Acute Kidney Injury: Pathogenesis & Risk Factors (Focus on ATN)

Overview of Acute Kidney Injury (AKI)

AKI is described as an abrupt decline in glomerular filtration rate (GFR) resulting from many potential insults. While clinicians often categorise causes into discrete groups to aid differential diagnosis, in practice AKI is frequently multifactorial, and overlap between mechanisms is common.

• Practical clinical distribution: the majority arise from prerenal mechanisms (≈ 70%70\%), a smaller slice from post-renal obstruction, and the least common from direct intrarenal damage.

• Core learning objective: understand causes, pathophysiology, and especially acute tubular necrosis (ATN)—the most frequent cause of AKI in hospitalised patients.

Prerenal Causes of AKI

Prerenal states reduce effective renal perfusion pressure or circulating blood volume, leading to adaptive renal vasoconstriction and a fall in GFR. They can be subdivided into:

1. Hypovolaemia

  • Haemorrhage (external or internal)
  • Extensive burns with fluid loss
  • Gastrointestinal losses (vomiting, diarrhoea)
  • Poor oral intake/dehydration

2. Reduced Cardiac Output

  • Congestive heart failure
  • Cardiac tamponade or large pericardial effusion

3. Systemic Vasodilation

  • Sepsis / systemic inflammatory response syndrome (SIRS)
  • Advanced liver disease → hepatorenal syndrome (HRS)

4. Impaired Renal Vasomotor Modulation / Shunting

Medications or metabolic states that blunt the kidney’s autoregulation by altering afferent or efferent arteriolar tone:

  • Non-steroidal anti-inflammatory drugs (NSAIDs) ↓ prostaglandin-mediated afferent dilation
  • ACE inhibitors / ARBs ↓ angiotensin-II-mediated efferent constriction
  • Hypercalcaemia, abdominal compartment syndrome, and severe liver disease contribute via complex haemodynamic effects.

Postrenal Causes of AKI

Obstruction anywhere from renal pelvis to urethra causes tubular back-pressure, ↑ intratubular hydrostatic pressure, and ↓ net filtration.

• Bilateral ureteric obstruction (stone, tumour, fibrosis)
• Bladder outlet obstruction (e.g. enlarged prostate, neurogenic bladder)
• Urethral strictures or external compression

NOTE: Significant GFR reduction generally requires bilateral obstruction above the bladder or complete obstruction below it.

Intrarenal (Intrinsic) Causes of AKI

The least common but often most complex, subdivided by anatomic compartment:

1. Vascular

a. Large Vessel
  • Renal artery stenosis (atherosclerotic, fibromuscular dysplasia)
  • Renal artery/vein thrombosis or embolism
b. Microvascular
  • Thrombotic microangiopathy (TMA): encompasses HUS, TTP, malignant hypertension, drug-induced TMAs
  • Vasculitides affecting small renal arteries/arterioles

2. Glomerular

  • Rapidly progressive glomerulonephritis (RPGN) – umbrella for immune-mediated necrotising crescentic GN (ANCA-associated, anti-GBM, immune-complex)
  • Other primary GNs: minimal-change, FSGS, membranous, IgA nephropathy
  • Secondary GNs: lupus nephritis, post-infectious, malignancy-associated, etc.

3. Tubulo-interstitial

  • Acute interstitial nephritis (AIN): classically a drug-induced hypersensitivity; also infections, systemic diseases (sarcoid, Sjögren’s) or toxins
  • Acute tubular necrosis (ATN): ischaemic or nephrotoxic injury to tubular epithelium (focus below)

Acute Tubular Necrosis (ATN)

Most common intrinsic AKI in hospitals. Results from prolonged hypoperfusion or direct toxic injury to tubular epithelium.

A. Epidemiology & Risk Factors

  1. Age > 6565 years – decreased nephron reserve
  2. Pre-existing chronic kidney disease (CKD) or proteinuria (marker of nephron stress)
  3. Exposure to nephrotoxins: aminoglycosides, radiocontrast, cisplatin, amphotericin, myoglobin, haemoglobin, heavy metals
  4. Drugs impairing autoregulation: NSAIDs, ACE-I, ARBs, calcineurin inhibitors
  5. Comorbidities that affect vasculature and perfusion: diabetes, hypertension, heart failure, cirrhosis

B. Pathophysiology

• The kidney receives a high fraction of cardiac output and has steep metabolic demands.
• Medullary thick ascending limb and S3 segment of proximal tubule are extremely sensitive to hypoxia.

Ischaemic ATN sequence:

  1. Hypoperfusion → ↓ ATP production.
  2. Loss of Na⁺/K⁺-ATPase function → cellular swelling, Ca²⁺ influx.
  3. Generation of reactive oxygen species (ROS) → oxidative damage.
  4. Tubular epithelial cell apoptosis/necrosis → cells slough into lumen forming obstructive casts.
  5. Inflammatory cascade (neutrophils, macrophages) + endothelial injury promotes vasoconstriction → further microcirculatory compromise (vicious cycle).

Nephrotoxic ATN follows similar histology but toxin causes direct epithelial injury ± vasoconstriction.

C. Histopathology & Imaging Correlate

• Normal tubule: plump epithelial lining, patent lumen.
• ATN: epithelial flattening, loss of brush border, granular casts, intracellular debris, “muddy brown” casts in urine.

D. Classical Phases

  1. Prerenal/Initiation (hours–days)
    – Perfusion falls; still reversible if corrected.
  2. Extension (ongoing ischaemia/inflammation)
    – Cellular injury worsens, tubular obstruction forms.
  3. Maintenance/Established ATN (11–88 weeks)
    – GFR low; oliguria or anuria common; FENa often > 2%2\%.
  4. Recovery (days–weeks)
    – Tubular cells regenerate; polyuric phase with large urine volumes yet impaired concentrating ability, risking electrolyte disturbances.

E. Clinical Significance

• ATN underpins many hospital-acquired AKI cases—contrast studies, surgery, hypotensive episodes.
• Preventive strategies: ensure euvolaemia, avoid nephrotoxins, monitor drug levels, use iso-osmolar contrast, permit renal perfusion (MAP > 6565–7070 mmHg).

Integrative Points & Real-World Relevance

• Overlap: A patient with sepsis (systemic vasodilation), on NSAIDs, and catheter blockage may have prerenal, intrinsic (ATN), and post-renal components simultaneously.
• Ethical/practical: Early recognition of AKI can prevent progression, avoid dialysis, and improve outcomes—highlighting the importance of iatrogenic harm reduction.
• Pathophysiology insight is based mostly on animal/ischaemia-reperfusion or toxin models; translation to humans remains imperfect, urging cautious interpretation.

Key Take-Home Messages

  1. Categorise AKI into prerenal, postrenal, and intrarenal, but search for multifactorial roots.
  2. Prerenal states (≈ 70%70\%) hinge on hypovolaemia, low cardiac output, systemic vasodilation, or deranged autoregulation.
  3. Postrenal obstruction must be bilateral (above bladder) or complete (lower) to critically drop GFR.
  4. Intrarenal causes sort by vascular, glomerular, and tubulo-interstitial compartments—ATN is the leading entity.
  5. ATN pathogenesis: hypoxia ± toxins → tubular cell death → obstruction, back-pressure, inflammation → sustained GFR decline.
  6. Clinical course features initiation, maintenance (oliguria), and recovery (polyuria); supportive care and avoidance of further insults are crucial while nephrons heal.