TB Flashcards
What happens to the aerosolised droplets?
Droplets evaporate to form droplet nuclei which disperse in the air without settling
The organisms they contain remain viable for extended periods of time
How is TB transmitted?
Between people via aersolised droplets
They are produced in coughing, sneezing, talking etc.
How are the droplets eventually eliminated?
Infinite dilution
UV radiation
What happens when the mycobacteria droplets reach the pulmonary alveoli?
Engulfed by alveolar macrophages
Mycobacterium replicates within them.
Macrophages carry the bacteria to hilar lymph nodes to try and control the infection.
Why are macrophages unable to kill mycobacteria?
The bacteria’s cell wall contains lipids which block fusion of phagosomes and lysosomes
How does a granulomatous reaction occur?
Macrophages ingest mycobacterium
Lymphocytes and fibroblasts surround the infected macrophages to prevent dissemination of bacteria
What is caseous necrosis?
Epithelioid macrophages, Langerhans giant cells and lymphocytes surround a cheese-like core of necrosis
Form a tubercle
What forms the primary complex?
Ghon’s nodes - sub pleural focus of tubercles
Draining lymph nodes
Classifications of mycobacterium tuberculosis?
Bacilli, aerobic, acid and alcohol fast
Slow growing on culture (2-6 weeks)
Why are mycobacteria Gram resistant?
Cell wall is hydrophobic
How does haematogenous spread occur?
Before healing of the primary complex the TB bacilli can enter the bloodstream and sorehead to other parts of the lung and other organs (extra-pulmonary sites)
What is latent TB?
When the primary infection is contained, primary complex heals but some organisms remain viable
What can cause deactivation of latent TB?
When immune system is compromised
HIV, age, malnutrition, immunosuppression
What does latent TB become if reactivated?
Primary progressive TB
What happens to the granulomas in primary progressive TB?
They form but necrotic tissue undergoes liquefaction and the fibrous wall breaks down.
Necrotic material drains into the bronchi, is coughed up and can infect others.
Can drain into blood vessels and become extra-pulmonary TB.
What is primary TB?
When you get formation of the primary complex and it resolves with local scarring
Where does primary infection of TB commonly occur?
Upper lobe
Why is post-primary pulmonary TB common in the upper lung zones?
High ventilation/perfusion
High pO2
What can post-primary pulmonary TB lead to and how?
Proliferation of TB bacilli in caseous centres followed by softening and liquefaction of caseous material
Can discharge into the bronchus and cause cavity formation
Fibrous tissue forms around periphery of TB regions but unable to limit extension of TB process
Haemorrhage of caseous process into vessels in cavity walls - haemotypsis
How does post primary pulmonary TB lead to TB pneumonia?
Get spread of caseous/liquefactive material through the bronchial Rees causing infection in other lung zones
See lots of inflammatory exudate which fills the alveoli to form pneumonia
How does miliary TB occur?
When you get rupture of caseous pulmonary focus into a blood vessel
Get formation of miliary TB foci in lung and other organs.
Signs and symptoms of TB?
Persistent cough Haemotypsis Shortness of breath Fever and chills Fatigue Loss of appetite and weight loss Lymphadenopathy
What can be seen on a CXR?
Pulmonary shadowing which be patchy, solid lesions
Streaky fibrosis or flecks of calcification
How is latent TB diagnosed?
Positive tuberculin skin test
Patient has type IV hypersensitivity reaction to proteins derived from mycobacteria
