Nephrotic Syndrome

Definition

Pathophysiology

The Tetrad of Features

Mechanisms of Nephrotic Edema

• Underfill: Hypoalbuminemia lowers plasma oncotic pressure → fluid shifts to interstitium → reduced plasma volume → RAAS activation → aldosterone-driven Na+ retention → worsens edema
• Overfill (more common): Primary defect in distal nephron Na+ excretion (likely ENaC activation by proteolytic enzymes in tubular fluid) → Na+ and water retention → expanded plasma volume, suppressed RAAS

Fig. 16.9 - Mechanisms of Nephrotic Edema (Comprehensive Clinical Nephrology, 7th Ed.)

Causes

Primary (Idiopathic) Glomerular Diseases

Secondary Causes

• Diabetes mellitus - Kimmelstiel-Wilson nodular glomerulosclerosis; most common secondary cause
• Lupus (SLE) - can mirror membranous or other patterns (lupus nephritis class V for nephrotic)
• Preeclampsia - nephrotic-range proteinuria + hypertension in pregnancy
• Infections - Hepatitis B (membranous pattern), hepatitis C, HIV, malaria, infective endocarditis
• Malignancy - Hodgkin lymphoma (classically associated with MCD), carcinomas (membranous)
• Drugs - Gold, penicillamine (membranous), heroin, NSAIDs, captopril
• Amyloidosis - AL or AA amyloid deposited in glomeruli; Congo red staining with apple-green birefringence

Clinical Features

Symptoms

• Foamy urine (frothy due to proteinuria)
• Peripheral edema - often worse in the morning, periorbital edema (especially in children); may progress to anasarca (generalized severe edema including ascites, scrotal/vulvar edema, pleural effusions)
• Fatigue, dyspnea, orthopnea
• Nausea/vomiting (from bowel wall edema or ascites)

Signs

• Hypertension, pitting edema
• Ascites, pleural effusions
• Muehrcke's lines - white transverse bands on nails (hypoalbuminemia sign)
• Eruptive xanthomata, xanthelasma (hyperlipidemia)

Laboratory Findings

• Proteinuria >3.5 g/day (or spot urine protein:creatinine ratio >3.5)
• Serum albumin <2.5-3 g/dL
• Total cholesterol usually >180 mg/dL; raised LDL and triglycerides
• Lipiduria on urinalysis (fatty casts, oval fat bodies)
• Serum creatinine variable

Complications

1. Thromboembolism (up to 40% VTE risk)

• Loss of anticoagulant proteins in urine: antithrombin III, proteins C and S
• Increased hepatic synthesis of procoagulant proteins (fibrinogen, factors V, VIII)
• Hyperlipidemia causes hyperviscosity; fibrinolytic inhibitors increase clot risk
• Most common sites: renal vein thrombosis, DVT, pulmonary embolism
• Arterial thrombosis also possible

2. Infection

• Loss of IgG and complement factors in urine
• Reduced T-cell function
• Increased risk of cellulitis, spontaneous bacterial peritonitis, pneumococcal infections
• Steroid therapy adds further immunosuppression

3. Atherosclerosis / Cardiovascular Disease

• Prolonged hyperlipidemia accelerates vascular disease

4. Protein malnutrition

• Loss of vitamin D-binding protein → vitamin D deficiency, secondary hyperparathyroidism
• Loss of transferrin → iron-deficiency anemia
• Loss of hormone-binding proteins → thyroid/hormonal disturbances

Diagnosis

1. Urinalysis with microscopy: proteinuria, lipiduria, fatty casts, oval fat bodies (no RBC casts - those suggest nephritic)
2. 24-hour urine or spot urine protein:creatinine ratio to quantify proteinuria
3. Serum: albumin, lipid panel, creatinine, BUN, electrolytes, CBC
4. Complement (C3, C4, CH50) - low in MPGN, lupus nephritis
5. Serologies: ANA, anti-dsDNA (SLE), anti-PLA2R (primary membranous), SPEP (amyloid), hepatitis B/C serology, HIV
6. Renal biopsy - indicated in adults (most cases); children usually treated empirically for MCD first

Treatment

General / Supportive

Disease-Specific Treatment

KDIGO Pediatric Protocol (2021)

• Initial episode: Prednisone 60 mg/m²/day (max 60 mg/day) for 4-6 weeks, then 40 mg/m² on alternate days for another 4-6 weeks
• Relapse: Prednisone until remission, then taper
• Frequent relapsers/steroid-dependent: Calcineurin inhibitors (cyclosporine, tacrolimus) or mycophenolate mofetil; rituximab for refractory cases

Key Distinctions: Nephrotic vs. Nephritic

Recent Evidence Note

SGLT2 Inhibitors in Nephrotic Syndrome Progression

Background: What SGLT2 Inhibitors Do in the Kidney

Core Mechanisms of Renoprotection

Evidence Base: From Diabetic to Non-Diabetic Proteinuric Disease

Landmark Trials

DAPA-CKD: FSGS Pre-specified Subgroup

Cutting-Edge 2026 Meta-Analysis Evidence

1. Neuen et al., JAMA 2026 (PMID 41203232)

• SGLT2 inhibitors reduced CKD progression risk by 38% overall (HR 0.62, 95% CI 0.57-0.68) across 70,361 participants
• Benefit was preserved regardless of baseline eGFR - including stage 4 CKD (eGFR <30): HR 0.71
• Benefit was preserved regardless of albuminuria level - including those with >300 mg/g (nephrotic-range proteinuria): HR 0.57 (95% CI 0.52-0.64)
• Annual eGFR decline was slowed across all UACR subgroups, in both diabetic and non-diabetic patients

2. Staplin et al., JAMA 2026 (PMID 41202026)

• Benefits on kidney disease progression were clear in both diabetic and non-diabetic patients (HR 0.65 vs. 0.74)
• Absolute benefits were largest in those with UACR ≥200 mg/g - the heavy proteinuria range that overlaps with nephrotic syndrome
• Benefits also extended to reduced AKI risk, hospitalizations, and mortality

Specific Relevance to Nephrotic Syndrome Causes

Diabetic Nephropathy (most common secondary NS)

IgA Nephropathy

FSGS

Minimal Change Disease / Primary NS

Practical Role in the Nephrotic Syndrome Management Algorithm

Nephrotic Syndrome
       |
       ├─ Identify underlying cause (biopsy, serologies)
       |
       ├─ ALL patients with persistent proteinuria:
       |    ├─ RAAS blockade (ACE-i or ARB) ← first line
       |    └─ SGLT2 inhibitor ← now standard adjunct if eGFR ≥20
       |         (particularly if UACR still >200 mg/g on RAAS alone)
       |
       ├─ Diabetic nephropathy: SGLT2i + RAAS + finerenone
       ├─ FSGS/IgA: SGLT2i + RAAS ± specific immunosuppression
       └─ MCD: Steroids first; SGLT2i not established

Key Practical Points

• eGFR dip: An initial acute drop of ~2-3 mL/min/1.73m² is expected and acceptable - this reflects the TGF-mediated afferent arteriole constriction and does not indicate harm. The long-term chronic slope benefit is preserved.
• SGLT2i + RAAS combination is additive - each reduces intraglomerular pressure via different mechanisms
• Volume effects: SGLT2i-driven natriuresis can exacerbate volume depletion in underfill-type nephrotic syndrome (particularly MCD) - monitor closely
• Furosemide interaction: The natriuretic effect of SGLT2i may reduce diuretic requirements; dose adjustment may be needed
• Infections: Genital mycotic infections are an adverse effect; already immunocompromised nephrotic patients require vigilance

Summary

• Strongest evidence: Diabetic nephropathy, IgA nephropathy, FSGS with CKD
• Mechanism: Primarily hemodynamic (TGF-mediated reduction in glomerular hyperfiltration) + anti-fibrotic + metabolic
• Not yet established: Primary MCD, MPGN, membranous nephropathy as dedicated indications
• Current guideline position: KDIGO 2024 recommends SGLT2 inhibitors for CKD with eGFR ≥20 and UACR ≥200 mg/g, regardless of diabetes status - which captures the majority of patients with clinically significant nephrotic syndrome

Sickle Cell Disease

Definition and Genetics

• Globally affects millions, particularly in sub-Saharan Africa, the Middle East, India, and the Mediterranean
• In the United States, ~8% of people of African descent are HbS carriers; ~1 in 600 have sickle cell anemia (HbSS)
• The HbS allele is maintained by selective pressure because heterozygous carriers are protected against falciparum malaria

Genotypes and Severity

Pathophysiology

Step 1: HbS Polymerization

• Sickling is initially reversible on reoxygenation
• Repeated cycles of sickling cause cumulative membrane damage (Ca²+ influx, K+ and water loss) → irreversibly sickled cells prone to hemolysis

• High intracellular HbS concentration
• Low pH (acidosis)
• High temperature
• Slow transit through the microvasculature (delayed deoxygenation)

• Presence of HbF or HbA (dilutes HbS, weak interaction)
• Good hydration (reduces intracellular HbS concentration)

FIGURE 149-2. Pathophysiology of SCD (Goldman-Cecil Medicine) - showing the β6 mutation, HbS polymerization, cell heterogeneity, hemolysis, vaso-occlusion, and NO-scavenging cascade

Step 2: Two Downstream Cascades

A. Hemolysis

• Extravascular: Macrophages phagocytose deformed sickled cells in the spleen and liver
• Intravascular: Direct red cell lysis releases free hemoglobin into plasma, which:
  • Depletes haptoglobin
  • Scavenges nitric oxide (NO) → vasoconstriction, platelet activation, endothelial inflammation
  • Releases arginase → destroys L-arginine (the NO synthase substrate) → further reduces NO
  • Produces reactive oxygen species

B. Vaso-Occlusion

• Microvascular obstruction → tissue ischemia
• Reperfusion injury → further endothelial activation
• Inflammation → more sickling

Peripheral blood smear: Panel A shows sickle cell anemia with typical sickled forms; Panel B shows high-power view of the crescentic sickle shape (Robbins & Kumar Basic Pathology)

Clinical Manifestations

Acute Complications

1. Vaso-Occlusive Crisis (VOC) / Painful Crisis

• Most frequent acute complication; caused by microvascular occlusion
• Severe pain in bones (especially long bones, back, chest, abdomen)
• Triggers: cold, infection, dehydration, hypoxia, stress, alcohol
• Management: IV fluids, analgesia (NSAIDs + opioids), warmth, oxygen if hypoxic

2. Acute Chest Syndrome (ACS)

• New pulmonary infiltrate + fever + respiratory symptoms
• Caused by fat embolism (from bone marrow infarction), infection, or in-situ sickling in pulmonary vasculature
• Most common cause of death in SCD
• Treatment: broad-spectrum antibiotics + exchange transfusion + respiratory support; hydroxyurea for recurrent ACS

3. Stroke and Cerebrovascular Disease

• Children: ischemic stroke from large vessel intracranial stenosis
• Adults: silent infarcts, hemorrhagic stroke
• Transcranial Doppler (TCD) screening identifies high-risk children; chronic transfusion program reduces stroke risk by ~90%
• Hydroxyurea is an alternative to transfusion in lower-resource settings

4. Aplastic Crisis

• Triggered by Parvovirus B19 infection (infects erythroid precursors)
• Sudden severe drop in hemoglobin + reticulocytopenia
• Self-limited; supportive transfusion

5. Splenic Sequestration

• Rapid pooling of blood in the spleen → acute splenomegaly + severe anemia + cardiovascular collapse
• Most common in children (before functional asplenia develops)
• Emergency splenectomy or transfusion; elective splenectomy after recurrence

6. Acute Kidney Injury

• Occurs in 2-10% of acute painful crises
• Precipitated by dehydration, sepsis, drugs (NSAIDs)

Chronic Complications (Organ Damage)

Diagnosis

Blood Tests

• CBC: Hemoglobin 5-10 g/dL (HbSS); normochromic normocytic anemia; reticulocytosis (10-25%); leukocytosis; thrombocytosis
• Peripheral smear: Sickle cells, target cells, Howell-Jolly bodies (asplenia), nucleated RBCs
• Hemoglobin electrophoresis / HPLC: Gold standard for diagnosis; identifies HbS, HbA, HbF, HbA2 percentages
• Newborn screening: Now mandatory in most high-income countries (isoelectric focusing or HPLC of cord blood)

Additional Tests

• Elevated bilirubin, LDH (hemolysis markers)
• Low haptoglobin
• TCD ultrasound for stroke risk stratification
• Echocardiography (pulmonary hypertension screening via tricuspid regurgitation velocity)

Treatment

Disease-Modifying Therapy

1. Hydroxyurea (Hydroxycarbamide) - First-line

• Mechanism: Increases HbF production (inhibits ribonucleotide reductase, activates HbF gene transcription via stress erythropoiesis); reduces WBC and platelet adhesion; decreases MCHC
• Benefits: 50% reduction in painful crises, ACS, hospitalizations; improved survival; reduces renal hyperfiltration in children
• Dose: 15-35 mg/kg/day in adults; uptitrate to maximum tolerated dose
• Monitoring: CBC every 4-8 weeks (myelosuppression)
• Pregnancy: Teratogenic - contraindicated in pregnancy; use with effective contraception
• Administered under supervision of an experienced provider; now being expanded in low-resource settings

2. L-Glutamine

• FDA approved 2017; reduces oxidative stress in sickle cells; reduces VOC frequency

3. Crizanlizumab (Adakveo)

• Mechanism: Anti-P-selectin monoclonal antibody; blocks sickle cell-endothelium adhesion
• FDA approved 2019; reduces VOC frequency
• Note: In 2023, the FDA withdrew crizanlizumab's accelerated approval pending confirmatory trial data - its status as of 2026 requires current guidelines review

4. Voxelotor (Oxbryta)

• Mechanism: Allosterically stabilizes HbS in the oxygenated state, preventing polymerization; raises hemoglobin
• FDA approved 2019; also withdrawn from market in 2024 by manufacturer following safety data review - prescribers should check current status

5. Chronic Transfusion Program

• Monthly exchange or simple transfusions to maintain HbS <30%
• Indications: stroke prevention (TCD-positive children), recurrent ACS, recurrent VOC on hydroxyurea
• Risks: alloimmunization, iron overload (requires chelation), transfusion reactions

Curative Therapies

6. Allogeneic Hematopoietic Stem Cell Transplantation (HSCT)

• Only established cure prior to gene therapies
• Best results with HLA-matched sibling donor (<15% graft failure, >90% event-free survival in children)
• Expanding use of haploidentical and unrelated donors
• Risks: graft-versus-host disease, graft failure, transplant-related mortality

7. Gene Therapies (2023-2024 FDA Approvals)

Supportive and Preventive Care

Special Situations

• Increased maternal mortality (6-fold vs. controls), preterm labor, placental infarction, SGA infants
• Hydroxyurea is contraindicated (teratogenic); NSAIDs avoided after 30 weeks
• Regional anesthesia preferred over general for cesarean
• Therapeutic transfusion goal: HbS <40%, Hb ~10 g/dL

• Preoperative transfusion to Hb 10 g/dL (simple transfusion, not exchange) reduces perioperative complications
• Maintain hydration, normothermia, normoxia

Prognosis

• Median life expectancy currently in the fifth decade for both sexes in high-income countries
• Major causes of death: ACS, infection, stroke, multiorgan failure
• Quality and length of life have improved significantly with hydroxyurea, transfusion programs, and now gene therapies
• The 2026 Lancet review identifies the transition from pediatric to adult care and equitable access to gene therapies as key ongoing challenges

Refeeding Syndrome

Definition

Pathophysiology

During Starvation

• Insulin secretion is low
• Intracellular electrolytes (phosphate, potassium, magnesium) become severely depleted from cells due to lack of intake and ongoing losses
• Serum levels of phosphate, potassium, and magnesium can appear normal because bone mineral stores release phosphate to maintain a near-normal extracellular concentration - this masks the true total body depletion
• Thiamine (vitamin B1) stores are also depleted

On Reintroduction of Carbohydrates

1. Na+/K+ ATPase is stimulated → potassium (and sodium) are driven intracellularly → hypokalemia
2. Phosphate is consumed for glycolysis (synthesis of ATP, glucose-6-phosphate, fructose-1,6-bisphosphate) and rapidly shifts intracellularly → hypophosphatemia (the hallmark)
3. Magnesium shifts intracellularly via incompletely understood mechanisms; hypomagnesemia also exacerbates hypokalemia (Mg²+ is required for K+ transport)
4. Insulin promotes Na+ and water retention → volume expansion → dilutional electrolyte drop + risk of pulmonary edema and heart failure
5. Thiamine is consumed as a cofactor for pyruvate dehydrogenase in the reactivated Krebs cycle → if already depleted, pyruvate cannot enter the cycle → Wernicke encephalopathy

Figure 23.5 - Pathogenesis of refeeding syndrome (Yamada's Textbook of Gastroenterology, 7th Ed.)

Who is at Risk? (NICE Criteria)

High Risk - One criterion is sufficient:

Moderate Risk - Two or more criteria needed:

Clinical Conditions Commonly Associated with High Risk

• Anorexia nervosa (prolonged severe restriction)
• Chronic alcoholism (poor intake + thiamine depletion)
• Cancer (cachexia, chemotherapy, poor intake)
• Post-bariatric surgery (malabsorption, restriction)
• Prolonged nil-by-mouth (ICU, post-surgical, dysphagia)
• Chronic malnutrition from inflammatory bowel disease, short bowel syndrome, malabsorption
• Elderly patients with poor intake or acute illness
• HIV/AIDS

Clinical Features

Electrolyte Disturbances and Their Effects

Other Features

• Sodium and water retention (insulin effect) → peripheral edema, acute heart failure, pulmonary edema
• Hyperglycemia → osmotic diuresis, worsening electrolyte losses
• Thiamine (B1) deficiency → Wernicke encephalopathy (ophthalmoplegia, ataxia, confusion) or Wernicke-Korsakoff syndrome; also cardiovascular beriberi (wet beriberi - high-output cardiac failure)
• Rhabdomyolysis → acute kidney injury
• Respiratory failure → ventilator dependence in ICU patients

Diagnosis

1. The patient has had adequate nutrition intake (≥50% of estimated requirements) commenced
2. Electrolyte imbalances emerge after its commencement
3. Clinical symptoms are present

Laboratory Monitoring

Prevention and Management

Step 1 - Correct Pre-Existing Deficiencies BEFORE Feeding

• Replace low phosphate, potassium, and magnesium to normal levels before starting nutrition
• Give thiamine 100-200 mg IV/IM before initiating feeds or any glucose-containing fluids - this is non-negotiable
• Continue thiamine 100 mg/day for 5-7 days (longer in alcoholics and severely starved patients)
• Give multivitamins and trace elements

Step 2 - Start Nutrition Slowly

Step 3 - Monitor Electrolytes Aggressively and Replace

• Check electrolytes (phosphate, potassium, magnesium, glucose) daily during the first week
• Replace electrolytes aggressively according to severity:

Step 4 - Multidisciplinary Approach

• Nutrition support team (dietitian, gastroenterologist/intensivist)
• Pharmacist (electrolyte replacement protocols)
• Nursing (monitoring, fluid balance)
• Medicine/ICU (cardiac monitoring, respiratory support if needed)

Specific Routes of Nutrition

Special Populations

Summary: The "Refeeding Checklist"

Before feeding a malnourished patient:
  1. Assess risk (NICE criteria)
  2. Check baseline electrolytes (PO4, K+, Mg2+, Ca2+)
  3. Correct deficiencies BEFORE starting
  4. Give thiamine 100-200mg IV before any glucose/feeds
  5. Start at 10 kcal/kg/day if highest risk; ~50% of needs if moderate risk
  6. Monitor daily electrolytes + glucose for first week
  7. Replace electrolytes aggressively as needed
  8. Escalate calories gradually over 4-7 days
  9. Ensure multidisciplinary oversight