Bronchiectasis - Definition

Core Definition (from Authoritative Sources)

"Bronchiectasis is the permanent dilation of bronchi and bronchioles caused by destruction of smooth muscle and supporting elastic tissue; it typically results from or is associated with chronic necrotizing infections."

• Robbins & Kumar Basic Pathology

"In bronchiectasis, there is, by definition, an abnormal and permanent dilatation of the bronchi; this is usually present to a much greater degree than is found in COPD and is often accompanied by airway distortion."

• Murray & Nadel's Textbook of Respiratory Medicine

"In bronchiectasis, there is, by definition, an abnormal and permanent dilatation of the bronchi... often accompanied by airway distortion. There is exaggeration of the muscular ridges and the presence of multiple bronchial gland-based pits. The large airway walls can be thickened and/or irregularly thinned as a result of inflammation and fibrosis, and there is often inspissated mucus or actual purulent material."

• Murray & Nadel's Textbook of Respiratory Medicine

Key Characteristics

Pathogenesis in Brief

1. Obstruction (foreign body, mucus, tumor) impairs secretion clearance, allowing infection to take hold
2. Chronic necrotizing infection damages the bronchial wall, causes peribronchial fibrosis, and progressively distends the airways

Classic Clinical Features

• Chronic cough producing copious purulent sputum (typically >30 mL/day)
• Hemoptysis
• Recurrent pulmonary infections
• Common pathogens: Haemophilus influenzae, Staphylococcus aureus, Pseudomonas aeruginosa

Common Predisposing Conditions

• Bronchial obstruction (tumor, foreign body, mucus impaction)
• Cystic fibrosis (most common cause of severe widespread bronchiectasis)
• Immunodeficiency states (especially immunoglobulin deficiencies)
• Primary ciliary dyskinesia (immobile cilia syndrome)
• Necrotizing/suppurative pneumonia (S. aureus, Klebsiella)
• Post-tuberculosis (significant in endemic areas)
• Post-COVID-19 pneumonia (increasingly reported)

Nephrotoxic Antibiotics

1. Aminoglycosides (Most Common Antibiotic Nephrotoxin)

• Freely filtered at the glomerulus and accumulate in proximal tubular cells (cortex) via megalin-mediated endocytosis
• Cause formation of myeloid bodies (membrane fragments in lysosomes) from inhibition of lysosomal enzymes
• Lead to oxidative stress, disruption of subcellular organelles, and mitochondrial dysfunction
• Result in acute tubular necrosis (ATN)

• Nonoliguric AKI in 10-30% of courses (even with therapeutic levels)
• Rising serum creatinine after 5-7 days of therapy
• Low-grade proteinuria, renal tubular epithelial (RTE) cells and granular casts in urine before clinical AKI
• Hypomagnesemia, hypokalemia (tubular wasting)
• In severe cases: Fanconi syndrome or Bartter-like syndrome

2. Vancomycin

• Excreted by glomerular filtration; accumulates in proximal tubular cells via basolateral secretion
• High-dose causes oxidative stress and triggers intrarenal apoptotic pathways
• ATN is the predominant lesion; rarely causes acute interstitial nephritis (AIN) or DRESS syndrome
• Can crystallize in tubules and cause intratubular obstruction

• AKI associated with trough levels >15 µg/mL (dose-dependent)
• Co-administration with piperacillin-tazobactam significantly increases AKI risk
• Generally reversible; high-flux hemodialysis can remove drug if levels are very high

3. Amphotericin B (Antifungal Antibiotic)

• Binds to cholesterol in tubular cell membranes (and ergosterol in fungal membranes), creating aqueous pores
• Sodium influx increases Na⁺/K⁺-ATPase activity and depletes cellular energy
• Causes renal vasoconstriction via tubuloglomerular feedback
• Direct tubular toxicity via reactive oxygen species

• Degree of injury proportional to cumulative dose (usually after >2-3 g)
• Polyuria (loss of concentrating ability), hypomagnesemia, hypokalemia, hypocalcemia
• Distal renal tubular acidosis (non-anion gap metabolic acidosis)
• Nonoliguric, progressive AKI

4. Polymyxins (Colistin / Polymyxin B)

• Dose-dependent, reversible nephrotoxicity - the most important adverse effect
• Also causes neurotoxicity (paresthesias, muscle weakness) but less commonly

5. Antibiotics Causing Acute Interstitial Nephritis (AIN)

6. Antibiotics Causing Crystal Nephropathy / Intratubular Obstruction

7. Other Antimicrobials

Summary Table

Hepatorenal Syndrome (HRS)

Definition

• Occurs in ~10% of patients with advanced cirrhosis or acute liver failure
• AKI occurs in 15-25% of all hospitalized cirrhotic patients; HRS accounts for 10-30% of these
• Annual frequency of HRS in cirrhotic patients with ascites: ~8% (up to 40% in some reports)

Pathophysiology

• Portal hypertension triggers release of vasoactive mediators: nitric oxide (NO), carbon monoxide, glucagon, prostacyclin, adrenomedullin, endogenous opiates
• These cause marked splanchnic and peripheral vasodilation
• Results in decreased effective circulating blood volume and reduced blood pressure
• Early compensation: increased heart rate and cardiac output (hyperdynamic circulation)

• As splanchnic vasodilation progresses beyond compensation, the body activates:
  • Renin-angiotensin-aldosterone system (RAAS)
  • Sympathetic nervous system (SNS)
  • Non-osmotic ADH release
• These responses cause intense renal vasoconstriction - a shift in the balance between systemic vasodilation and renal vasoconstriction
• Leads to increased renal vascular resistance, decreased renal perfusion, and reduced GFR
• Mediators involved: endothelins, altered prostaglandins, kallikreins, F2-isoprostanes
• Result: Na⁺/H₂O retention → ascites and hyponatremia

• Impaired cardiac output further worsens renal hypoperfusion
• Patients who develop HRS have lower cardiac output and higher circulating norepinephrine/renin than those who do not

Key precipitants: SBP/sepsis (30%), severe alcoholic hepatitis (25%), large-volume paracentesis without albumin (10%), GI bleeding, over-diuresis

Types of HRS (Updated Nomenclature)

Diagnostic Criteria (International Club of Ascites, 2015)

1. Cirrhosis with ascites
2. AKI by ICA criteria: rise in serum creatinine ≥0.3 mg/dL within 48 h OR ≥50% rise from baseline within 7 days
3. No response after ≥48 h of diuretic withdrawal + IV albumin (1 g/kg/day, max 100 g/day)
4. Absence of shock
5. No recent nephrotoxic drugs (NSAIDs, ACEIs, ARBs, certain antibiotics)
6. No intrinsic renal disease: proteinuria <500 mg/day, no microhematuria (>50 RBC/hpf), normal renal ultrasound

Note: HRS is a diagnosis of exclusion - ATN, AIN, glomerulonephritis, and obstructive uropathy must all be ruled out.

Clinical Features

• Most patients are asymptomatic except for decreased urine output
• Oliguria (urine output often <500 mL/day in HRS-AKI)
• Urinary sodium typically very low (<10 mEq/L) - reflects avid sodium retention
• Urine osmolality > plasma osmolality
• Rising serum creatinine and BUN
• Hepatic encephalopathy may be the presenting feature
• Background: jaundice, ascites, spider angiomata, asterixis

Management

• Avoid volume depletion (over-diuresis, lactulose diarrhea, large-volume paracentesis without albumin)
• Avoid nephrotoxins (NSAIDs, ACEIs, ARBs, certain antibiotics)
• Treat infections promptly (especially SBP)
• IV albumin 1.5 g/kg on day 1 + 1 g/kg on day 3 for SBP patients (prevents HRS)
• Beta-blockers + band ligation to prevent variceal bleeding

Duration of vasoconstrictor therapy: maximum 2 weeks, until HRS reverses or liver transplant is performed.

Prognosis

• Without treatment: median survival of Type 1 HRS is 2 weeks
• With terlipressin + albumin: reversal in ~40-50% of cases
• Liver transplantation is the only modality that can reverse both liver dysfunction and HRS; 35% still require long-term renal replacement therapy post-transplant