Lung Abscess - Harrison's Principles of Internal Medicine (22nd Ed.)

Definition

Classification

Etiology / Microbiology (Harrison's Table 132-1)

Epidemiology & Risk Factors

• More common in middle-aged men
• Major risk factor: aspiration (the common pathway)
• High-risk groups:
  • Altered mental status, alcoholism, drug overdose, seizures
  • Bulbar dysfunction, prior cerebrovascular events, neuromuscular disease
  • Esophageal dysmotility, strictures, or tumors
  • Gastroesophageal reflux; recumbent position
  • Gingivitis/periodontal disease - gingival crevices colonized by anaerobes serve as the bacterial nidus; lung abscess is considered extremely rare in edentulous patients

Pathogenesis

Clinical Features

Symptoms

• Subacute presentation typical - symptoms develop over days to weeks
• Cough - initially non-productive, later productive
• Foul-smelling / putrid sputum - hallmark feature indicating anaerobic infection
• Pleuritic chest pain
• Fever and chills
• Night sweats
• Hemoptysis - can occur, rarely massive
• Weight loss and fatigue (especially chronic cases)
• Dyspnea

Signs

• Fever (high-grade, often >39°C)
• Tachycardia, tachypnea
• Dullness to percussion over involved area
• Bronchial or amphoric breath sounds over the cavity
• Decreased breath sounds, crackles
• Clubbing may be present in chronic cases
• Signs of periodontal disease / poor dentition commonly present
• Altered mental status in high-risk (aspirating) patients

CT Scan Image (Harrison's Fig. 132-1)

Diagnosis

Laboratory

• CBC: Leukocytosis with left shift; leukopenia may suggest immunocompromise or overwhelming sepsis
• Elevated ESR, CRP
• Blood cultures (positive in ~15% of cases; more likely in secondary/embolic abscesses)
• HIV testing in appropriate patients
• Serum LDH may be elevated

Imaging

• CXR: Thick-walled cavity, often with air-fluid level; predilection for posterior segments of upper lobes and superior segments of lower lobes (gravity-dependent zones in recumbent patients)
• CT chest (preferred): Better defines cavity wall thickness, identifies satellite lesions, distinguishes abscess from empyema, detects endobronchial obstruction. Abscesses have thick irregular walls, surrounded by consolidation. CT is critical for ruling out underlying malignancy.
• Classic location: Right lower lobe > right upper lobe posterior segment > left lower lobe (gravity-dependent aspiration)

Microbiological Workup

• Sputum Gram stain and culture (including anaerobic cultures, AFB, fungal stains)
• Bronchoscopy with BAL - recommended, especially in: immunocompromised patients, failure to respond to empirical therapy, suspicion of endobronchial obstruction, or need to rule out malignancy
  • BAL specimens should be sent for aerobic, anaerobic (if feasible), mycobacterial, fungal, Nocardia cultures
  • CT-guided biopsy of cavity wall if malignancy suspected
• Blood cultures x2
• Thoracentesis if pleural effusion/empyema present

Management

Antibiotic Therapy

Drainage

• Postural drainage / chest physiotherapy - important adjunct; position patient to drain dependent segments
• Bronchoscopic drainage - can be used to facilitate drainage and obtain cultures; generally not first-line but useful in non-resolving cases
• Percutaneous catheter drainage - indicated when:
  • Abscess fails to respond to antibiotics after 4-6 weeks
  • Large abscess (>6-8 cm)
  • Communication with pleural space
  • Immunocompromised host
  • CT-guided; effective in selected patients
• Surgical resection (lobectomy) - reserved for:
  • Failure of medical therapy and percutaneous drainage
  • Massive hemoptysis
  • Underlying malignancy
  • Suspected drug-resistant organisms requiring tissue diagnosis
  • Rare in the antibiotic era; carries significant morbidity

Supportive Care

• Adequate nutrition (often need supplemental nutrition in chronic cases)
• Bronchodilators if bronchospasm present
• Avoid bronchoscopy early if concern for cavity rupture
• Treat underlying conditions (periodontal disease, GER, etc.)

When to Reassess / Escalate

• Repeat imaging
• Bronchoscopy to rule out obstruction or malignancy
• Review of microbiology; culture-directed therapy
• Consideration of percutaneous or surgical drainage

Differential Diagnosis

• Thin-walled cavity - favors cyst, bulla, or resolved infarct
• Thick irregular wall - favors malignancy or active infection
• Air-fluid level - strongly supports abscess
• Location in dependent zones - strongly supports aspiration-related abscess
• Satellite lesions, lymphadenopathy - may suggest malignancy or TB
• Foul sputum + periodontal disease - virtually pathognomonic of anaerobic lung abscess

Complications

• Hemoptysis (can be massive/life-threatening)
• Bronchopleural fistula
• Empyema thoracis
• Mediastinitis (rare, very serious)
• Septicemia / metastatic infection
• Amyloidosis (rare, chronic cases)

Prognosis

• Primary lung abscess with anaerobes: excellent - ~85-90% cure with antibiotics alone
• Poor prognostic factors:
  • Secondary abscess (underlying malignancy, immunocompromise)
  • Cavity >6 cm
  • Aerobic gram-negative or S. aureus causative organisms
  • Age extremes
  • Delayed diagnosis and treatment
  • Obstruction from malignancy

Liver Abscess - Harrison's Principles of Internal Medicine (22nd Ed.)

PYOGENIC LIVER ABSCESS

Etiology & Pathogenesis

Clinical Features of Pyogenic Liver Abscess

• Fever (hectic, with chills/rigors) - most consistent finding
• Right upper quadrant (RUQ) pain - dull or colicky; may radiate to right shoulder
• Nausea and vomiting
• Anorexia and weight loss
• Malaise and fatigue
• Symptoms of underlying source (appendicitis, cholangitis, etc.)

• Hepatomegaly - tender
• RUQ tenderness on palpation
• Jaundice - in biliary source or large abscesses compressing bile ducts (~25%)
• High fever (>38.5°C)
• Reduced breath sounds at right base (reactive pleural effusion)
• Rarely: palpable mass

• Leukocytosis with left shift
• Elevated alkaline phosphatase (most sensitive LFT - elevated in ~90%)
• Elevated bilirubin, elevated transaminases (moderate)
• Elevated ESR, CRP
• Hypoalbuminemia (chronic/severe cases)
• Blood cultures positive in ~50% of cases

Diagnosis of Pyogenic Liver Abscess

• Ultrasound - first-line; shows hypoechoic lesion; can guide drainage; sensitivity ~85%
• CT abdomen with contrast - gold standard; shows ring-enhancing hypodense lesion with surrounding hepatic edema; better for small lesions; identifies source (biliary/appendix)
• MRI - useful for complex cases
• CXR - elevated right hemidiaphragm, right pleural effusion, atelectasis

• Blood cultures (aerobic + anaerobic) x2 before antibiotics
• Abscess aspirate: Gram stain + aerobic, anaerobic, AFB, fungal cultures
• Serology for E. histolytica to exclude amebic abscess

Management of Pyogenic Liver Abscess

1. Antibiotics

• Transition to oral therapy after clinical improvement
• Duration: typically 4-6 weeks total (longer for large or multiple abscesses)
• Treat underlying source simultaneously (biliary drainage, appendectomy, etc.)

2. Drainage

• Indicated for most pyogenic abscesses >3-4 cm
• Needle aspiration (single or repeated) - suitable for smaller lesions (<5 cm)
• Catheter drainage - for large (>5 cm) or multiloculated abscesses; left in situ until output minimal

• Failed percutaneous drainage
• Ruptured abscess with peritonitis
• Multiple lesions not amenable to percutaneous approach
• Need to address underlying surgical source (perforated viscus, etc.)

3. Treat Underlying Cause

• Biliary: ERCP with sphincterotomy/stenting, cholecystectomy
• Appendicular: appendectomy
• Diverticular: colectomy if needed

AMEBIC LIVER ABSCESS

Etiology & Epidemiology

• Caused by Entamoeba histolytica (not E. dispar which is non-pathogenic)
• Most common extraintestinal manifestation of amebiasis
• Predominantly affects men aged 30-60 years - 10-12x more common than in women of same age (testosterone increases susceptibility via modulation of NK T-cell IFN-γ secretion)
• Travelers to endemic areas: 95% develop amebic liver abscess within 5 months of returning
• Endemic areas: South Asia, Sub-Saharan Africa, Central/South America, Mexico

Pathogenesis

1. Ingestion of E. histolytica cysts via fecal-oral route
2. Cysts excyst in intestine → trophozoites colonize colon
3. Trophozoites invade colonic mucosa → portal venous invasion
4. E. histolytica is resistant to complement-mediated lysis (survives in bloodstream)
5. In liver: acute neutrophilic infiltrate → neutrophils lysed by amebae (neutrophil toxins contribute to hepatocyte necrosis)
6. Liver parenchyma replaced by necrotic material - classically described as "anchovy paste" (reddish-brown, bacteriologically sterile fluid, composed of granular debris with few cells)
7. Amebae tend to be found near the capsule of the abscess (not centrally)

Clinical Features of Amebic Liver Abscess

• Fever - present in most patients (>38.5°C)
• RUQ pain - dull or pleuritic, may radiate to the right shoulder
• Younger patients: acute presentation (<10 days' symptoms)
• Older patients from endemic areas: subacute/chronic course (up to 6 months) with weight loss and hepatomegaly; only ~1/3 febrile in chronic presentation
• Diarrhea - present in <1/3 of patients (colon is primary infection site but active diarrhea is often absent)
• Anorexia, malaise
• 10-15% present with fever alone → amebic liver abscess must be in differential for fever of unknown origin (FUO)

• Point tenderness over the liver - characteristic
• Right-sided pleural effusion - common
• Hepatomegaly
• Jaundice - rare (differentiates from pyogenic in which jaundice is more common)
• Generally less toxic-appearing than pyogenic liver abscess

Complications of Amebic Liver Abscess

• Pleuropulmonary involvement - most frequent complication (20-30%):
  • Sterile effusions (resolve with medical therapy)
  • Contiguous spread from liver
  • Rupture into pleural space (requires drainage)
  • Hepatobronchial fistula - cough productive of large amounts of necrotic material ("anchovy paste sputum"); carries good prognosis
• Rupture into peritoneum - may be indolent or acute abdomen; requires percutaneous catheter drainage + medical therapy
• Pericardial rupture (left lobe abscess) - gravest prognosis; requires surgical drainage; can occur even during active medical therapy
• Genitourinary involvement - direct extension or hematogenous spread; painful genital ulcers
• Cerebral amebiasis - <0.1% of cases

Diagnosis of Amebic Liver Abscess

• Leukocytosis >10,000/μL in ~75% (especially acute presentations)
• Elevated alkaline phosphatase, mild transaminase elevation
• Elevated ESR, CRP
• Mild normochromic anemia

• Wet mounts + iodine-stained concentrates + trichrome stain → confirms diagnosis in 75-95% if cysts/trophozoites found
• Examine at least 3 fresh stool specimens (trophozoites die quickly)
• Hematophagous trophozoites (containing ingested RBCs) are diagnostic

• Indirect hemagglutination (IHA), ELISA, immunodiffusion - positive in >90% of patients with liver abscess
• Results revert to negative within 6-12 months - positive serology = active disease in the appropriate clinical context
• In highly endemic areas, serology less useful (prior exposure common)
• Specific antigen-detection tests (TechLab E. histolytica II) - can distinguish E. histolytica from E. dispar
• PCR available in some centres

• Ultrasound - first-line; amebic abscess typically is a single, round, hypoechoic lesion in the right lobe; no internal echoes
• CT/MRI - homogeneous round/oval lesion, well-defined walls; right lobe predilection

1. Need to rule out pyogenic abscess, especially with multiple lesions
2. Lack of clinical response to metronidazole within 3-5 days
3. Left lobe abscess (risk of pericardial rupture)
4. Imminent rupture (large abscess, bulging)

Management of Amebic Liver Abscess

Drug Therapy

• Iodoquinol 650 mg TID x 20 days, OR
• Paromomycin 500 mg TID x 10 days

Aspiration/Drainage

• Usually NOT required for uncomplicated amebic liver abscess (responds to metronidazole)
• Indications (as above): rule out pyogenic, no response in 3-5 days, left lobe, imminent rupture
• Pericardial rupture → surgical drainage required

Prognosis

• Mortality <1% with early diagnosis and metronidazole therapy
• Amebic liver abscess rarely recurs

Differential Diagnosis of Liver Abscess