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Tuberculosis

Background knowledge 🧠

Definition

  • An infectious disease caused by the bacterium Mycobacterium tuberculosis, primarily affecting the lungs but can involve other organs

Epidemiology

  • One of the top 10 causes of death worldwide
  • Higher prevalence in low and middle income countries, particularly parts of Africa, Asia, and Latin America
  • Risk factors
    • HIV infection
    • Immunocompromise
    • TNF-alpha inhibitors and some other biologics
    • Older age
    • Homelessness
    • Alcohol excess

Transmission

  • Primarily spread via airborne droplets from coughs or sneezes of infected people

Pathophysiology

  • When a person becomes infected, TB bacilli enter the lungs
  • There are then three possibilities:
    • 5% of people develop active TB soon after infection (primary TB)
    • The remainder will develop latent TB. Of these:
      • 90% will never reactivate (and some will clear infection spontaneously)
      • 5% will develop active TB at some point in their lifetime – post-primary TB (risk increases with immunosupression)
  • These risks recur with each new infection

Clinical Features 🌑️

Symptoms

  • Pulmonary TB: chronic cough, haemoptysis, night sweats, fever, weight loss
  • Extrapulmonary TB: symptoms depend on the organ involved (e.g. lymphadenopathy, meningitis, back pain)
  • Miliary TB: disseminated form; non-specific systemic symptoms

Signs

  • Cachexia
  • Cough
  • Signs of other organs affected in extra-pulmonary TB
  • Rare signs
    • Poncet’s disease: reactive polyarthritis associated with active TB
    • Lupus vulgaris: painful cutaneous skin lesions, usually on the face

Investigations πŸ§ͺ

Diagnosis of active TB

  • Chest X-ray: upper lobe infiltrates, cavitation, hilar lymphadenopathy
  • Chest CT
  • Sputum AFB staining and culture: confirms diagnosis; allows for drug sensitivity testing; staining for acid fast bacilli may be via Ziehl–Neelsen stain or Auramine stains
  • Sputum TB PCR: Rapid molecular test for TB and rifampicin-resistance
  • Biopsy of affected tissue
    • TB culture and PCR
    • Histology: characteristically necrotizing granulomas

Screening for latent TB

  • Mantoux test (Tuberculin skin test): Measures immune response to TB antigen by cutaneous injection
  • Interferon-gamma release assays (IGRA): blood tests that looks for a cellular immune response to tuberculosis antigens

Management πŸ₯Ό

Treatment of active TB

  • First-line anti-TB drugs
    • Rifampicin β†’ hepatitis, turns bodily fluids red
    • Isoniazid β†’ hepatitis, peripheral neuropathy (given with pyridoxine to prevent this)
    • Pyrazinamide β†’ hepatitis
    • Ethambutol β†’ optic neuritis
  • Duration: 6 months for pulmonary tuberculosis (longer courses for some other types, e.g. CNS TB, spinal TB, ocular TB)
  • Multidrug-resistant TB (MDR-TB) = tuberculosis with resistance to rifampicin and isoniazid
    • Requires longer treatment durations with second-line drugs

Treatment of latent TB

Latent TB is defined as where there is documented previous exposure to TB without any evidence of active disease. Options for chemoprophylaxis to reduce the risk of reactivation include:

  • 3 months rifampicin + isoniazid
  • 6 months isoniazid

Complications

  • Drug resistance
  • Chronic pulmonary cavitation
  • Aspergillomas
  • Dissemination to other organs

Prevention

  • BCG vaccination in high-risk populations or areas
  • Early detection and treatment of cases
  • Screening and treating contacts
  • Isolation of infectious patients

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