Nephrotic Syndrome

Nephrotic Syndrome – High-Yield Study Guide for Medical Students

Definition

Nephrotic syndrome is a clinical syndrome characterized by:

• Heavy proteinuria (>3–3.5 g/24 hours in adults; >40 mg/m²/h in children)
• Hypoalbuminemia (typically <3.0 g/dL)
• Generalized edema
• Hyperlipidemia and lipiduria

It reflects significant injury to the glomerular filtration barrier, leading predominantly to protein loss rather than inflammatory hematuria.

Epidemiology

Nephrotic syndrome occurs in both children and adults, but the etiologic spectrum differs by age:

• Children (especially 2–6 years): most commonly minimal change disease (MCD).
• Adults: more often due to focal segmental glomerulosclerosis (FSGS), membranous nephropathy, or systemic diseases such as diabetic nephropathy and lupus nephritis.

There is a higher prevalence in males for MCD in childhood, while membranous nephropathy shows a slight male predominance in adults. Certain causes (e.g., APOL1-associated FSGS) are more common in individuals of African ancestry.

Pathophysiology

The hallmark of nephrotic syndrome is increased permeability of the glomerular filtration barrier to plasma proteins. This barrier is composed of:

• Fenestrated glomerular endothelium
• Glomerular basement membrane (GBM)
• Visceral epithelial cells (podocytes) with slit diaphragms

Key mechanisms include:

• Podocyte injury and effacement: seen in MCD and FSGS; disruption of slit diaphragm proteins (e.g., nephrin) causes massive albumin leak.
• Immune complex deposition: as in membranous nephropathy, where subepithelial immune complexes cause complement activation and GBM damage.
• Metabolic and hemodynamic injury: as in diabetic nephropathy, with GBM thickening, mesangial expansion, and podocyte loss.

Protein loss leads to hypoalbuminemia, reducing plasma oncotic pressure and causing fluid to shift into the interstitial space, resulting in edema. In response, the liver increases synthesis of proteins and lipids, causing hyperlipidemia and lipiduria (oval fat bodies, fatty casts).

Loss of anticoagulant proteins (e.g., antithrombin III, protein S) and changes in platelet function induce a hypercoagulable state. There is also urinary loss of immunoglobulins and complement components, leading to increased susceptibility to infections.

Etiology and Major Causes

Nephrotic syndrome can be primary (idiopathic glomerular diseases) or secondary to systemic conditions, drugs, or infections.

Primary (idiopathic) glomerular causes:

• Minimal change disease (MCD)
  • Most common cause in children; can occur in adults.
  • Often triggered by viral infection, immunization, allergy, or NSAIDs.
  • Light microscopy usually normal; electron microscopy shows diffuse podocyte foot process effacement.
• Focal segmental glomerulosclerosis (FSGS)
  • Common cause in adults and adolescents.
  • Associated with HIV, heroin use, obesity, sickle cell disease, and reduced nephron mass.
  • Segmental sclerosis and hyalinosis in some glomeruli.
• Membranous nephropathy
  • Common cause of nephrotic syndrome in White adults.
  • Primary form often associated with antibodies against PLA2R on podocytes.
  • Secondary causes: malignancies (solid tumors), infections (HBV, syphilis), autoimmune diseases, drugs (gold, penicillamine, NSAIDs).
  • Diffuse capillary wall thickening, subepithelial immune deposits, “spike and dome” pattern on EM.
• Membranoproliferative glomerulonephritis (MPGN)
  • Can present with mixed nephritic and nephrotic features.
  • Associated with chronic infections (e.g., hepatitis C), complement dysregulation.

Secondary causes:

• Diabetic nephropathy (most common cause of nephrotic syndrome in adults in many regions)
  • Longstanding diabetes with microalbuminuria progressing to overt proteinuria.
  • Diffuse and nodular (Kimmelstiel–Wilson) glomerulosclerosis.
• Systemic lupus erythematosus (SLE) – class V membranous lupus nephritis can cause nephrotic syndrome.
• Amyloidosis – deposition of amyloid fibrils in glomeruli.
• Infections – hepatitis B, hepatitis C, HIV, malaria, syphilis.
• Drugs – NSAIDs, gold, penicillamine, lithium, interferon, some biologics.
• Malignancies – solid tumors (e.g., lung, colon, breast), hematologic malignancies (Hodgkin lymphoma, CLL).
• Hereditary podocytopathies – mutations in podocin, nephrin, etc.

Clinical Presentation

Typical features of nephrotic syndrome include:

• Edema
  • Periorbital edema (especially in the morning, classic in children).
  • Dependent edema (legs, sacrum), scrotal or labial swelling.
  • Can progress to generalized anasarca and ascites.
• Weight gain due to fluid retention.
• Foamy urine from heavy proteinuria.
• Hypertension – more common in adults and in secondary causes (e.g., diabetic nephropathy).
• Signs of hyperlipidemia – xanthelasmas, rarely xanthomas.
• Symptoms related to complications
  • Dyspnea (pleural effusions, pulmonary edema, or pulmonary embolism).
  • Abdominal pain (thrombosis of renal vein or mesenteric vessels, SBP in cirrhotic overlap).
  • Recurrent infections (e.g., cellulitis, spontaneous bacterial peritonitis in heavy ascites).

Diagnostic Evaluation

The diagnosis is based on demonstrating nephrotic-range proteinuria and characteristic lab abnormalities, then identifying the underlying cause.

Initial Laboratory Assessment

• Urinalysis
  • 3+ or 4+ protein on dipstick.
  • Oval fat bodies and fatty casts (“Maltese cross” under polarized light).
  • Typically bland sediment (few RBCs or WBCs) unless a mixed nephritic-nephrotic process is present.
• Quantification of proteinuria
  • 24-hour urine protein >3–3.5 g/24 h in adults.
  • Spot urine protein-to-creatinine ratio (UPCR) >3–3.5 g/g (approximate correlation with 24-hour collection).
• Serum tests
  • Low serum albumin.
  • Elevated total cholesterol, LDL, and triglycerides.
  • Renal function: serum creatinine and eGFR (may be normal early, decreased with chronicity or severe disease).
  • Electrolytes, calcium, and vitamin D (may be low due to protein binding losses).

Workup for Underlying Etiology

Targeted investigations depend on clinical context, but commonly include:

• Serologies
  • ANA, anti-dsDNA, complement levels (C3, C4) – for SLE and other immune causes.
  • HBsAg, anti-HBc, HCV antibodies, HIV testing – for infectious etiologies.
  • Serum and urine protein electrophoresis, free light chains – for amyloidosis and plasma cell disorders.
  • PLA2R antibodies – in suspected primary membranous nephropathy.
• Imaging
  • Renal ultrasound to assess kidney size, parenchymal echogenicity, and rule out obstruction.

Renal Biopsy

Renal biopsy is often required in adults to establish the precise histologic diagnosis, particularly when the cause is unclear or when management requires immunosuppression. Exceptions include classic diabetic nephropathy with longstanding diabetes and typical retinopathy, where biopsy may be unnecessary.

Biopsy provides:

• Light microscopy (LM) for overall architecture and sclerosis.
• Immunofluorescence (IF) for immune complex deposition.
• Electron microscopy (EM) for ultrastructural changes (e.g., podocyte effacement, subepithelial deposits).

Management Principles

Management of nephrotic syndrome involves both general measures (supportive care and complication prevention) and disease-specific therapy based on etiology.

General / Supportive Management

• Edema control
  • Dietary sodium restriction (typically <2 g/day of sodium).
  • Loop diuretics (e.g., furosemide), sometimes combined with thiazide-type diuretics in resistant cases.
  • Careful monitoring of volume status, blood pressure, electrolytes, and renal function.
• Proteinuria and blood pressure control
  • ACE inhibitors or ARBs are first-line to reduce intraglomerular pressure and proteinuria, and to slow progression of CKD.
  • Blood pressure goal typically <130/80 mmHg in most patients with proteinuric CKD.
• Lipid management
  • Statins are commonly used to treat hyperlipidemia and reduce cardiovascular risk.
  • Lipid abnormalities may improve with remission of nephrotic syndrome but often require pharmacologic therapy.
• Anticoagulation / thromboembolism prevention
  • Patients with nephrotic syndrome, especially membranous nephropathy, albumin <2–2.5 g/dL, or known thrombophilia, have a high risk of venous thromboembolism (VTE).
  • Consider prophylactic anticoagulation in high-risk patients after weighing bleeding risk.
• Infection prevention
  • Ensure age-appropriate immunizations, including pneumococcal and influenza vaccines.
  • Maintain vigilance for bacterial infections due to urinary loss of immunoglobulins and complement.
• Nutritional support
  • Avoid severe protein restriction; moderate protein intake is typically recommended.
  • Monitor for deficiencies in vitamin D, calcium, and trace elements.

Disease-Specific Therapy

Treatment depends on the underlying cause and histologic type:

• Minimal change disease (MCD)
  • First-line: corticosteroids (e.g., high-dose oral prednisone).
  • Most children and many adults respond with complete remission of proteinuria.
  • Relapsing or steroid-dependent disease may require steroid-sparing agents (e.g., calcineurin inhibitors, cyclophosphamide, rituximab).
• Focal segmental glomerulosclerosis (FSGS)
  • Initial approach often includes high-dose steroids; response is variable.
  • Calcineurin inhibitors (e.g., cyclosporine, tacrolimus) are frequently used in steroid-resistant cases.
  • Address secondary contributors (control obesity, treat HIV, stop offending drugs).
• Membranous nephropathy
  • Risk-stratified according to degree and duration of proteinuria, renal function, and PLA2R antibody levels.
  • Supportive therapy alone may be sufficient in low-risk patients.
  • Moderate/high-risk patients often receive immunosuppressive regimens (e.g., rituximab-based protocols, or alternating steroids and alkylating agents, or calcineurin inhibitor-based regimens).
• Diabetic nephropathy
  • Intensive glycemic control and renin-angiotensin system blockade (ACEi/ARB).
  • Consider SGLT2 inhibitors and nonsteroidal mineralocorticoid receptor antagonists (e.g., finerenone) depending on guidelines and comorbidities.
• Lupus nephritis (class V or mixed lesions)
  • Requires immunosuppression tailored to class and severity (e.g., mycophenolate mofetil or cyclophosphamide plus glucocorticoids; newer biologics depending on guideline).
• Amyloidosis
  • Treat underlying cause (e.g., AL amyloid with hematologic chemotherapy, AA amyloid with control of inflammatory disease).

Advanced cases with progression to end-stage kidney disease (ESKD) may require dialysis and eventual kidney transplantation. Certain primary diseases (e.g., FSGS) may recur post-transplant.

Complications

Nephrotic syndrome is associated with significant complications that must be anticipated and managed:

• Thromboembolism
  • Renal vein thrombosis (classic in membranous nephropathy).
  • Deep vein thrombosis and pulmonary embolism.
• Infections
  • Increased risk of bacterial infections due to urinary loss of IgG and complement.
  • Spontaneous bacterial peritonitis in patients with heavy ascites.
• Acute kidney injury (AKI)
  • Due to intravascular volume depletion, overdiuresis, or superimposed insults.
• Progressive chronic kidney disease (CKD)
  • Particularly in FSGS, membranous nephropathy, and diabetic nephropathy if proteinuria is not controlled.
• Cardiovascular disease
  • Accelerated atherosclerosis from dyslipidemia and chronic inflammation.
• Endocrine and metabolic disturbances
  • Loss of vitamin D–binding protein can cause hypovitaminosis D and secondary hyperparathyroidism.
  • Loss of thyroid-binding globulin may alter thyroid hormone measurements.

Prognosis

Prognosis depends on the underlying disease, response to therapy, and burden of complications:

• MCD – excellent prognosis, especially in children; many achieve complete remission.
• FSGS – variable; high risk of progression to ESKD if proteinuria persists.
• Membranous nephropathy – some patients undergo spontaneous or treatment-induced remission; others progress over years; PLA2R antibody trends can help with risk stratification.
• Diabetic nephropathy – typically progressive without optimal risk factor control.

Key Clinical Pearls for Exams

• Classic triad for nephrotic syndrome: heavy proteinuria, hypoalbuminemia, edema, with hyperlipidemia and lipiduria as additional hallmarks.
• Most common cause in children: minimal change disease; often post-infectious, responsive to steroids.
• Most common cause in adults in many regions: diabetic nephropathy; think of long-standing diabetes, retinopathy, and progressive albuminuria.
• Membranous nephropathy: common adult cause; associated with solid tumors, HBV, drugs; high risk of renal vein thrombosis.
• FSGS: associated with HIV, heroin use, obesity, reduced nephron number; often steroid-resistant and progressive.
• Minimal change disease: normal light microscopy, negative IF, effaced foot processes on EM; dramatic response to corticosteroids.
• Hypercoagulability in nephrotic syndrome: due to urinary loss of antithrombin III and other anticoagulant factors; consider VTE prophylaxis in high-risk patients.
• ACE inhibitors/ARBs are cornerstone therapy to reduce proteinuria and slow CKD progression in proteinuric kidney disease.
• Oval fat bodies and fatty casts (“Maltese cross”) on urine microscopy are classic for nephrotic-range proteinuria.
• Always look for a secondary cause (diabetes, SLE, infections, drugs, malignancy) in adults presenting with nephrotic syndrome.

Summary

Nephrotic syndrome is a high-yield topic that integrates glomerular pathology, systemic disease, and complex complications. For exam purposes, focus on the cardinal clinical features, the major etiologic patterns by age (MCD in children, FSGS/membranous/diabetes in adults), typical histologic findings, and the rationale for ACEi/ARB therapy and anticoagulation in high-risk patients. Clinically, early recognition and aggressive management of proteinuria and complications are key to preserving renal function and reducing morbidity.