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Chronic Kidney Disease for USMLE Step 3

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Chronic Kidney Disease (CKD) for the USMLE Step 3 Exam
Definition
  • Chronic Kidney Disease (CKD): A progressive and irreversible decline in kidney function over at least 3 months, characterized by reduced glomerular filtration rate (GFR) or evidence of kidney damage (e.g., albuminuria, abnormal urine sediment, or structural changes).
  • Diagnosis:
    • GFR <60 mL/min/1.73 m² for ≥3 months, or
    • Evidence of kidney damage such as albuminuria ≥30 mg/g or abnormal imaging.
Staging (KDIGO)
  • Stage 1: GFR ≥90 mL/min/1.73 m² with evidence of kidney damage.
  • Stage 2: GFR 60–89 mL/min/1.73 m² with evidence of kidney damage.
  • Stage 3:
    • 3a: GFR 45–59 mL/min/1.73 m².
    • 3b: GFR 30–44 mL/min/1.73 m².
  • Stage 4: GFR 15–29 mL/min/1.73 m².
  • Stage 5: GFR <15 mL/min/1.73 m² (end-stage renal disease, ESRD).
Etiology
  • Diabetes Mellitus (DM): The leading cause of CKD, where chronic hyperglycemia leads to glomerular damage, hyperfiltration, and eventual nephron loss.
  • Hypertension: Prolonged high blood pressure results in nephrosclerosis, a major contributor to CKD progression.
  • Glomerulonephritis: Inflammatory diseases of the glomeruli (e.g., IgA nephropathy, lupus nephritis) cause scarring and nephron loss.
  • Polycystic Kidney Disease (PKD): Genetic disorder causing multiple renal cysts, progressively damaging normal kidney tissue.
  • Other Causes:
    • Obstructive uropathy (e.g., kidney stones, benign prostatic hypertrophy).
    • Long-term nephrotoxin exposure (e.g., NSAIDs, certain antibiotics).
Pathophysiology
  • Hyperfiltration Injury: Initial nephron loss leads to compensatory hyperfiltration in remaining nephrons, which accelerates glomerular damage, eventually causing further nephron loss.
  • Glomerulosclerosis and Tubulointerstitial Fibrosis: Chronic kidney damage results in scarring (glomerulosclerosis) and fibrosis of the interstitium, leading to decreased renal function.
  • Uremia: As kidney function declines, uremic toxins accumulate, causing systemic symptoms like nausea, pruritus, fatigue, and cognitive impairment.
Clinical Presentation
  • Early Stages (1–3):
    • Asymptomatic or mild symptoms, including fatigue and hypertension.
    • Proteinuria or albuminuria is often the earliest sign of kidney damage.
    • Hypertension: Results from sodium retention and increased RAAS activation.
  • Late Stages (4–5):
    • Uremia: Nausea, vomiting, pruritus, cognitive changes, and anorexia.
    • Volume Overload: Dyspnea, peripheral edema, and pulmonary edema due to fluid retention.
    • Anemia: Decreased erythropoietin production.
    • Bone Disease: Secondary hyperparathyroidism, renal osteodystrophy, and increased fracture risk.
Signs & Symptoms of Chronic Kidney Disease
Diagnostic Evaluation
  • Laboratory Tests:
    • Serum Creatinine and GFR: Used to monitor disease progression.
    • Urinalysis: Proteinuria, hematuria, and albumin-to-creatinine ratio (UACR) assess kidney damage.
    • Electrolytes: Hyperkalemia, hyperphosphatemia, and metabolic acidosis in advanced CKD.
  • Imaging:
    • Renal Ultrasound: Useful to assess kidney size (typically small in advanced CKD) or detect polycystic kidneys in PKD.
Management
  • Control of Underlying Causes:
    • Diabetes: Tight glycemic control (HbA1c <7%) to reduce CKD progression.
    • Hypertension: Use of ACE inhibitors or ARBs to reduce proteinuria and slow disease progression. Target BP <130/80 mmHg.
  • Dietary Management:
    • Protein Restriction: Limiting protein intake (0.8 g/kg/day) to reduce nitrogenous waste.
    • Sodium Restriction: Limiting sodium intake to manage hypertension and prevent fluid overload.
    • Phosphorus Restriction: To prevent secondary hyperparathyroidism.
  • Management of Complications:
    • Anemia: Erythropoiesis-stimulating agents (ESAs) and iron supplementation.
    • Hyperkalemia: Dietary restriction, loop diuretics, and potassium binders.
    • Metabolic Acidosis: Sodium bicarbonate supplementation.
    • Bone Disease: Phosphate binders, vitamin D analogs, and calcimimetics to manage secondary hyperparathyroidism and renal osteodystrophy.
  • End-Stage Renal Disease (ESRD):
    • Dialysis: Initiated for uremic symptoms, hyperkalemia, fluid overload, or metabolic acidosis unresponsive to medical treatment.
    • Kidney Transplantation: The definitive treatment for ESRD, offering the best long-term outcomes.
Complications
  • Cardiovascular Disease: The leading cause of death in CKD patients, exacerbated by hypertension, hyperlipidemia, and uremia.
  • Hyperkalemia and Metabolic Acidosis: Life-threatening if untreated.
  • Bone and Mineral Disorders: Renal osteodystrophy increases the risk of fractures and bone pain.
Key Points
  • CKD is defined by reduced GFR (<60 mL/min/1.73 m²) or kidney damage lasting ≥3 months.
  • Leading causes include diabetes, hypertension, and glomerulonephritis.
  • Early detection focuses on managing underlying conditions like diabetes and hypertension to slow disease progression.
  • Key complications include cardiovascular disease, electrolyte imbalances, anemia, and bone disorders.
  • End-stage renal disease (ESRD) requires dialysis or kidney transplantation.

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