1. Fenestrated endothelium (innermost, blood side)

   • Endothelial cells with fenestrae (pores ~70 nm). These are lined by a glycocalyx (negatively charged).
   • Allows passage of water and small solutes but repels large proteins partly by charge.

2. Glomerular basement membrane (GBM) (middle layer)

   • A dense, acellular matrix of type IV collagen, laminin, and heparan sulfate proteoglycans.
   • Acts as both a size barrier and a charge barrier (negative charge from heparan sulfate).

3. Podocytes with foot processes and slit diaphragms (outermost, urinary side)

   • Podocytes ("podo" = foot, "cyte" = cell) are highly specialised epithelial cells.
   • They extend interdigitating foot processes that wrap around capillaries.
   • Between adjacent foot processes lies the slit diaphragm — a zipper-like structure composed of proteins such as nephrin, podocin, and CD2AP. This is the final and most critical barrier to protein filtration.

In MCD, the primary insult is to the podocyte foot processes. When podocytes are injured:

• Foot processes retract, flatten, and fuse → effacement
• The slit diaphragm is disrupted → loss of the final size-selective barrier
• Additionally, the negative charge barrier (from podocyte surface glycoproteins like podocalyxin) is lost
• Result: massive selective proteinuria, predominantly albumin (because albumin is relatively small at ~66 kDa and negatively charged — loss of charge barrier allows it through preferentially)

Idiopathic MCD is the most common cause ^[1][2]. No identifiable secondary trigger is found. This is especially true in children aged 2–6 years.

Secondary causes of MCD ^[1][2]:

The pathogenesis of MCD is unclear but probably related to systemic T-cell dysfunction ^[1][2].

Here is the current understanding, explained step by step:

1. T-cell dysregulation: MCD is not a classic antibody-mediated or immune-complex disease (IF is negative — no immunoglobulin deposits). Instead, it is believed to involve aberrant T lymphocytes (possibly of immature, undifferentiated CD34+ stem cell origin) ^[1][2].

2. Circulating permeability factor: These dysfunctional T cells are thought to produce a "glomerular permeability factor" — a soluble mediator (cytokine or lymphokine) that alters glomerular permeability ^[1][2].

   • Candidate mediators include: IL-13, IL-8, hemopexin, cardiotrophin-like cytokine-1 (CLC-1), soluble urokinase plasminogen activator receptor (suPAR), and angiopoietin-like 4 (ANGPTL4).
   • None have been definitively proven as "the" factor.

3. Podocyte injury: The permeability factor damages podocytes:

   • Effacement of foot processes — the podocyte cytoskeleton (actin filaments) reorganises, foot processes flatten and fuse ^[1][2][3]
   • Disruption of the slit diaphragm — nephrin and podocin expression is downregulated
   • Loss of the negative charge barrier — podocalyxin and other anionic glycoproteins are lost from the podocyte surface

4. Consequence → massive proteinuria: With the slit diaphragm disrupted and charge barrier lost, albumin (and other proteins) leak freely into the urine.

This is a key conceptual point. Glucocorticoids are potent immunosuppressants that suppress T-cell activation and cytokine production. If MCD is driven by T-cell-derived permeability factors, then:

• Steroids suppress the aberrant T cells → ↓ permeability factor → podocytes recover → foot processes reform → proteinuria resolves.
• This explains the excellent steroid responsiveness (> 90% in children, ~80–85% in adults) ^[3].
• It also explains why MCD frequently relapses when steroids are tapered — the underlying T-cell abnormality returns.

This classification is critical in clinical practice, especially in children ^[4]:

Steroid-sensitive nephrotic syndrome (SSNS): MCD is the prototypical SSNS ^[1].

Some authorities consider MCD and primary FSGS as two ends of the same disease spectrum ^[1][2]:

• LM shows normal glomerular appearance ^[1][2][3]
• This is why the disease is called "minimal change" — under the standard light microscope, nothing looks wrong
• Tubules may show lipid droplets (from lipiduria) and protein reabsorption droplets — but these are non-specific

• EM reveals diffuse effacement (fusion) of epithelial foot processes ^[1][2][3]
• Retraction, widening, and shortening of foot processes ^[1][2]
• The foot processes lose their normal interdigitating architecture and become flattened, broad, and fused
• The slit diaphragms are obliterated
• EM is the only imaging modality that can visualise the histopathological changes in MCD ^[6]

• IF shows no immunoglobulin or complement deposition ^[1][2][3]
• This distinguishes MCD from immune-complex-mediated glomerulonephritides (e.g., membranous nephropathy, IgA nephropathy, lupus nephritis)

Summary: LM = normal, EM = foot process effacement, IF = negative — this triad is pathognomonic for MCD ^[1][2][3][4].

• Dipstick: Protein 3–4+; RBC usually negative or trace
• 24-hour urine protein or urine protein:creatinine ratio (uPCR): Nephrotic range (> 3.5 g/day in adults; > 40 mg/h/m² in children)
• Urine microscopy: Bland sediment — oval fat bodies, fatty casts; NO dysmorphic RBCs, NO RBC casts (these would suggest nephritic disease)
• Selectivity index: Can formally measure using the ratio of IgG clearance to transferrin clearance. A highly selective proteinuria (selectivity index < 0.2) suggests MCD

• Serum albumin: Low (< 30 g/L, often < 20 g/L)
• Lipid profile: ↑ Total cholesterol, ↑ LDL, ↑ TG, ↓ HDL
• Renal function: Serum creatinine may be modestly elevated in 30–40% ^[1][2]
• CBC: May show ↑ Hb (haemoconcentration from intravascular volume depletion)
• Immunological workup (in adults): ANA, anti-dsDNA, complement levels (C3, C4), anti-PLA2R (to exclude other causes)
• HBV/HCV serology (important in Hong Kong)

In children:

• Usually clinical diagnosis → empirical glucocorticoids without biopsy ^[1][2]
• Biopsy is indicated if: atypical features (e.g. age < 1 or > 12 years, gross haematuria, low C3, hypertension, renal impairment, steroid resistance) ^[4]

In adults:

• Renal biopsy is generally required because MCD is less common (only 10–15% of adult nephrotic syndrome), and other causes (membranous nephropathy, FSGS, diabetic nephropathy, amyloidosis) must be excluded ^[2][4]
• Exception: if anti-PLA2R is strongly positive → clinical diagnosis of membranous nephropathy possible without biopsy

Age-stratified causes of nephrotic and nephritic syndrome ^[4]:

In a child aged 2–6 with typical nephrotic syndrome, MCD is so overwhelmingly likely (> 90%) that empirical steroids are given without biopsy ^[1][2][4]. In an adult, MCD is only 10–15%, so you must consider the broader differential and usually need a renal biopsy.

Classification of glomerulonephritis by proliferative vs non-proliferative, and primary vs secondary ^[8]:

Primary FSGS may be misdiagnosed as MCD if biopsy does not hit the pathological glomeruli ^[1][2].

Why FSGS can mimic MCD:

• Early FSGS affects only some glomeruli ("focal" = < 50% of glomeruli) and only parts of those glomeruli ("segmental"). If your biopsy needle samples a region of the kidney without sclerotic glomeruli, the LM looks completely normal — indistinguishable from MCD.
• Both MCD and FSGS show foot process effacement on EM.
• Both present with nephrotic syndrome.

Some consider primary FSGS and MCD as part of a spectrum of disease caused by glomerular permeability factors. FSGS patients may have defective glomerular repair mechanisms, leading to progressive sclerosis. On one end, MCD is very steroid responsive, associated with minimal glomerular changes, and usually does not progress into ESRD. On the other end, FSGS is not so steroid-responsive (only in ~1/2), associated with focal segmental sclerotic changes, and may result in progressive renal failure if severe proteinuria is persistent ^[1][2].

MCD Variants that can also cause confusion:

• MCD variants (e.g. IgM nephropathy, C1q nephropathy): usually more often associated with haematuria but may also show minimal LM findings on biopsy ^[1]. These have mesangial IgM or C1q deposits on IF respectively, but LM may look near-normal.

Membranous nephropathy is the most common cause of nephrotic syndrome in adults ^[4].

Why it enters the DDx:

• Both cause nephrotic syndrome with heavy proteinuria.
• Both are non-proliferative GN (no cellular proliferation on LM).

How to distinguish from MCD:

MPGN can occasionally present with nephrotic-range proteinuria, though it more classically presents as a nephritic–nephrotic overlap. It shows both GBM thickening and mesangial proliferation on LM ("tram-tracking" of GBM due to mesangial interposition), and granular deposits on IF. Complement (C3) is typically low, unlike MCD where complement is normal. MPGN is associated with HBV, HCV, autoimmune disease, and paraproteinaemia.

Diabetic nephropathy is the most common cause of renal impairment (51%) ^[9].

Why it enters the DDx: Diabetic nephropathy causes nephrotic-range proteinuria. In a diabetic patient with oedema and heavy proteinuria, the default assumption is diabetic nephropathy.

How to distinguish from MCD: The key is to look for red flags for non-diabetic nephropathy ^[9]:

• Gross haematuria
• Sudden drop / rapid reduction in GFR
• Short duration / recent diagnosis of diabetes (e.g. 2–3 years) — need 15–20 years for T2DM to cause nephropathy
• Absence of other microvascular complications (no diabetic retinopathy)

If any of these red flags are present → suspect a superimposed glomerular disease (including MCD) and consider renal biopsy.

Amyloidosis (AL or AA type) causes nephrotic syndrome by amyloid fibril deposition in the mesangium and along the GBM. Distinguished by Congo red staining showing apple-green birefringence under polarised light on LM. Clinical clues include macroglossia, periorbital purpura, carpal tunnel syndrome, hepatomegaly, restrictive cardiomyopathy — features absent in MCD.

In an elderly patient with nephrotic syndrome and AKI, the differential includes light and heavy chain deposition disease and amyloidosis as complications of monoclonal gammopathy ^[10]. Myeloma cast nephropathy classically causes AKI through tubular obstruction by casts, but light chains can also deposit in the glomerulus causing nodular glomerulosclerosis. Clues: monoclonal band on serum protein electrophoresis, abnormal immunoglobulin pattern, bone pain, anaemia, hypercalcaemia.

Lupus nephritis affects ~50% of SLE patients and can present as nephrotic syndrome (particularly class V — membranous pattern) or nephritic syndrome (class III/IV) ^[11]. Distinguished by:

• Systemic features: malar rash, arthralgia, oral ulcers, photosensitivity, alopecia
• Serological markers: ANA+, anti-dsDNA+, low C3/C4
• Renal biopsy: "full house" IF (IgG, IgA, IgM, C3, C1q all positive) — in stark contrast to MCD's negative IF

When a patient presents with nephrotic syndrome, urine no RBC and AKI, some eosinophilia — a top differential is NSAID-induced nephrotic syndrome, which comprises minimal change glomerulonephropathy + acute tubulointerstitial nephritis, with T-lymphocytes and eosinophils (40%), and is higher risk in the elderly ^[12].

This is a critical differential because:

• The glomerular component looks exactly like MCD (normal LM, foot process effacement on EM).
• But there is a concurrent tubulointerstitial nephritis (TIN) causing AKI, which you would not expect in pure MCD.
• Clue: peripheral eosinophilia, AKI out of proportion to the nephrotic state, history of NSAID use.
• The combination of MCD + AIN from NSAIDs is a classic exam scenario ^[12].

Before diagnosing nephrotic syndrome, non-renal causes of generalised oedema should be considered:

In children, MCD is usually a clinical diagnosis → empirical glucocorticoids ^[1][2][4].

Suggestive features (supporting empirical steroid trial without biopsy) ^[1]:

If all of these features are present → the child is treated as presumptive MCD with steroids. The diagnosis is confirmed retrospectively by the treatment response: complete remission within 4–8 weeks of steroids = steroid-sensitive nephrotic syndrome (SSNS), which is almost always MCD ^[4].

In adults, renal biopsy is generally required because the differential is broad ^[2][4].

The definitive diagnostic criteria for MCD on renal biopsy are the histological triad:

"Light microscopy demonstrates essentially normal glomerular morphology, hence the term minimal change. Immunofluorescence is usually negative for immunoglobulins. The only detectable abnormality is the total effacement of foot processes (podocytes) of visceral epithelial cells shown on ultrastructural examination by electron microscopy." ^[15]

Investigations to order when working up a case of potential nephrotic syndrome ^[6]:

Renal biopsy is still essential for definitive diagnosis of a number of renal diseases, including glomerulonephritis — e.g. minimal change disease, FSGS, IgAN, membranous GN, membranoproliferative GN ^[18].

When is renal biopsy indicated in MCD?

What does the biopsy involve?

Diagnosis of renal diseases requires ^[18][19]:

• Clinical history and manifestation (oliguria, haematuria, proteinuria, oedema, hypertension, loin pain)
• Basic laboratory investigations (blood RFT; urine microscopy; urine protein quantification)
• Renal biopsy — typically percutaneous under USG guidance. The tissue is processed for three types of examination:

When a patient presents with nephrotic syndrome, urine no RBC and AKI, some eosinophilia → a top differential is NSAID-induced nephrotic syndrome, which comprises: minimal change glomerulonephropathy + acute tubulointerstitial nephritis, with T-lymphocytes and eosinophils (40%), and is higher risk in the elderly ^[12].

The biopsy in this scenario shows:

• Glomeruli: Normal on LM, negative IF, foot process effacement on EM (= MCD pattern)
• Tubulointerstitium: Intense inflammation with T-lymphocyte and eosinophil infiltration (= AIN)
• The coexistence of both is the diagnostic hallmark of NSAID-induced disease

Drugs associated with MCD: NSAIDs, lithium, bisphosphonates ^[12].

If a child is treated empirically for MCD but fails to respond after 4 weeks of full-dose prednisolone:

• The most important concern is that the child actually has FSGS, not MCD
• Primary FSGS may be misdiagnosed as MCD if biopsy does not hit the pathological glomeruli ^[1][2]
• A renal biopsy (or repeat biopsy) with adequate glomeruli sampled is mandatory
• The biopsy should sample the juxtamedullary region (where FSGS lesions preferentially begin)

"In elderly subjects, the onset of MCNS should arouse the suspicion of a possible underlying malignancy, particularly that of lymphoid origin, e.g. Hodgkin's disease" ^[15].

Additional investigations in the elderly:

• CT chest/abdomen/pelvis (lymphadenopathy, mass)
• PET-CT if lymphoma suspected
• Bone marrow biopsy if haematological malignancy suspected
• LDH, β2-microglobulin (lymphoma markers)
• Tumour screening: CXR, stool occult blood, other relevant markers ^[6]

Monitoring of blood pressure (BP) and body weight (BW) ^[3]:

• Daily body weight — aim for weight loss of ~1 kg/day when treating oedema ^[4]. Faster losses risk hypovolaemia.
• Daily intake/output (I/O) charting ^[4]
• Daily vitals including BP
• Home urine dipstick monitoring ^[3] — patients and parents are taught to check urine protein at home to detect relapses early. This is particularly important in children who relapse frequently.

Na⁺ restriction to 2–3 mEq/kg/day (maximum = 2 g/day) ^[3]:

• Why: Sodium retention (via RAAS activation from underfilling + intrinsic tubular retention) is a key driver of oedema. Restricting sodium intake reduces the substrate for fluid retention.

Fluid restriction (~50% of maintenance) can be considered but generally not needed ^[3]:

• Usually reserved for severe hyponatraemia or refractory oedema.

Nutritious and relatively low-fat diet is recommended in patients on steroids ^[3]:

• Why: Steroids increase appetite and cause weight gain; a healthy diet counteracts this. High-fat diets worsen the hyperlipidaemia already present from nephrotic syndrome.

Note that protein restriction is NOT recommended due to heavy urinary protein loss — should have normal protein intake as ↑ albumin excretion is associated with poorer outcomes ^[20]. This is counter-intuitive: you might think restricting protein reduces proteinuria, but it actually worsens malnutrition and albumin levels in a patient who is already losing massive amounts of protein in the urine.

Furosemide ± Spironolactone ^[3][4][20]:

Management of resistant oedema / anasarca ^[4]:

Why does gut wall oedema cause frusemide malabsorption? Because frusemide is an oral medication absorbed in the GI tract. If the intestinal wall is oedematous (from low oncotic pressure), the drug cannot cross the mucosa efficiently → bioavailability drops → inadequate diuresis. Switching to IV bypasses this problem entirely.

IV albumin infusion can be used as adjunct to diuretics only for diuretic-resistant + oliguria/uraemia in absence of severe glomerular damage ^[20]. The rationale: albumin transiently raises plasma oncotic pressure, pulling fluid back intravascularly, where frusemide can then act on it. However, the infused albumin is rapidly lost in the urine in severe nephrotic syndrome, so the effect is temporary.

Anti-proteinuric therapy by angiotensin inhibitor (ACEI/ARB): indicated in ALL glomerulopathies ^[4][20].

Lipid-lowering drugs by statins (drug of choice): should be considered if hyperlipidaemia persists after treatment of underlying disorder (by immunosuppressive Tx) and/or ACEI/ARB ^[20].

Why not treat immediately? Because if the underlying disease (MCD) responds to steroids and proteinuria resolves, the hyperlipidaemia will correct itself as hepatic synthetic machinery returns to normal. Statins are reserved for persistent hyperlipidaemia.

Anti-thrombotic therapy by anticoagulants and ↓ immobilisation ^[20]:

• Pneumococcal vaccination: indicated for ALL as pneumococcal infection is common ^[20]
• Why: urinary loss of IgG and complement predisposes to encapsulated organism infections, especially Streptococcus pneumoniae → SBP in children, pneumonia in adults

1st line: high-dose prednisolone (1 mg/kg) × 4–8 weeks ^[4].

This is the cornerstone of MCD treatment. The rationale is straightforward: MCD is driven by T-cell dysfunction producing a circulating permeability factor. Glucocorticoids are potent immunosuppressants that:

1. Suppress T-cell activation and cytokine production → ↓ permeability factor
2. Have direct podocyte-stabilising effects → upregulate nephrin expression and stabilise the actin cytoskeleton
3. Result: podocytes recover → foot processes reform → slit diaphragm reconstituted → proteinuria resolves

Regimen details:

Overall prognosis is good. Good response to steroids ^[3][15].

Steroid contraindications (relative):

• Uncontrolled diabetes (steroids worsen glycaemia)
• Active peptic ulcer disease (add PPI prophylaxis)
• Active untreated infection (steroids immunosuppress)
• Severe osteoporosis (add bisphosphonate + calcium/vitamin D)
• Psychiatric vulnerability (monitor closely)

Clinical course definitions ^[4]:

Indicated when: the patient is frequently relapsing, steroid-dependent, or experiencing unacceptable steroid side effects (e.g. growth retardation in children, Cushing's). The goal is to maintain remission while reducing cumulative steroid exposure.

2nd line: cyclophosphamide, cyclosporine ^[4]:

Steroid resistant (10%): persistent proteinuria despite full-dose prednisolone × 4 weeks → repeat renal biopsy ^[4].

The first and most critical step is to repeat the renal biopsy — because the most common reason for "steroid-resistant MCD" is actually misdiagnosed FSGS (sampling error on the original biopsy).

If re-biopsy confirms true MCD:

• Calcineurin inhibitor (ciclosporin or tacrolimus) is the treatment of choice ± low-dose prednisolone
• If CNI fails → consider rituximab
• If all fail → consider experimental agents (e.g. ACTH analogues, voclosporin)

If MCD is secondary to an identifiable cause, the primary treatment is removing the offending trigger:

Prothrombotic state from increased hepatic clotting factor synthesis and urinary antithrombin III loss predisposing to deep vein and renal vein thrombosis ^[5].

Pathophysiology — explained from first principles:

The liver responds to the drop in plasma oncotic pressure (from hypoalbuminaemia) by ramping up protein synthesis non-specifically. This increased synthetic drive produces not only lipoproteins (causing hyperlipidaemia) but also clotting factors — particularly fibrinogen and factors V, VII, VIII, and X. Simultaneously, the damaged glomerular barrier allows urinary loss of anticoagulant proteins — most importantly antithrombin III (AT III, a small ~58 kDa protein that is readily filtered) and protein S. The net result is a gross imbalance: pro-coagulant factors are elevated while natural anticoagulants are depleted.

Additional pro-thrombotic factors:

• Thrombocytosis — reactive; thrombopoietin production increases with hypoalbuminaemia
• Platelet hyperaggregability — hyperlipidaemia and hypoalbuminaemia alter platelet membrane composition
• Hyperviscosity — haemoconcentration from intravascular volume depletion (underfill)
• Immobility — bed-bound oedematous patients with reduced physical activity

Management ^[4]:

Infection risk from immunoglobulin loss and complement dysfunction ^[5].

Pathophysiology:

Why are nephrotic patients immunocompromised? Three reasons:

1. Urinary loss of immunoglobulins (especially IgG) — IgG (~150 kDa) is relatively large but still leaks through the severely damaged glomerular barrier. IgG is the principal opsonising antibody → its depletion impairs humoral immunity against encapsulated bacteria.
2. Urinary loss of complement factors — particularly factor B and factor D of the alternative complement pathway (these are small proteins readily lost in urine). Complement is critical for bacterial killing.
3. Oedematous tissue acts as a culture medium — fluid-laden subcutaneous tissue and peritoneal fluid provide an excellent environment for bacterial growth.
4. Immunosuppressive therapy (steroids, cyclophosphamide, CNI, rituximab) — further suppresses both humoral and cellular immunity.

Key infections in MCD:

Modestly ↑ serum Cr in 30–40% ± AKI ^[1][2].

AKI in MCD can arise through multiple mechanisms:

Hyperlipidaemia from increased hepatic LDL and VLDL synthesis with urinary HDL loss ^[5].

Pathophysiology:

• ↓ Plasma oncotic pressure → liver compensatory ↑ synthesis of all proteins/lipoproteins → ↑ VLDL, ↑ LDL production
• ↓ Lipoprotein lipase activity → impaired catabolism of circulating lipoproteins
• Urinary loss of HDL (small, ~200 kDa) → ↓ HDL
• Result: atherogenic lipid profile — ↑ total cholesterol, ↑ LDL, ↑ TG, ↓ HDL

Clinical significance:

• In MCD, hyperlipidaemia is usually transient and resolves once proteinuria remits with steroid therapy
• However, in patients with frequently relapsing or steroid-dependent MCD who remain nephrotic for prolonged periods, chronic hyperlipidaemia contributes to accelerated atherosclerosis and cardiovascular disease ^[4]
• CV disease: long-term complications → CV risk modifications ^[4]

Management:

• If lipids normalise after treatment → no specific lipid-lowering needed
• If hyperlipidaemia persists despite treatment → statins (first-line)
• Lifestyle: diet, exercise, smoking cessation

Resistant oedema / anasarca ^[4]:

Why does "sequential nephron blockade" work? Chronic loop diuretic use causes compensatory Na⁺ reabsorption in the distal convoluted tubule (DCT) via hypertrophy of DCT cells. Adding a thiazide blocks this compensatory mechanism. Similarly, aldosterone-driven Na⁺ reabsorption in the collecting duct can be blocked by spironolactone. By hitting three segments of the nephron simultaneously (loop + DCT + collecting duct), you maximise natriuresis.

• Heavy proteinuria → ongoing urinary loss of albumin and other plasma proteins → negative nitrogen balance
• If nephrotic state is prolonged or frequently relapsing → protein malnutrition, muscle wasting
• In children: growth retardation from both chronic protein loss AND repeated courses of high-dose steroids (steroids suppress growth hormone axis and linear growth)
• This is one of the main reasons steroid-sparing agents are used in FRNS/SDNS children

• Thyroxine-binding globulin (TBG) is lost in urine (along with bound T4) → ↓ total T4
• Free T4 is initially maintained by increased TSH drive, but with very severe/prolonged nephrotic syndrome, functional hypothyroidism can develop
• Screen with TSH in patients with prolonged nephrotic state, fatigue, or unexplained symptoms

• 25-hydroxyvitamin D is bound to vitamin D-binding protein (DBP) → both are lost in urine
• → ↓ 25-OH vitamin D → ↓ 1,25-dihydroxyvitamin D (active form)
• → ↓ intestinal calcium absorption → secondary hyperparathyroidism → bone resorption
• Compounded by steroid therapy (which further suppresses osteoblasts and intestinal calcium absorption)
• Clinical: osteopenia, pathological fractures, rickets (children)
• Management: vitamin D supplementation + calcium; bone densitometry if prolonged steroid use

• Transferrin (~80 kDa) is readily lost in urine → ↓ serum iron transport → functional iron deficiency → microcytic hypochromic anaemia
• May compound the relative anaemia from haemodilution (if fluid-overloaded) or AKI

These are critically important because most MCD patients receive multiple steroid courses: