Resp - TB Flashcards
suggest risk factors for TB
- close contact with TB pt
- ethnic minority group
- homeless pts, alcoholics and other drug abusers
- HIV +ve and other immunocompromised pts
- elderly pts and children
how is TB usually transmitted?
person-to-person by inhalation of infected droplets
describe the process of primary TB infection
- Mtb inhaled and deposited in alveoli…
- organisms phagocytosed by alveolar macrophages and carried to hilar LNs…
- Mtb prevents phagolysosome fusion but macrophages initiate cell-mediated immunity…
- over 6 wks, Th cell response mounted against Mtb, causing IFNy production and activation of macrophages to become bactericidal and produce TNF…
- monocyte recruitment and differentiation into ‘epithelioid histiocytes’…
- formation of caseating granulomas, i.e. Ghon’s foci.
describe the microscopic appearance of a TB granuloma
Caseous necrosis core surrounded by epithelioid macrophages, Langerhans giant cells and lymphocytes.
describe the possible outcomes of primary TB infection
- spontaneous healing and bacteria elimination (80%) - may form Ranke complexes (healed Ghon foci that have calcified)
- latent infection - some organisms may disseminate via lymphatics or bloodstream to distant sites, and persist in otherwise healthy individual
- primary active disease (5%)
name 3 differences between active and latent TB
Active
- active, multiplying bacilli in body
- symptomatic
- infectious
Latent
- inactive contained bacilli in body
- asymptomatic
- non-infectious
why can secondary infection occur? how is it different to primary active TB?
Re-activation of Mtb usually precipitated by impaired immune function, e.g. malnutrition, steroids, AIDS DM.
- Usually occurs in apex of lungs (higher alveolar pO2) - can spread locally or to distant sites.
- Usually more severe as involves activation of secondary immune response (stronger so more tissue damage) and bacteria has time to mutate and adapt.
describe the complications/lung damage that can occur as a result of pulmonary TB
- cavity formation - softening and liquefaction of caseous material which is discharged into bronchus, resulting in cavity formation which may then fibrose.
- haemorrhage - from extension of caesous process into vessels of cavity walls causing haemoptysis
- spread to rest of lung - caseous and liquefied material spread infection through bronchial tree to other lung zones
- pleural effusion (exudate), lobar collapse, beonchiectasis, pneumonia
what is miliary TB?
Haematogenous spread of Mtb causing widespread infection - lungs always involved as well as multiple other organs.
describe examples of how TB can affect extra-pulmonary organs
- CNS (tuberculous meningitis): initially non-specific symptoms (headache, vomiting, altered behaviour) followed by diminishing consciousness +/- focal neurological signs
2. cervical lymph nodes (scrofula)
3. pericardial: initially non-specific, may be signs of pericardial effusion (pulsus paradoxus, elevated JVP) or constrictive pericarditis
4. GI or peritoneal: mainly ileocaecal lesions (abdo. pain, bloating, obstruction and simulating appendicitis) but occasionally peritoneal spread causes ascites
5. genito-urinary: slow progression to renal disease and subsequent spreading to lowe urinary tract, infertility in females, epididymal swelling in males
6. bones and joints, esp. spine (Pott’s disease): pain, arthritis, osteomyelitis and absecess formation
7. skin: erythema nodosum (represents early immunological response to infection), skin sinus formation (scrofuloderma)
describe the symtpoms associated with pulmonary TB
- chronic productive cough
- purulent +/- bloodstained sputum
- dyspnoea (if pleural effusion)
- general symptoms: fever, night sweats, weight loss, anorexia, tiredness and malaise
how would you diagnose active TB?
Imaging
- CXR: essential even in non-pulmonary disease as there may have been pulmonary infection
Bedside tests
- sputum culture and microscopy (or BAL if not possible):
~ Ziehl-Neelsen stain and rapid direct microscopy for acid/alcohol-fast bacilli
~ culture on Lowenstein-Jensen slope (gold-standard but slow, 4-8 wks)
~ antibiotic sensitivity cultures (further 3-4 wks)
- TB culture of other samples if relevant, e.g. LN biopsies, urine…
which CXR features suggest TB infection?
- typical TB appearance: patchy or nodular shadows in upper zones, loss of volume, fibrosis +/- cavitations
- primary TB usually appears as central apical portion with L lower lobe infiltrate or pleural effusion
- reactivated TB: no pleural effusion and lesions are apical in position
- miliary TB: millet seed-like pattern indicating severe disease with poor immune response with uniform 1-10 mm shadows throughout the lung
how would you diagnose latent TB?
- tuberculin skin test (Mantoux): type IV hypersensitivity reaction (induration) if positive
- interferon-gamma release assay: results within 24 hrs and no cross-reaction with BCG but cannot distinguish between latent and active TB
which drug regimen would you use to treat active TB?
- RIFAMPICIN (6 mths)
- ISONIAZID (6 mths) + vit B6
- PYRAZINAMIDE (2 mths)
- ETHAMBUTOL (2 mths)
