17 - Tuberculosis

Question 1

What is TB caused by?

Answer 1

Mycobacterium tuberculosis

• Aerobic
• Acid and alcohol fast bacilli
• Slow growing

Question 2

How can we stain for tuberculosis?

Answer 2

Sputum smear stained with Ziehl-Nielsen method

Takes 2-6 weeks to grow colonies

Question 3

How does TB transmit from person to person?

Answer 3

- Infected droplets in the air

• Need quite a prolonged exposure
• Patients sputum is infective until 2 weeks of treatment with anti TB chemotherapy

Question 4

Where is the most common site of pulmonary TB?

Answer 4

Right lung apex as high pO₂ in these areas compared to the rest of the lungs

Question 5

What is the pathogenesis of TB?

Answer 5

• Alveolar macrophages phagocytose MTB but cannot kill them as cell wall lipids of MTB block fusion of phagosome and lysosome
• Macrophages initiate cell mediated immunity so activated macrophages can come and kill MTB, takes about 6 weeks
• Granulomatous reaction from macrophages

Question 6

What does TB look like microscopically?

Answer 6

• Granuloma with central caseous necrosis (cheese) surround by epitheliod macrophages, langhans giant cells and lymphocytes

Question 7

What could cause a latent TB infection to reactivate?

Answer 7

• HIV
• Chemotherapy
• Malnutrition
• Old age

Question 8

How can we test for latent infection?

Answer 8

IGRA (QuantiFERON)

MTB antigens can make the body produce interferon gamma. Lymphocytes from the patient are cultured with MTB antigens and if T lymphocytes have been exposed before they will produce interferon gamma. The MTB antigen is not present in BCG or atypical mycobacteria so can distinguish latent from BCG vaccine

Tuberculin Skin Test:

Protein from MTB injected intradermally. Skin reaction 48-72 hours later indicates previous TB exposure, type IV hypersensitivity reaction to MTB

Question 9

What is the likelihood of someone being infected with TB actually developing the active disease?

Answer 9

10% lifetime risk

• 5% develop primary TB at initial infection when primary complex does not heal
• 5% develop post primary TB up to 60 years after initial infection

Question 10

What are some of the changes a patient may have with post primary TB?

Answer 10

- Cavity formation: liquefaction of caseous material. Fibrous tissue usually around periphery of lesions

- Haemorraghe: extension of caseous process into vessels. leads to haemoptysis

- Spread to rest of lung

• Pleural effusion: seeding of TB into pleura or hypersensitivity

- Miliary TB: rupture of caseous pulomnary focus into blood vessel so widespread dissemination through body

Question 11

What are some sites of extrapulmonary TB?

Answer 11

• Lymph nodes
• Bones
• Joints
• CNS
• GI tract
• Urinary tract

Question 12

What are some clinical features of pulmonary TB?

Answer 12

• Gradual onset over weeks or months
• Tiredness
• Malaise
• Weight loss
• Fever
• Sweats
• Cough with haemoptysis
• Can be asymptomatic even when CXR abnormal
• Crackles may be present
• Signs of pleural effusion or fibrosis

Question 13

What does a CXR of TB look like?

Answer 13

• Patchy solid lesions
• Cavity solid lesions
• Streaky fibrosis
• Flecks of calcification

Question 14

How do we diagnose active TB?

Answer 14

• Need to stain or culture
• Isolate organisma and determine drug susceptibility as well

Question 15

How do we treat TB?

Answer 15

- Rifampicin (red urine)

- Isoniazid (INAH)

- Pyrazinamide

- Ethambutol

All 4 drugs for 2 months and then just R and INAH for a further 4 months. Give pyridoxine with INAH to stop peripheral nerve damage

Question 16

Why do we give 4 drugs for TB?

Answer 16

One drug would allow selection of resistant strains, less likely to be resistant to all three drugs

Question 17

What should you do if you diagnose a patient with TB?

Answer 17

• Isolate and notify public health
• PPE
• Contact trace and vaccinate

Question 18

What are the differences between a latent and active TB infection?

Answer 18

Question 19

What are some groups of people at high risk of contracting TB?

Answer 19

• Non-UK born
• HIV
• Homeless
• Drug users

Question 20

What are some advantages and disadvantages of the tuberculin skin test?

Answer 20

+ Cheap

+ Lab infrastructure not needed

• False positives in BCG
• False negatives in immunocompromised
• Subjective interpretation

Question 21

What are some of the adverse effects of TB drugs?

Answer 21

Question 22

What are some extra-pulmonary signs of miliary TB?

Answer 22

• Headaches as meningeal involvement
• Pericardial and pleural effusions
• Retinal involvement
• Ascites

Question 23

Why does a premature baby have difficulty breathing?

Answer 23

• Lack of surfactant
• Increased surface tension and decreased lung compliance
• Increased effort needed to breathe

Question 24

Why does a deficiency of surfactant lead to hypoxia?

Answer 24

• Poor lung compliance
• Hypoventilation of lungs so less oxygen reaches alveoli
• Lack of surfactant leads to alveoli collapsing so no gas exchange in these alveoli