Nephrotic Syndrome in Children

• Podocyte dysfunction and loss of charge selectivity: In minimal change disease, the pathologic hallmark involves effacement of podocyte foot processes visualized on electron microscopy, yet no immune deposits are present on immunofluorescence—hence "minimal change." The podocytes normally maintain a negatively charged glycocalyx rich in heparan sulfate proteoglycans (particularly perlecan and agrin), which repel negatively charged plasma proteins like albumin. In MCD, loss of this negative charge barrier occurs through mechanisms involving cytokine dysregulation, particularly increased production of vascular permeability factor (VPF)/VEGF and altered T-cell function. This leads to disruption of the slit diaphragm, a zipper-like structure composed of adhesion molecules (nephrin, podocin, CD2AP) that normally prevents protein passage. The consequence is loss of both charge and size selectivity, resulting in selective proteinuria (predominantly albumin and small proteins, with relative preservation of larger globulins). This is why serum complement levels and serologic markers remain normal in MCD—the underlying mechanism is functional rather than immune-mediated.

• Massive proteinuria and its metabolic consequences: The proteinuria in nephrotic syndrome typically exceeds 3.5 g/day in children (compared to the normal excretion of 4-8 mg/kg/day) and can reach 5-10 g/day or higher. This massive protein loss exceeds the liver's capacity for albumin synthesis, despite upregulation of hepatic production, resulting in hypoalbuminemia (<2.5 g/dL) that is the pathophysiologic cornerstone of the syndrome. The oncotic pressure gradient is determined by the Starling equation: Net filtration pressure = (Pc - Pif) − σ(πc − πif), where Pc is capillary hydrostatic pressure, Pif is interstitial fluid pressure, σ is reflection coefficient, and π represents oncotic pressure. Reduction in plasma albumin concentration decreases plasma oncotic pressure (πc), shifting the balance toward net fluid filtration into the interstitium. Simultaneously, loss of albumin to urine and compensatory hepatic synthesis of fibrinogen, immunoglobulins, and lipoproteins (including apoB-containing particles) result in qualitative changes in serum proteins. The liver also increases synthesis of cholesterol and triglycerides in response to perceived protein depletion and reduced feedback inhibition, explaining the hyperlipidemia (elevated LDL, VLDL, and sometimes triglycerides) characteristic of nephrotic syndrome. These lipid abnormalities promote endothelial dysfunction and contribute to thrombosis risk.

• Edema formation through altered Starling forces and sodium retention: The decreased plasma oncotic pressure (πc) due to hypoalbuminemia creates a hydrostatic pressure gradient favoring fluid movement into the interstitium across all capillary beds. Additionally, sodium retention occurs through multiple mechanisms: (1) activation of the renin-angiotensin-aldosterone system (RAAS) triggered by perceived hypovolemia despite clinical hypervolemia (the "underfill hypothesis"), (2) sympathetic nervous system activation leading to renal vasoconstriction and reduced glomerular filtration rate, and (3) direct tubular sodium reabsorption via activation of the collecting duct epithelial sodium channel (ENaC) through aldosterone signaling. Some evidence also supports an "overfill hypothesis" wherein primary renal sodium retention occurs independent of RAAS activation. The net result is expansion of the extracellular fluid compartment, with preferential distribution to the interstitium due to low plasma oncotic pressure. Edema accumulates in dependent areas (periorbital, peripheral) and potential spaces (pleural, peritoneal, pericardial), with severity correlating to the degree of hypoalbuminemia.

• Immunologic dysregulation in minimal change disease: Compelling evidence suggests that MCD involves abnormal T-cell-derived cytokine production, with increased secretion of IL-13, VEGF, and eotaxin by activated T cells. These cytokines act on podocytes via IL-13 receptor and VEGF receptor 2, triggering changes in the actin cytoskeleton and disruption of tight junctions. The mechanism explains why MCD shows excellent response to corticosteroids, which suppress T-cell function and cytokine production. Additionally, some children with MCD demonstrate abnormalities in regulatory T cells (Tregs), suggesting a loss of immune tolerance. This immunologic basis is evidenced by the association of MCD with Hodgkin lymphoma (rare), allergic reactions, NSAIDs, and infections—all conditions that can trigger T-cell activation.

• Secondary nephrotic syndromes: distinct pathophysiology: In contrast to MCD, focal segmental glomerulosclerosis (FSGS) involves true structural glomerular injury with sclerosis of some glomeruli and segments thereof, visible on light microscopy and often accompanied by immune deposits. Membranoproliferative glomerulonephritis (MPGN) involves immune complex deposition (seen on immunofluorescence) with characteristic electron-dense deposits on electron microscopy. IgA nephropathy, the most common primary glomerulonephritis worldwide, involves IgA-dominant immune complex deposition in the glomerulus. Post-streptococcal glomerulonephritis (PSGN) occurs 1-3 weeks after Group A Streptococcal (GAS) infection and involves immune complex deposition with granular IgG and C3 pattern on immunofluorescence. Lupus nephritis in the context of systemic lupus erythematosus (SLE) involves autoimmune complex deposition. These secondary causes are distinguished from MCD by presence of active urinary sediment (dysmorphic RBCs, RBC casts), hematuria, hypertension, and reduced serum complement levels.

• Minimal change disease (MCD): 85-90% of pediatric nephrotic syndrome: MCD is the predominant cause across childhood, presenting typically between ages 2-6 years with a 1.5-2:1 male predominance. It is termed "minimal change" because light microscopy reveals normal glomerular architecture, immunofluorescence shows no immune deposits (hence "nil disease"), yet electron microscopy demonstrates foot process effacement. MCD is characterized as steroid-responsive in ~95% of cases, with 85-90% achieving complete remission with initial corticosteroid therapy. The etiology remains incompletely understood but involves T-cell dysregulation, as evidenced by association with allergic conditions (atopy, asthma), recent viral infections (especially respiratory), and rare associations with Hodgkin lymphoma and other lymphoproliferative disorders. Notably, MCD can present acutely with nephrotic syndrome without preceding systemic illness in most cases.

• Focal segmental glomerulosclerosis (FSGS): 10-15% of pediatric nephrotic syndrome, increasing with age: FSGS involves segmental sclerosis of some glomeruli visible on light microscopy, representing a spectrum from collapsing variant (with podocyte hyperplasia and capillary collapse) to tip lesion, perihilar, and not-otherwise-specified variants. In children, FSGS presents clinically in two patterns: (1) primary idiopathic FSGS in children age >6 years, often with higher-grade proteinuria, hematuria, hypertension, and reduced GFR at presentation compared to MCD, and (2) secondary FSGS associated with systemic conditions. Primary FSGS is steroid-resistant in 40-50% of cases, with worse long-term prognosis. Genetic mutations in NPHS1 (nephrin), NPHS2 (podocin), WT1 (Wilms tumor suppressor), and other podocyte genes cause steroid-resistant nephrotic syndrome (SRNS) in 10-15% of pediatric cases, often presenting in infancy or early childhood. Secondary FSGS occurs with HIV infection, heroin abuse (rare in pediatrics but important), anabolic steroids (in adolescents), reflux nephropathy, and massive obesity.

• Membranoproliferative glomerulonephritis (MPGN) and C3 glomerulopathy: MPGN has become increasingly recognized in children and accounts for 3-5% of pediatric nephrotic syndrome. Recent reclassification divides MPGN into (1) immune complex MPGN with immune deposits (post-infectious, lupus, hepatitis C), and (2) C3 glomerulopathy driven by dysregulation of the alternative complement pathway (C3 glomerulonephritis, post-infectious GN with isolated C3 deposits). Clinically, children often present with hematuria and/or hypertension alongside proteinuria, and low serum C3 levels support post-infectious or C3-mediated disease. These children are typically steroid-resistant and have intermediate-term renal prognosis.

• Post-streptococcal glomerulonephritis (PSGN): While PSGN more commonly presents with nephritic syndrome (hematuria, RBC casts, hypertension, azotemia) than nephrotic syndrome, some children (~10-15%) develop nephrotic-range proteinuria concurrent with the acute nephritic picture. PSGN occurs 1-3 weeks after pharyngeal infection with nephritogenic strains of Group A Streptococcus (commonly M1 and M12) or 3-6 weeks after skin infection (impetigo). The mechanism involves immune complex deposition of streptococcal antigen-antibody complexes in the glomerulus, with characteristic findings of "starry sky" appearance on immunofluorescence (diffuse granular IgG and C3), subepithelial "humps" on electron microscopy, and low C3 levels (typically normalized within 6-8 weeks). Unlike MCD, PSGN has excellent overall prognosis with spontaneous resolution in >95% of children.

• IgA nephropathy and other primary glomerulonephritis: IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and occasionally presents with nephrotic syndrome in children, though more typically with hematuria (gross or persistent microscopic). IgAN is characterized by IgA-dominant deposits on immunofluorescence, with diagnosis confirmed by kidney biopsy. Prognosis is variable, with some children progressing to chronic kidney disease. Other primary glomerulonephritides (e.g., membranous nephropathy, rare in children) present with nephrotic syndrome.

• Secondary nephrotic syndrome: systemic and infectious causes: Systemic lupus erythematosus (SLE), particularly lupus nephritis Class IV, presents with nephrotic syndrome in children with SLE, accompanied by systemic features (malar rash, photosensitivity, arthritis), positive antinuclear antibodies (ANA), anti-dsDNA, and anti-Smith antibodies. Infections including hepatitis B, hepatitis C, HIV, malaria, and syphilis can cause nephrotic syndrome through immune complex-mediated glomerulonephritis. Medications including NSAIDs (can trigger MCD or FSGS), ACE inhibitors/ARBs (paradoxically, in some populations), and ampicillin have been implicated. Malignancy-associated nephrotic syndrome is rare in children but occurs with Hodgkin lymphoma, lymphoma, and solid tumors. Allergic/anaphylactic reactions to foods, insect venoms, or medications can precipitate nephrotic syndrome. Congenital and hereditary conditions including congenital nephrotic syndrome (CNS), Alport syndrome (with progressive hematuria), and thin basement membrane disease present with nephrotic syndrome or persistent proteinuria.

• Nephrotic-range proteinuria and edema: cardinal presenting features: The hallmark of nephrotic syndrome is nephrotic-range proteinuria (≥3.5 g/day in older children, or urine protein-to-creatinine ratio ≥3.5 g/g, or ≥50 mg/kg/day in younger children). Children and parents typically seek medical attention because of edema, which develops insidiously over days to weeks as hypoalbuminemia and sodium retention accumulate. Edema is usually first noted in dependent areas: periorbital edema (puffiness around the eyes, often described by parents as "puffy face"), followed by ankle and lower extremity edema that worsens throughout the day. In more severe cases, edema becomes generalized and anasarca develops, with ascites, pleural effusions, and genital edema (scrotal edema in boys, labia and perineal edema in girls). Weight gain accompanies edema formation, typically 5-15 kg over the course of a few weeks, and parents often describe a recent viral illness or URI symptoms preceding onset by 1-2 weeks.

• Abdominal pain and gastrointestinal symptoms: Many children present with abdominal pain, attributed to intestinal edema and ascites. The pain is often diffuse, nonspecific, and may mimic acute abdomen, leading to unnecessary imaging or even surgical consultation. Diarrhea occurs in some children due to intestinal mucosal edema, while poor appetite and failure to thrive may accompany more severe disease. Scrotal pain can occur with severe scrotal edema. Rarely, spontaneous bacterial peritonitis develops in children with large ascites, presenting with fever and acute abdominal pain.

• Constitutional symptoms and infection: Many children present with anorexia, malaise, and fatigue related to hypoproteinemia and metabolic derangements. Importantly, children with nephrotic syndrome have an increased susceptibility to infection, particularly spontaneous bacterial peritonitis (SBP) caused by Streptococcus pneumoniae and other encapsulated organisms. This increased infection risk is attributed to loss of immunoglobulins (particularly IgG) in urine, loss of complement proteins (especially C3 and properdin), and impaired opsonization. Recurrent infections, cellulitis, and pneumonia may complicate the course.

• Respiratory symptoms: Dyspnea may develop acutely if pleural effusions accumulate (more common in nephrotic syndrome than in other childhood conditions causing pleural effusion) or if pulmonary edema develops from severe volume overload. Some children present with cough attributed to URI but actually related to pulmonary edema. Orthopnea

Initial testing

• Urine dipstick and microscopy: the first test. Dipstick shows 3+ to 4+ protein with a bland sediment; oval fat bodies and lipiduria produce the classic "Maltese cross" under polarized light. Dysmorphic RBCs or RBC casts point away from minimal change disease toward a nephritic process.
• Quantification of proteinuria: a first-morning urine protein-to-creatinine ratio is preferred over 24-hour collection in children. KDIGO's 2021 glomerular disease guideline frames pediatric nephrotic-range proteinuria in terms of a markedly elevated first-morning ratio or a timed excretion rate well above normal; the ratio also avoids false positives from orthostatic (postural) proteinuria, which resolves on a recumbent first-void specimen.
• Serum studies: albumin (low, the biochemical cornerstone), total cholesterol and triglycerides (elevated), creatinine and electrolytes, and total calcium (falsely low because it is albumin-bound — check ionized calcium).

Studies that sort primary from secondary disease

• C3 and C4: normal in minimal change disease. A depressed C3 redirects toward post-infectious glomerulonephritis, C3 glomerulopathy, or lupus nephritis.
• ANA, anti-dsDNA, hepatitis B and C serologies, HIV: obtained selectively, largely in older children, those with hematuria/hypertension, or atypical features.

Biopsy — the gold standard, but usually deferred

• In a child aged roughly 1–10 years with steroid-sensitive-appearing idiopathic nephrotic syndrome, normal complement, normal renal function, and no gross hematuria, KDIGO endorses empiric corticosteroids without biopsy because the pretest probability of minimal change disease is overwhelming.
• Biopsy is indicated for: infants under 1 year, adolescents/atypical presentations, gross hematuria, persistent hypertension, low C3, impaired GFR, or steroid resistance. Light microscopy is normal; immunofluorescence is negative ("nil disease"); electron microscopy shows diffuse podocyte foot process effacement.

Immediate stabilization (only if symptomatic)

• Sodium and fluid restriction: first-line for edema; most children need nothing more.
• IV albumin (25%) followed by a loop diuretic (furosemide): reserved for anasarca, symptomatic pleural effusion, scrotal/labial edema, or skin breakdown. Albumin transiently restores oncotic pressure so the diuretic can work. Diuretics given alone to an intravascularly depleted child risk hypovolemic shock, AKI, and thrombosis.

First-line therapy

• Corticosteroids: oral prednisone or prednisolone 60 mg/m²/day (max 60 mg) daily for 4–6 weeks, then 40 mg/m² every other day with taper over a total course of ~8–12 weeks, per KDIGO 2021. Roughly 90% of children with minimal change disease remit, most within the first 2 weeks — the response itself is diagnostic.
• Relapse: re-treat with daily corticosteroids until remission, then alternate-day taper.

Escalation

• Steroid-sparing agents for frequently relapsing or steroid-dependent disease (KDIGO 2021): calcineurin inhibitors (tacrolimus or cyclosporine), mycophenolate mofetil, cyclophosphamide, or rituximab (anti-CD20). Choice is driven by toxicity profile and desire to avoid cumulative steroid exposure.
• Steroid-resistant nephrotic syndrome: obtain kidney biopsy and genetic testing (NPHS1, NPHS2, WT1). KDIGO favors a calcineurin inhibitor as initial therapy; monogenic disease is generally immunotherapy-unresponsive.
• RAAS blockade (ACE inhibitor such as enalapril, or an ARB) as antiproteinuric therapy in steroid-resistant disease — not in steroid-sensitive relapsing disease.

Prevention and contraindications

• Pneumococcal immunization (conjugate plus polysaccharide) and annual influenza vaccine are recommended by the AAP/ACIP given encapsulated-organism risk from urinary IgG and complement loss.
• Live vaccines (MMR, varicella) are contraindicated during high-dose corticosteroid or other immunosuppressive therapy; give during remission off therapy.
• Avoid NSAIDs. Routine prophylactic antibiotics and routine anticoagulation are not recommended in children.
• ACE inhibitors are contraindicated in pregnancy — relevant when counseling adolescent patients.

Infectious — the leading cause of death

• Spontaneous bacterial peritonitis (emergency): urinary loss of IgG, factor B, and properdin impairs opsonization of encapsulated organisms; ascitic fluid is a culture medium. Streptococcus pneumoniae and E. coli predominate. Signals: fever, diffuse abdominal pain, and tenderness in an edematous child — diagnostic paracentesis and empiric third-generation cephalosporin (ceftriaxone).
• Cellulitis, pneumonia, and bacteremia: same opsonic defect, compounded by immunosuppressive therapy.

Thrombotic

• Venous thromboembolism (emergency): urinary loss of antithrombin III, hemoconcentration from diuresis, thrombocytosis, and increased hepatic fibrinogen synthesis create a hypercoagulable state. Renal vein thrombosis is the classic pediatric example — sudden flank pain, gross hematuria, worsening proteinuria, and a palpable kidney. Pulmonary embolism and cerebral sinus venous thrombosis also occur.

Hemodynamic and metabolic

• Hypovolemia and prerenal AKI (emergency): precipitated by aggressive diuresis or sepsis despite total-body volume overload; look for tachycardia, delayed capillary refill, and rising creatinine.
• Hyperlipidemia, hypothyroidism (loss of thyroxine-binding globulin), vitamin D deficiency (loss of vitamin D–binding protein), and iron-resistant anemia (transferrin loss) all follow from nonselective urinary protein wasting.

Treatment-related

• Corticosteroids: linear growth suppression, cushingoid habitus, hypertension, hyperglycemia, cataracts, osteopenia, behavioral change, and adrenal suppression — the reason steroid-sparing agents exist.
• Calcineurin inhibitors: chronic nephrotoxicity with interstitial fibrosis, hypertension, hyperkalemia, tremor; gingival hyperplasia and hirsutism with cyclosporine, new-onset diabetes with tacrolimus.
• Cyclophosphamide: gonadal toxicity/infertility, hemorrhagic cystitis, myelosuppression, later malignancy risk.
• Rituximab: infusion reactions, prolonged hypogammaglobulinemia, hepatitis B reactivation, and rare progressive multifocal leukoencephalopathy.
• Progression to CKD: seen with steroid-resistant FSGS and genetic disease, not with steroid-sensitive minimal change disease.

• The prototype stem: a 3–6-year-old boy with periorbital edema on waking, frothy urine, 4+ proteinuria, normal blood pressure, normal creatinine, and normal C3/C4. The single best next step is empiric prednisone, not kidney biopsy — biopsy is the classic distractor.
• Buzzwords for minimal change disease: normal light microscopy, negative immunofluorescence ("nil disease", lipoid nephrosis), and diffuse foot process effacement on electron microscopy. In urine, oval fat bodies with a "Maltese cross" under polarized light.
• The complement question separates the syndromes: normal C3 → minimal change disease; low C3 → post-streptococcal glomerulonephritis, C3 glomerulopathy, or lupus nephritis. Hematuria, RBC casts, hypertension, and azotemia mean nephritic, not nephrotic.
• The association examiners love: minimal change disease with Hodgkin lymphoma (paraneoplastic, cytokine-mediated) and with NSAIDs. Focal segmental glomerulosclerosis with HIV, obesity, sickle cell disease, and heroin use.
• Renal vein thrombosis: sudden flank pain, gross hematuria, and a palpable flank mass in a nephrotic child. Mechanism is urinary antithrombin III loss — this is the single most tested laboratory-to-clinical link in the topic.
• Fever plus diffuse abdominal pain in a child with ascites = spontaneous bacterial peritonitis until proven otherwise; S. pneumoniae is the classic organism. Paracentesis first, then ceftriaxone.
• Failure to remit after an adequate daily corticosteroid course defines steroid resistance — now biopsy, and expect FSGS or a podocyte gene mutation (NPHS1, NPHS2, WT1). Nephrotic syndrome in the first 3 months of life is congenital (Finnish type, NPHS1) and will not respond to steroids.
• Do not give live vaccines during high-dose immunosuppression, and do not reflexively anticoagulate or give prophylactic antibiotics to every nephrotic child — neither is routine in pediatrics.