TB

Question 1

Describe the process of TB infection

(Primary)

Answer 1

Primary TB

• Produced by M. Tuberculosis infection in those not previously infected
• Mild inflammatory response at site of infection followed by spread to regional lymph nodes
• Combination of infective focus and lymph node involvement is known as the primary complex
• The infective focus is known as the ‘Ghon focus’
• 1-2 weeks after the infection, with the onset of immune sensitivity, the tissue reaction at both sites of the primary complex changes form characteristic caseating granulomas
• Pts usually asymptomatic
• Viable bacteria may remain walled off within the primary complex giving latent TB

Question 3

What methods might primary TB become symptomatic?

Answer 3

• The Ghon focus can erode through the visceral pleura to discharge organisms and cause TB pleurisy/pleural effusion
• Enlarged hilar lymph nodes can also erode into the bronchus and rupture, causing TB bronchopneumonia
• The enlarging nodes can also erode into vessels, giving miliary dissemination to the lung (pulmonary arteries) or systemic dissemination (pulmonary vein)
• Erythema nodosum (red lumps on skin) is also common in primary disease

Question 4

Describe how post-primary (secondary) TB infection occurs

Answer 4

• M. Tuberculosis re-infection in tuberculin-sensitive individuals
• Infection can be from exogenous sources, or more commonly ‘reactivation’ from a healed primary complex
• There is an immediate granulomatous response to the disease, thus regional lymph node involvement is not common
• In the lung, this creates a classical apical lesion termed Assmann focus, with destruction of lung parenchyma leading to caviation
• Again the lesion may heal with fibrosis and calcification if the immune system is strong or will progressively enlarge in those with poor immune systems
• This has greater risks of eroding into vessels/airways and causing complications

Question 5

What are symptoms of post-primary TB?

Signs on examination?

Answer 5

• Earliest symptoms non-specific:
  
  • malaise, night sweats, anorexia, weight loss
• Specific symptoms occur late (only in establish disease)
  
  • Productive mucoid cough
  • Repeated small haemoptysis
  • Pleural pain
• Can present with pleural effusion

Signs on examination:

• Fever and apical crepitations with late signs of consolidation or pleural effusion
• May be clubbed in advanced disease

Question 6

What are common predisposing factors for a TB infection?

Answer 6

• High risk patients are those
  
  • born in endemic areas
  • Previously treated for TB (can come back - post-primary)
  • Close contacts with TB
  • Immunosupressive co-morbidites or drug treatments
  • Live in overcrowded conditions
  • Alcohol/drug abusers

Question 7

What investigations would you do if a patient has suspected active pulmonary TB?

Answer 7

• Sputum samples: take at least 3, including one morning sample
  
  • Microscopy: for acid-fast bacilli, results within 24h
  • PCR: if rapid diagnostic results are required or suspected MDR-TB (however this will not differentiate between active & latent TB)
  • Culture: gold standard diagnostic test, but takes 6 weeks, on Lowenstein-Jensen medium
• If sputum samples negative, bronchoscopy with biopsy
• CXR
  
  • upper lobe cavitation
  • pleural effusions
  • lymphadenopathy

Question 8

What investigations would you do for a patient with suspected latent TB?

Answer 8

• Mantoux test
  
  • TB antigen injected, size of wheal reaction monitored (is there a reaction)
• Interferon-gamma release assay: IGRA blood test also required to diagnose latent TB in immunocompromised individuals, as they can have false negatives in skin tests
• Always do standard bloods
  
  • FBC, U&Es, LFTs to rule out other causes and will affect treatment given

Question 9

List the common sites and pathological features of non-pumonary TB

Answer 9

When small numbers of tubercle bacilli escape into the blood, most die if host defence mechanisms are effective, yet for unknown reasons some bacilli settle in specific organs and may remain dormant for many years, appearing later to cause disease:

• Painless lymphadenopathy: lymphatic TB
• Monoarthritis: joint/spinal TB
• Sterile pyuria: renal TB
• Meningitic syndrome: TB meningitis
• Erythema nodosum/lupus vulgaris: cutaneous TB
• Chest pain: TB pericarditis