What is the pathophysiology of nephrotic syndrome?

Pathophysiology of Nephrotic Syndrome

Nephrotic syndrome results from increased glomerular basement membrane permeability to proteins, primarily driven by podocyte injury, leading to massive proteinuria (>3.5 g/day in adults, >40 mg/m²/h in children), hypoalbuminemia (<3.0 g/dL in adults, <2.5 g/dL in children), edema, and hypercholesterolemia 1.

Primary Mechanism: Podocyte Dysfunction

The fundamental pathophysiologic defect centers on podocyte injury and dysfunction of the glomerular filtration barrier (GFB). The GFB consists of three layers: glomerular endothelial cells with glycocalyx, the basement membrane, and podocytes with foot processes connected by slit diaphragms 2.

Immune-Mediated Injury (MCD and FSGS)

Evidence strongly suggests a primary T-cell disorder drives minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS) 1. The proposed mechanism involves:

• T-cell-driven circulating "permeability factor" that interferes with glomerular permselectivity to albumin 1
• Despite 30+ years of research, the identity of this glomerular permeability factor remains elusive, though experimental observations consistently support its existence 1
• Interleukin-2 (IL-2) levels increase during proteinuria and normalize during remission in adults with idiopathic nephrotic syndrome and children with MCD or FSGS 1
• This IL-2 pattern reflects a widespread cellular immunity disorder rather than being directly causal of proteinuria 1

Autoimmune Mechanisms (Membranous Nephropathy)

The cause of idiopathic membranous nephropathy remains equally elusive 1, though recent evidence confirms it is an autoimmune disease with specific antigens and antibodies involved 3.

Consequences of Proteinuria

Barrier Dysfunction

The proteinuria in nephrotic syndrome results from:

• Loss of charge selectivity: In MCD, epithelial cell damage alters metabolism of polyanions (heparan sulfates) that constitute the normal charge barrier 4
• Loss of size selectivity: In FSGS, increased large pores in the glomerular basement membrane impair size selectivity 4
• Slit diaphragm disruption: Disorders of the slit diaphragm structure contribute to protein leakage 5

Progressive Kidney Damage

Prolonged nephrotic proteinuria leads to renal scarring and eventual renal failure, though the precise mechanism remains unresolved 1. The pathophysiology includes:

• Proteinuria may cause tubular epithelial cell overload and dysfunction, resulting in tubular atrophy and interstitial fibrosis 6
• Formation of cellular or fibrous crescents with development of rapidly progressive glomerulonephritis or focal glomerulosclerosis 6
• Adults with proteinuria >3.8 g/day have 35% risk of ESRD within 2 years, compared to only 4% risk with proteinuria <2.0 g/day 1

Systemic Complications

Hypoalbuminemia Impact

Hypoalbuminemia carries severe prognostic implications: A meta-analysis demonstrated that each 1.0 g/dL decrease in serum albumin increases odds of morbidity by 89% and mortality by 137% 1.

Edema Formation

Edema pathogenesis varies widely between patients 7:

• May occur via intravascular volume underfilling (classic "underfill" hypothesis)
• Or via intravascular volume overfilling (primary sodium retention)
• Most patients with nephrotic edema have primary salt retention 5

Hypercoagulability

Nephrotic syndrome creates a prothrombotic state through multiple mechanisms 1, 8:

• Urinary loss of anticoagulants (antithrombin III, protein C, protein S)
• Overproduction of coagulation factors by the liver
• Prevalence of thrombotic complications in membranous nephropathy: 29% renal vein thrombosis, 17-28% pulmonary embolism, 11% deep vein thrombosis 1
• Thromboembolism occurs most commonly within first 6 months of diagnosis, particularly with serum albumin <2.9 g/dL 8

Hyperlipidemia

Hyperlipidemia results from both overproduction and impaired catabolism of lipoproteins 5. This contributes to:

• Accelerated coronary heart disease (4-fold increased risk versus age-matched controls) 1
• Podocyte dysfunction 6
• Vascular thrombosis 6
• Multiple cardiovascular risk factors: hypercholesterolemia, hypertension, hypercoagulability, steroid exposure 1

Infection Risk

Increased infection susceptibility occurs through 1, 8:

• Hypogammaglobulinemia and urinary loss of immunoglobulins
• Loss of complement factors in urine
• Immunosuppressive therapy effects
• In children: particular risk of cellulitis and spontaneous bacterial peritonitis 1

Clinical Implications

The duration and severity of proteinuria serve as surrogate markers for glomerular disease progression 1. The main prognostic factor across all histologic variants is the response of proteinuria to therapy 1.

Obtaining and maintaining the lowest level of proteinuria—when carefully balanced against therapy risks—results in the best quality and quantity of both renal and patient survival 1.