-1), and ultimately tubular epithelial cell apoptosis and dedifferentiation. The result is progressive tubulointerstitial fibrosis and tubular atrophy independent of ongoing glomerular disease. Additionally, proteinuria itself is a known predictor of glomerulosclerosis progression through these maladaptive pathways.

• Immune-mediated glomerular injury mechanisms: While the proteinuria and barrier disruption are the final common pathway in all nephrotic conditions, the primary immune mechanisms differ by etiology. In immune complex-mediated disease (such as lupus nephritis and post-infectious glomerulonephritis), circulating or in situ immune complex deposition triggers complement activation (classical pathway) and recruitment of inflammatory cells. In antibody-mediated disease, such as anti-glomerular basement membrane disease or certain presentations of membranous nephropathy (particularly PLA2R-positive disease), IgG autoantibodies directly target glomerular antigens, activating complement and cellular immunity. In FSGS and minimal change disease, non-immune mechanisms predominate, often involving circulating permeability factors (such as circulating vascular permeability factor, a cleaved form of VEGF, or other unidentified factors) that directly alter podocyte function without immune complex deposition.

Primary (Idiopathic) Nephrotic Syndrome — accounts for 80-90% of nephrotic cases in developed countries

• Minimal change disease (MCD): Represents 85-90% of nephrotic syndrome in children and 10-15% in adults. Characterized by normal light microscopy, electron microscopy showing diffuse podocyte foot process effacement, and clinical features of highly selective proteinuria (predominantly albumin). Associated with Hodgkin lymphoma and NSAIDs. Excellent response to corticosteroid therapy (90-95% remission in children, 50-60% in adults). Pathophysiology involves circulating permeability factors that reduce nephrin and podocin expression.

• Focal segmental glomerulosclerosis (FSGS): Accounts for 20-30% of primary nephrotic syndrome in adults and is rising in prevalence. Characterized by sclerosis affecting some (focal) glomeruli and only portions (segmental) of affected glomeruli. Five morphologic variants exist (perihilar, cellular, tip lesion, collapsing, and not-otherwise-specified). Strong association with circulating permeability factors (apolipoprotein L1 [APOL1] mutations confer 3-7 fold increased risk in African Americans); also associated with HIV infection, heroin use, obesity, and reflux nephropathy. More resistant to corticosteroid therapy; 30-40% of patients progress to ESRD within 10 years. Recurrence in renal allografts occurs in up to 30% of cases.

• Membranous nephropathy (MN): Most common primary cause of nephrotic syndrome in adults (30-40% of cases), with peak incidence in 5th-6th decades. Characterized by thickened basement membrane with subepithelial "spike and dome" deposits on electron microscopy. Positive anti-PLA2R antibodies in 70-75% of patients with primary MN, and anti-THSD7A in 3-5%. Clinical presentation ranges from asymptomatic proteinuria to overt nephrotic syndrome; spontaneous remission occurs in 25-30% of cases. Significant risk of renal vein thrombosis (10-40% with imaging surveillance).

• Membranoproliferative glomerulonephritis (MPGN): Represents 10-15% of primary nephrotic cases. Subdivided into immune-complex MPGN (associated with hepatitis C, autoimmune diseases, and cryoglobulinemia) and C3 glomerulonephritis (isolated C3 deposition without significant immunoglobulin, associated with C3 nephritic factor and complement dysregulation). Progressive course with 40-50% developing ESRD within 10-20 years.

Secondary Nephrotic Syndrome — accounts for 10-20% of adult cases

• Diabetes mellitus: Most common cause of nephrotic syndrome globally; responsible for ~30-35% of ESRD in developed countries. Diabetic nephropathy progresses through stages: hyperfiltration, glomerular basement membrane thickening, nodular glomerulosclerosis (Kimmelstiel-Wilson lesions), and progressive decline. Risk factors include poor glycemic control (elevated HbA1c), hypertension, and genetic predisposition. Presents with slowly progressive proteinuria, typically non-nephrotic initially, evolving to nephrotic range proteinuria. Screening with urine albumin-to-creatinine ratio is standard.

• Systemic lupus erythematosus (SLE) and lupus nephritis: Glomerulonephritis develops in 40-50% of SLE patients, with Classes III (focal proliferative), IV (diffuse proliferative), and V (membranous) most commonly presenting with nephrotic proteinuria. Pathophysiology involves immune complex deposition in lupus membranous patterns and proliferative lesions. Other systemic manifestations (malar rash, arthritis, serositis, cytopenias) usually accompany lupus nephritis.

• Infections: Post-infectious glomerulonephritis (particularly post-streptococcal GN, though other organisms including hepatitis C, hepatitis B, endocarditis-associated, and HIV-associated can cause nephrotic syndrome). Hepatitis C-associated glomerulonephritis frequently presents with nephrotic-range proteinuria; MPGN and cryoglobulinemic vasculitis are common patterns.

• Malignancy: Solid tumors (especially lung, gastric, breast, and ovarian cancers) and hematologic malignancies (multiple myeloma, Hodgkin lymphoma, lymphomas) can trigger nephrotic syndrome through paraneoplastic immune phenomena, notably through production of circulating immune complexes or direct tumor infiltration. Membranous nephropathy is the most common pattern with solid tumors. Screening for occult malignancy is critical in adults >50 years presenting with nephrotic syndrome.

• Medications: NSAIDs (particularly associated with MCD and FSGS), ACE inhibitors and ARBs (can unmask pre-existing proteinuria), gold salts, penicillamine, and bisphosphonates are well-documented triggers. The mechanism often involves immune sensitization.

• Allergic reactions and environmental exposures: Exposure to allergens (pollen, insect stings, environmental antigens) can occasionally trigger MCD. Heroin use (particularly with repeated injections) is associated with FSGS, particularly in African American populations.

• Systemic diseases: Amyloidosis (both AL and AA types, with nephrotic syndrome indicating advanced renal involvement), sarcoidosis (rare, usually with concomitant systemic involvement), and light-chain deposition disease present with nephrotic-range proteinuria and represent important diagnostic considerations in older patients with systemic symptoms.

• Proteinuria: Patients may report foamy or frothy urine, representing increased urinary protein content and surfactant properties of urinary proteins. The massive proteinuria (>3.5 g/day, often 5-20 g/day in active disease) may be clinically silent, discovered incidentally on urinalysis, or associated with symptoms when accompanied by acute changes in proteinuria magnitude. Selective proteinuria (predominantly albumin and small proteins) suggests minimal change disease or membranous nephropathy, while non-selective proteinuria (broader protein size range including immunoglobulins) indicates more severe barrier disruption, typical of proliferative diseases.

• Edema: Typically the most bothersome symptom, edema begins in dependent areas (ankles and lower legs in ambulatory patients, sacral and periorbital areas in bedridden patients) and progresses to massive generalized edema (anasarca) in severe cases. Edema develops insidiously over weeks to months and may worsen throughout the day with dependency. Pathophysiology reflects both decreased plasma oncotic pressure (from hypoalbuminemia) and often secondary activation of the renin-angiotensin-aldosterone system, leading to sodium retention and expansion of total body water. Patients report difficulty with ambulation, inability to fit into shoes, and weight gain (10-20 lbs is typical); severe edema can cause skin breakdown and secondary cellulitis.

• Fatigue and malaise: Non-specific but common, attributed to ongoing urinary losses of amino acids, trace metals (zinc, iron), vitamins (vitamin D from loss of DBP), and fat-soluble vitamins, combined with uremia if renal function is compromised and the systemic inflammatory state associated with active glomerulonephritis.

• Dyspnea and orthopnea: Can occur from severe hypoalbuminemia leading to pulmonary edema, though in the absence of significant cardiac dysfunction this is less common than in acute kidney injury. Ascites from edema can impair diaphragmatic excursion. Patients with severe hypoalbuminemia (albumin <1.5 g/dL) are at particular risk.

• Abdominal pain and gastrointestinal symptoms: Mesenteric edema from massive fluid retention can cause abdominal pain and nausea. Additionally, patients on corticosteroid therapy (standard treatment) frequently experience GI upset, and those with hypoalbuminemia and malnutrition may develop constipation.

• Infection-related symptoms: Patients may present with bacterial infections (particularly pneumonia with encapsulated organisms like Streptococcus pneumoniae, or spontaneous peritonitis) due to loss of immunoglobulins and complement components. Fever and localized infection symptoms may herald these complications.

• Thrombotic symptoms: Patients may present acutely with flank pain, hematuria, and/or progressive renal dysfunction from renal vein thrombosis (occurs in 10-40% of membranous nephropathy). More insidiously, leg swelling and pain may indicate deep vein thrombosis or pulmonary symptoms from pulmonary embolism.

Physical Examination Findings

• Edema on examination: Pitting peripheral edema, often in dependent areas. In severe cases, skin tightness, difficulty with mobility, and visible edema of face (periorbital puffiness), genitalia, and dependent areas. Skin may appear pale from anemia and poor nutritional status. Skin integrity should be assessed for breakdown and cellulitis risk.

• Ascites: Detected by fluid wave, shifting dullness, or bulging flanks in patients with severe hypoalbuminemia and total body fluid overload. Ascites in nephrotic syndrome without cirrhosis or cardiac disease is relatively specific for severe nephrotic syndrome.

• Elevated blood pressure: Present in 30-50% of nephrotic patients, particularly those with secondary causes (especially diabetes and SLE). Hypertension reflects sodium retention and RAAS activation; absence of hypertension does not exclude significant renal disease and may actually indicate more severe proteinuria-related glomerular disease in primary nephrotic conditions.

• Other systemic findings: Examination findings depend on the underlying etiology—malar rash or arthritis in lupus nephritis, signs of diabetes (retinopathy, neuropathy), stigmata of hepatic disease in hepatitis-associated nephrotic syndrome, or lymphadenopathy/hepatosplenomegaly in malignancy-associated cases.

Important Clinical Variants and Presentations

• Asymptomatic nephrotic-range proteinuria: Some patients (particularly those with membranous nephropathy or early diabetic nephropathy) may have minimal edema despite nephrotic-range proteinuria, discovered during routine screening or investigation for other conditions. These patients require careful risk stratification and monitoring.

• Nephrotic syndrome with acute kidney injury: Some patients present with concurrent acute kidney injury (elevated creatinine) rather than merely reduced GFR from chronic

Step 1 — confirm and quantify proteinuria

• Urine dipstick: detects albumin only (charge-based); a negative dipstick with heavy proteinuria suggests light chains, so order urine protein electrophoresis if myeloma is suspected.
• Spot urine protein-to-creatinine ratio (UPCR): the practical first quantitative test; a ratio above ~3.5 g/g approximates 3.5 g/day. KDIGO's 2021 glomerular diseases guideline accepts first-morning UPCR in place of a 24-hour collection, which remains the reference standard when precision matters.
• Urine microscopy: oval fat bodies and fatty casts; free lipid droplets show a Maltese cross under polarized light. Dysmorphic RBCs or RBC casts point away from pure nephrotic syndrome toward a nephritic/proliferative lesion.

Step 2 — document the syndrome and its metabolic signature

• Serum albumin and lipid panel: hypoalbuminemia (typically <3 g/dL, often <2.5 g/dL) plus hypercholesterolemia complete the tetrad with proteinuria and edema.
• Basic metabolic panel with eGFR to separate isolated nephrotic proteinuria from concurrent kidney failure.

Step 3 — search for a secondary cause before biopsy

• Serologies: ANA and anti-dsDNA with C3/C4, hepatitis B and C, HIV, and syphilis where relevant; SPEP/UPEP with serum free light chains in adults over 50.
• HbA1c and dilated eye exam: diabetic nephropathy is essentially always accompanied by retinopathy in type 1 disease; nephrotic proteinuria without retinopathy argues for biopsy.
• Anti-PLA2R antibody: in a patient with preserved GFR and nephrotic syndrome, a positive titer can establish primary membranous nephropathy without tissue, per KDIGO.

Immediate supportive care (all patients, regardless of lesion)

• Dietary sodium restriction plus a loop diuretic: furosemide is first-line for edema. Gut wall edema impairs oral absorption, so switch to IV dosing or a continuous infusion when oral therapy fails; adding a thiazide-like agent (metolazone) produces sequential nephron blockade. IV albumin paired with a loop diuretic is reserved for refractory anasarca with severe hypoalbuminemia.
• RAAS blockade: an ACE inhibitor (lisinopril) or ARB lowers intraglomerular pressure and proteinuria independent of blood pressure. KDIGO recommends this for essentially all nephrotic proteinuria except steroid-responsive minimal change disease, where remission is rapid.
• Statin therapy for the nephrotic dyslipidemia, and pneumococcal and influenza vaccination before immunosuppression.

Lesion-specific first-line immunosuppression (KDIGO 2021)

• Minimal change disease: high-dose oral corticosteroids (prednisone); most children and roughly half of adults remit.
• FSGS (primary): prolonged corticosteroids; a calcineurin inhibitor (tacrolimus or cyclosporine) for steroid resistance or dependence.
• Membranous nephropathy: risk-stratify. Low-risk disease gets supportive therapy alone since spontaneous remission is common; moderate-to-high risk gets rituximab, cyclophosphamide alternating with corticosteroids (modified Ponticelli), or a calcineurin inhibitor. Anti-PLA2R titers track response.
• Lupus nephritis: glucocorticoids plus mycophenolate mofetil or low-dose IV cyclophosphamide, with belimumab or voclosporin as add-on — per ACR lupus nephritis guidance.
• Diabetic kidney disease: ACEI/ARB plus an SGLT2 inhibitor, with the nonsteroidal MRA finerenone for residual albuminuria (KDIGO diabetes-in-CKD guidance).

Contraindicated/avoid

• ACE inhibitors and ARBs in pregnancy — fetotoxic; all agents including captopril are contraindicated.
• Combined ACEI + ARB (hyperkalemia, AKI), NSAIDs, and live vaccines during immunosuppression.

Disease-related

• Venous thromboembolism (emergency): urinary antithrombin III loss plus hepatic overproduction of procoagulants. Renal vein thrombosis — classically membranous nephropathy — signals itself with flank pain, gross hematuria, and an abrupt creatinine rise; pulmonary embolism presents with pleuritic pain and hypoxemia. Antithrombin depletion can also produce apparent heparin resistance.
• Infection (emergency): urinary loss of IgG and complement factor B/properdin impairs opsonization of encapsulated organisms. Spontaneous bacterial peritonitis with Streptococcus pneumoniae in an edematous child with fever and abdominal pain is the classic scenario; cellulitis arises in stretched, broken edematous skin.
• Acute kidney injury: from overaggressive diuresis in an intravascularly underfilled patient, interstitial edema compressing tubules, bilateral renal vein thrombosis, or superimposed interstitial nephritis (NSAIDs).
• Accelerated atherosclerosis: driven by the LDL and lipoprotein(a) elevations described above.
• Endocrine and nutritional losses: vitamin D–binding protein loss → hypovitaminosis D and secondary hyperparathyroidism; thyroxine-binding globulin loss → abnormal thyroid indices; transferrin loss → microcytic, iron-refractory anemia; protein malnutrition with muscle wasting.
• Progression to ESRD via the tubulointerstitial fibrosis pathway.

Treatment-related

• Corticosteroids: hyperglycemia, osteoporosis, cataracts, avascular necrosis of the femoral head, adrenal suppression on abrupt withdrawal, and growth failure in children.
• Cyclophosphamide: hemorrhagic cystitis from acrolein (prevented with mesna and hydration), gonadal failure, myelosuppression, and later urothelial malignancy.
• Calcineurin inhibitors: dose-dependent nephrotoxicity with a rising creatinine, hypertension, tremor, and hyperkalemia — a rising creatinine on tacrolimus is drug toxicity until proven otherwise.
• Rituximab: hepatitis B reactivation (emergency — screen HBsAg and anti-HBc before dosing), hypogammaglobulinemia, infusion reactions, and rare PML.
• Diuretics: hypokalemia, metabolic alkalosis, and prerenal AKI.

• Maltese crosses under polarized light: the single most testable urinary finding — free lipid in oval fat bodies. Pair it with frothy urine and the diagnosis is nephrotic, not nephritic.
• A 2–6-year-old with periorbital edema after a viral URI: minimal change disease. The best next step is empiric prednisone, not biopsy — biopsy only if steroid-resistant (KDIGO). Selective albuminuria and normal light microscopy with foot process effacement on EM seal it.
• New membranous nephropathy in an adult over 50: check anti-PLA2R and pursue age-appropriate malignancy screening. Solid tumors (lung, GI) are the association examiners test.
• Flank pain + gross hematuria + sudden creatinine rise in membranous nephropathy: renal vein thrombosis. Next step is CT or MR venography, then anticoagulation.
• Infection associations to memorize: HIV → collapsing FSGS (large kidneys on ultrasound); hepatitis B → membranous, especially in children; hepatitis C → membranoproliferative pattern with cryoglobulins and low C3/C4.
• APOL1 risk variants in patients of West African ancestry explain the FSGS/hypertension-attributed ESRD excess — a favorite genetics-meets-nephrology stem.
• Common distractor: RBC casts and dysmorphic RBCs are nephritic. A stem offering "nephrotic-range proteinuria with RBC casts and hypertension" is a proliferative glomerulonephritis (or lupus class IV), not minimal change disease.
• Second common distractor: don't attribute failure of subcutaneous heparin to "non-compliance" — urinary antithrombin III loss produces genuine heparin resistance.
• Every nephrotic patient: sodium restriction, a loop diuretic, an ACE inhibitor or ARB for antiproteinuric effect, a statin, and vaccination against encapsulated organisms — supportive care is the answer stem before any immunosuppression choice.