Acute Kidney Injury – Three Categories:  1. Pre-Renal, 2. Intrarenal and 3. Post-Renal:

Pre-renal Kidney Injury:

• Definition:
  • Damage or impairment before the blood reaches the kidney.

• Causes:
  • Circulatory shock: Severe reduction in blood flow due to hemorrhage, burns, crush injuries, myocardial infarction, pulmonary embolism, or severe dehydration.
    • Cardiogenic Shock due to Heart failure or MI: Heart’s inability to pump effectively reduces blood pressure and flow to kidneys.
    • Hypovolemic Shock due to hemorrhaging, burns, or severe dehydration
  • Systemic vasodilation: Due to septic shock, severe infections, or drug effects, leading to low blood pressure and reduced renal perfusion.
  • Hemolytic transfusion reactions or severe muscle injury -resulting in large proteins (hemoglobin or myoglobin) damaging the glomeruli and nephron tubules.
    • Bad blood transfusions – cause hemolysis with free hemoglobin accumulating in glomeruli causing damage
  • Rhabdomyolysis (skeletal muscle damage) leading to accumulation of myoglobin in glomeruli causing damage

• Pathophysiology:
  • Ischemia: Reduced blood flow causes tubular necrosis.
  • Damage primarily affects nephrons, especially the glomerulus.
  • Damage can extend to surrounding capillaries and glomerular structures.

• Outcome:
  • Rapid cell death (necrosis), leading to acute tubular necrosis (ATN).
  • The damage is outside the kidney, but flow into the kidney is impeded.

• Link Between Damage and Function:
  • Damage to glomeruli or tubules reduces filtration.
  • Inflammation and edema further worsen renal tissue injury.
  • Blood flow impairment causes tissue cell dysfunction/death and damage to renal capillaries.

Summary: 

• Prerenal injury results from low blood flow or impaired perfusion causing ischemic damage to the nephron tissue, which can become irreversible if blood flow is not restored quickly.

• The damage occurs outside the kidney but affects its function directly.
• Early recognition and correction of blood flow issues can prevent permanent kidney damage.

Intra-Renal Kidney Injury (Intrinsic Renal Damage):

Definition:

• Intra-renal (within the kidney) injury refers to damage occurring directly inside the kidney itself.

Causes:

• Infection:
  • Post-Streptococcal Glomerulonephritis (PSGN):
    • Autoimmune response involving antibody deposition in the glomeruli.
    • Results in glomerular destruction and impaired filtration.
  • Pyelonephritis:
    • Bacterial infection that ascends from the urinary tract.
    • Causes inflammation and tissue damage within the kidney.
• Nephrotoxic drugs:
  • Chemicals or toxins that damage kidney tissue.
  • Examples include:
    • NSAIDs (e.g., ibuprofen), which impair blood flow and cause tubular necrosis.
    • Acetaminophen and aspirin.
    • Penicillin (especially in high doses or prolonged use).

Pathophysiology:

• Toxins or bacteria cause tubular necrosis, damaging the nephrons.
• White blood cell accumulation and pus formation occur as part of the inflammatory response.
• Edema and collateral damage further impair function.
• Ischemia worsens the injury, leading to tissue death.

Note:
This type of injury can be acute and severe, often reversible if identified early and the offending agents are removed or treated.

Summary:

Intra-renal kidney injury involves directly damaging processes inside the kidney, mainly due to infection or toxins. Recognizing causes like glomerulonephritis or drug toxicity is essential, as early intervention can prevent permanent damage leading to kidney failure.

Post-Renal Kidney Injury (Obstructive Kidney Damage):

Definition:

• Post-renal injury occurs after the kidneys, caused by an obstruction that prevents urine from draining.
• Unilateral obstruction (one ureter) typically does not cause significant loss of renal function because the other kidney compensates.
• Bilateral obstruction can be life-threatening, leading to acute kidney injury.

Causes:

• Obstruction of both ureters:
  • Critical for causing renal failure because both kidneys are blocked.
  • Could be caused by:
    • Stones (calculi): Impacted in both ureters.
    • Tumors affecting the ureters or bladder mass.
    • Strictures or fibrosis obstructing urine flow.
    • Clots or blood accumulation.

• Obstruction of lower urinary tract:
  • Bladder outlet obstruction:
    • Benign prostatic hyperplasia (BPH).
    • Bladder tumors.
  • Urethral blockage: Due to strictures or urethral trauma.

Pathophysiology:

• Obstruction causes urine retention proximal to the blockage.
• Urine backup increases pressure in the kidneys, leading to hydronephrosis.
• Increased pressure damages renal tissue, causing necrosis and scarring.
• If unchecked, this damage results in kidney failure.

Summary:

• Post-renal failure results from obstruction downstream of the kidneys—most critically when both ureters are blocked—leading to urine build-up, pressure damage, and potential kidney failure. Early detection and relieving the obstruction are essential to prevent irreversible renal damage.