Nephrotic Syndrome: A Comprehensive Essay

Definition and Diagnostic Criteria

Pathophysiology

1. Massive proteinuria - Increased glomerular permeability leads to urinary protein loss exceeding 3.5 g/day. The degree of selectivity matters: highly selective proteinuria consists mostly of low-molecular-weight proteins (albumin, transferrin), whereas poorly selective proteinuria also includes higher-molecular-weight globulins.
2. Hypoalbuminemia - Heavy proteinuria depletes serum albumin beyond the liver's compensatory synthetic rate. Increased renal catabolism of filtered albumin compounds this effect.
3. Edema - The fall in plasma oncotic pressure from hypoalbuminemia increases the flux of fluid into the interstitial spaces ("underfill" mechanism). Simultaneously, a primary renal salt retention can lead to "overfill" edema. Sodium retention occurs through several mechanisms: activation of the renin-angiotensin-aldosterone system (RAAS) driven by perceived hypovolemia, sympathetic nervous system stimulation, and reduced secretion of natriuretic peptides. Edema is characteristically soft, pitting, and most marked in the periorbital regions and dependent portions of the body. Severe hypoalbuminemia can produce pleural effusions and ascites.
4. Hyperlipidemia and lipiduria - Most patients develop elevated cholesterol, triglycerides, VLDL, LDL, and lipoprotein(a), with decreased HDL in some cases. This reflects a combination of increased hepatic lipoprotein synthesis (a compensatory response to low oncotic pressure), abnormal transport of circulating lipid particles, and decreased lipid catabolism. Lipiduria follows as lipoproteins also leak across the damaged glomerular wall, appearing in urine as free fat or oval fat bodies.
5. Hypercoagulability - The urinary loss of natural anticoagulants - protein C, protein S, plasminogen, and antithrombin III - creates a prothrombotic state. Renal vein thrombosis can occur when proteinuria exceeds 10 g/24 hr with severe hypoalbuminemia below 2 g/dL.
6. Susceptibility to infection - Loss of immunoglobulins in the urine predisposes to staphylococcal and pneumococcal infections. Spontaneous bacterial peritonitis occurs in 2-6% of children with nephrotic syndrome.
7. Additional deficiencies - Loss of transferrin causes iron deficiency anemia; loss of vitamin D-binding protein causes vitamin D deficiency; loss of thyroid-binding globulin affects thyroid hormone levels.

Aetiology and Classification

Primary (Idiopathic) Causes

Secondary Causes

• Diabetic kidney disease (DKD) - the most common cause of kidney failure in the United States
• Amyloidosis
• Lupus nephritis (SLE)
• Infections - hepatitis B, hepatitis C, HIV
• Drugs - NSAIDs, gold, penicillamine, heroin
• Malignancies - Hodgkin lymphoma (MCD), solid tumors (MN)
• Monoclonal immunoglobulin-associated disease
• Hereditary/genetic causes - mutations in podocyte genes

Major Causes of Primary Nephrotic Syndrome

1. Minimal Change Disease (MCD)

• Light microscopy (LM): Glomeruli appear entirely normal (hence the name)
• Immunofluorescence (IF): No immune deposits
• Electron microscopy (EM): Diffuse effacement of podocyte foot processes (the pathognomonic finding), along with vacuolization, microvillus formation, and focal detachments

• Abrupt onset of nephrotic syndrome in an otherwise healthy child
• Selective proteinuria (mainly albumin)
• Renal function usually preserved
• Hematuria and hypertension are not typical
• In children without atypical features, kidney biopsy is NOT required before treatment

• Mainstay: corticosteroids (prednisone 60 mg/m²/day or 2 mg/kg/day)
• Over 90% of children respond; response is defined as resolution of proteinuria within 8 weeks
• Proteinuria recurs in more than two-thirds of initial responders
• Steroid-sparing agents for frequent relapsers: cyclophosphamide, mycophenolate mofetil (MMF), rituximab (anti-CD20), calcineurin inhibitors (CNIs)
• Frequently-relapsing NS (FRNS): ≥2 relapses within 6 months or ≥4 relapses within 12 months
• Steroid-dependent NS (SDNS): Two consecutive relapses during steroid therapy, or inability to maintain remission without continuous steroids

2. Focal Segmental Glomerulosclerosis (FSGS)

• Primary FSGS: Due to an undiscovered circulating podocyte permeability factor; typically presents with acute onset of nephrotic syndrome, low albumin, and proteinuria >3.5 g/day
• Secondary FSGS:
  • Viral: HIV-associated nephropathy, parvovirus B19, CMV, COVID-19
  • Drug-induced: Heroin, interferons, pamidronate, sirolimus, calcineurin inhibitors, anabolic steroids
  • Adaptive: Reduced nephron mass, obesity-related glomerulopathy, low birth weight, unilateral renal agenesis, sickle cell anemia
• Genetic FSGS: Mutations in podocyte genes including NPHS1 (nephrin), NPHS2 (podocin), PLCE1, INF2, ACTN4, and APOL1 (strongly associated with FSGS in African descent)

• LM: Focal (affecting only some glomeruli) and segmental (involving a portion of the glomerular tuft) sclerosis. Five variants: cellular, tip, perihilar, collapsing, and NOS
• EM: Varying degrees of foot process effacement; >80% effacement favors primary FSGS; <80% favors secondary
• Collapsing variant: Strongly associated with viral etiologies (HIV, COVID-19); presents with abrupt onset of severe nephrotic syndrome and rapidly progressive loss of kidney function

• Immunosuppression (corticosteroids, calcineurin inhibitors, mycophenolate mofetil) for primary FSGS
• Secondary FSGS: address underlying cause; avoid unnecessary immunosuppression
• Primary FSGS recurs rapidly after kidney transplant (supporting the circulating factor hypothesis)
• Persistent hematuria or poor response to immunosuppression indicates progression toward ESKD
• The APOL1 gene polymorphism in patients of African descent confers increased risk of FSGS and advanced kidney disease

3. Membranous Nephropathy (MN)

• Anti-PLA2R antibodies (anti-phospholipase A2 receptor): present in ~70% of primary MN; a critical diagnostic and monitoring biomarker
• Anti-THSD7A antibodies (thrombospondin type-1 domain containing 7A): present in 1-3% of primary MN, also associated with malignancy
• Newer antigens: NELL-1, EXT1/2 (associated with autoimmune disease), Semaphorin 3B (associated with pediatric cases)
• Secondary causes: SLE (EXT1/2), hepatitis B infection, NSAIDs, solid tumors, paraproteinemias

• LM: Normal glomerular architecture initially; methenamine silver stain shows subepithelial spikes along capillary walls in later stages
• IF: Granular deposition of IgG and C3 along capillary walls (characteristic granular pattern)
• EM: Subepithelial and intramembranous immune complex deposits - the hallmark; plus foot process effacement shared with MCD and FSGS
• Biopsies can now be stained for PLA2R, THSD7A, EXT1/2, NELL-1, and Sema3B antigens

• One-third undergo spontaneous remission
• One-third achieve partial remission
• One-third progress (persistent nephrotic syndrome or declining GFR)

• Indications for immunosuppression: persistent nephrotic syndrome and/or declining kidney function
• Options: Rituximab (increasingly first-line), calcineurin inhibitors, cyclophosphamide + steroids
• PLA2R antibody titers guide response to immunosuppressive therapy
• If progressive course: evaluate for superimposed crescentic GN or acute interstitial nephritis

Clinical Features and Complications

Core Features

Complications

• Iron deficiency anemia (transferrin loss)
• Vitamin D deficiency (vitamin D-binding protein loss)
• Hypothyroidism (thyroid-binding globulin loss)
• Hypogammaglobulinemia predisposing to sepsis

Investigation

• 24-hour urinary protein (>3.5 g/day) or spot urine protein-to-creatinine ratio (>2-3.5 mg/mg)
• Urinalysis: heavy proteinuria, lipiduria, oval fat bodies, fatty casts, maltese crosses under polarized light
• Urinary microscopy: absence of red cell casts (distinguishes from nephritic syndrome)

• Serum albumin (decreased)
• Lipid profile: total cholesterol, LDL, triglycerides (elevated), HDL (may be decreased)
• Renal function: creatinine, urea, eGFR
• Complement levels (C3, C4) - may be low in secondary causes (SLE, MPGN)
• ANCA, ANA, anti-dsDNA (for secondary causes)
• Anti-PLA2R and anti-THSD7A antibodies (for membranous nephropathy)
• Serum protein electrophoresis, free light chains (to exclude paraproteinemia)
• HBsAg, anti-HCV, HIV serology
• Blood glucose, HbA1c (for diabetic nephropathy)

• Indicated in adults with nephrotic syndrome (essential for diagnosis)
• In children with typical presentation (age 1-8 years, no hematuria, normal complement, no hypertension), empirical steroid therapy is started without biopsy
• Atypical features warranting biopsy in children: age <1 year, macroscopic hematuria, hypertension, hypocomplementemia, extrarenal symptoms (rash, arthritis)

Management

General (Non-Disease-Specific) Measures

• Low-salt diet to reduce edema
• Protein restriction: 0.8 to 1 g/kg/day (excessive protein intake increases proteinuria without benefit)
• Hypercaloric diet in severe disease

• ACE inhibitors (ACEi) or angiotensin receptor blockers (ARBs) reduce intraglomerular pressure and proteinuria; considered cornerstone therapy regardless of etiology
• Adding a loop diuretic to ACEi/ARB reduces proteinuria further but may increase serum creatinine

• Loop diuretics (furosemide) - first-line; note that hypoalbuminemia reduces protein-binding of furosemide, increasing its volume of distribution
• Intravenous albumin + loop diuretic combination is used in severe diuretic-resistant cases (though iso-oncotic albumin infusion alone does not reliably induce negative sodium balance)
• Thiazide diuretics added as adjuncts

• Statin therapy for dyslipidemia

• Prophylactic anticoagulation considered for patients at high risk of thrombosis (severe proteinuria >10 g/day, serum albumin <2 g/dL), particularly in membranous nephropathy

• Pneumococcal and influenza vaccination
• Prompt treatment of infections

Disease-Specific Treatment (Summary)

Nephrotic vs. Nephritic Syndrome: Key Distinction

Special Populations

Children

Adults

Pregnancy

Prognosis

• MCD in children: Excellent prognosis; >90% initial steroid response; most achieve long-term remission though relapses are common
• Primary FSGS: Variable; patients with non-nephrotic proteinuria have the best outcomes; persistent nephrotic syndrome or hematuria and poor steroid response predict progression to ESKD
• Primary MN: Rule of thirds applies; anti-PLA2R titer is a reliable guide
• Secondary causes: Prognosis determined by the underlying systemic disease
• Complications (thromboembolism, infection, AKI) worsen overall outcomes

References and Sources

• National Kidneys Foundation Primer on Kidney Diseases, 8th Edition - Chapters 16, 18, 19, 20
• Robbins, Cotran & Kumar Pathologic Basis of Disease - Chapter on Glomerular Diseases
• Robbins & Kumar Basic Pathology - Chapter 12
• Comprehensive Clinical Nephrology, 7th Edition - Chapter on Minimal Change Disease
• Brenner and Rector's The Kidney, 2-Volume Set - Chapter 50 (Edema in Nephrotic Syndrome)
• Campbell Walsh Wein Urology, 3-Volume Set - Pediatric Nephrotic Syndrome

Recent Evidence Note: A 2025 Cochrane systematic review (PMID 40337980) on interventions for idiopathic steroid-resistant nephrotic syndrome in children updates treatment considerations for this subgroup. A 2026 meta-analysis (PMID 41843217) specifically reports rituximab-induced hypogammaglobulinemia as a significant complication in childhood nephrotic syndrome - clinicians should monitor immunoglobulin levels when using rituximab.