Nephrotic Syndrome: Etiology, Pathophysiology, and Clinical Features
Nephrotic syndrome is defined by heavy proteinuria (≥3.5 g/day in adults; ≥1.0 g/m²/day in children), hypoalbuminemia (<3.0 g/dL in adults; <2.5 g/dL in children), edema, and typically hypercholesterolemia. 1
Etiology
The causes of nephrotic syndrome divide into primary (idiopathic) glomerular diseases and secondary causes:
Primary Causes
The three leading histologic variants are 1:
- Minimal change disease (MCD) - most common in children
- Focal segmental glomerulosclerosis (FSGS) - most common in populations of African ancestry 2
- Membranous nephropathy - most common in white adults 2
The relative frequencies vary considerably by age and ethnicity 1.
Secondary Causes
- Diabetic nephropathy - the most common systemic disease causing nephrotic syndrome 2
- Systemic lupus erythematosus
- Amyloidosis
- Infections (hepatitis B, hepatitis C, HIV)
- Drugs and toxins
- Malignancies 3
Pathophysiology
Glomerular Injury Mechanism
Evidence suggests a primary T-cell disorder drives MCD and FSGS 1. A T-cell-driven circulating factor interferes with glomerular permselectivity to albumin, though the precise identity of this "glomerular permeability factor" remains elusive despite 30+ years of research 1.
Consequences of Proteinuria
Prolonged nephrotic proteinuria leads to 1:
- Renal scarring and eventual renal failure through tubular epithelial cell overload and dysfunction 4
- Formation of cellular or fibrous crescents
- Tubular atrophy and interstitial fibrosis 4
- Patients with proteinuria >3.8 g/day have a 35% risk of end-stage renal disease within 2 years, compared to only 4% risk for those with proteinuria <2.0 g/day 1
Hypoalbuminemia Impact
Each 1.0 g/dL decrease in serum albumin increases the odds of morbidity by 89% and mortality by 137% 1. This represents a critical prognostic marker across all pathologic conditions.
Common Clinical Features
Cardinal Manifestations
The classic tetrad consists of 3:
- Edema - periorbital (most noticeable in the morning) or dependent pitting edema (more common later in the day) 2
- Heavy proteinuria - documented by urine protein:creatinine ratio (PCR) >300-350 mg/mmol 2
- Hypoalbuminemia
- Hypercholesterolemia
Major Complications
Thromboembolism
Nephrotic syndrome creates a hypercoagulable state 1, 5. In membranous nephropathy specifically 1:
- 29% prevalence of renal vein thrombosis
- 17-28% pulmonary embolism
- 11% deep vein thrombosis
- Risk increases with serum albumin <2.9 g/dL 5
The mechanism involves urinary loss of anticoagulants and overproduction of coagulation factors 4.
Cardiovascular Disease
The risk of coronary heart disease is four times greater in nephrotic patients than age- and sex-matched controls 1. Multiple risk factors converge 1:
- Hypercholesterolemia
- Hypertension
- Hypercoagulability
- Exposure to atherogenic drugs (steroids)
- Dyslipidemia may cause podocyte dysfunction and contribute to vascular thrombosis 4
Infection Risk
Patients face increased infection susceptibility due to 1, 5:
- Hypogammaglobulinemia
- Loss of complement factors in urine
- Immunosuppressive therapy
- In children: particular risk of cellulitis and spontaneous bacterial peritonitis 1
Edema Pathophysiology
Recent evidence challenges the traditional "underfill hypothesis." A kidney-limited sodium-reabsorption mechanism exists, attributed to serine proteases in the tubular lumen activating ENaC channels 6. This explains why patients often exhibit diuretic resistance and why ENaC blockers emerge as potential add-on treatment 6.
Clinical Evaluation Approach
Distinguishing Primary vs. Secondary Causes
When evaluating nephrotic syndrome, look for these specific features 7:
Factors favoring primary diabetic nephropathy:
- Persistent albuminuria with bland urine sediment
- Slow disease progression
- Other diabetic complications present
- Diabetes duration >5 years
Factors suggesting alternative diagnosis:
- Extreme proteinuria (>6 g/day)
- Persistent hematuria or active urine sediment (RBC/WBC casts)
- Rapidly falling eGFR (≥5 mL/min/1.73 m² loss per year)
- Low eGFR with minimal proteinuria
- Diabetes duration <5 years
- Family history of non-diabetic renal disease
Urgent Referral Criteria
Urgent nephrologist referral (ideally within 2 weeks) is necessary 2. Renal biopsy is usually performed to establish the specific form of glomerular disease, except in children <12 years without syndromic features who can receive empiric glucocorticoid therapy 8.
Prognostic Implications
The main factor predicting prognosis across all histologic variants is the response of proteinuria to therapy 1. Treatment objectives are threefold 1:
- Lower proteinuria
- Reduce frequency of relapses
- Protect the kidney and prevent progression to renal failure
The duration and severity of proteinuria serve as surrogate markers of glomerular disease progression 1.