S9: lung radiology & tuberculosis Flashcards
What is the silhouette sign?
Adjacent structures of differing density form a crisp silhouette
Heart next to lung = white next to black
Loss of this contour can locate pathology
Describe x-ray findings of a mediastinal shift
Adequately centred image
Push = increase volume/pressure
Pull = decrease volume/pressure
Describe the causes of lobar lung collapse
Volume loss within lung lobe
Causes:
1) Luminal – aspirated foreign material, mucous plugging, iatrogenic
2) Mural – bronchogenic carcinoma
3) Extrinsic – compression by adjacent mass
Describe x-ray findings of lobar lung collapse
Elevation of the ipsilateral hemidiaphragm
Crowing of the ipsilateral ribs
Shift of the mediastinum towards the side of atelectasis
Crowding of pulmonary vessels
What is consolidation?
Filling of small airways/alveoli with stuff
- pus = pneumonia
- blood = haemorrhage
- fluid = oedema
- cells = cancer
Describe the causes of a space occupying lesion
Malignant – primary, metastases Benign mass lesion Inflammatory Congenital Mimics – bone lesion, cutaneous lesion, nipple shadow
Describe the cardiac index
Normal <50% - must be a PA image
BEWARE OVERESTIMATION ON AP IMAGE
Describe the microbiology of TB
Caused by bacteria belonging to the mycobacterium tuberculosis complex
Non-motile rod-shaped bacteria
Obligate aerobe
Long-chain fatty acids, complex waves & glycolipids in cell wall -> structural rigidity, resists decolourisation by acids during staining procedures (acid-fast)
Relatively slow-growing compared to other bacteria
List groups at high risk of TB
Non-UK born/recent migrants HIV, other immunocompromised conditions Homelessness Drug use, prisoners Close contacts of TB Young adults (also higher incidence in elderly)
How is TB transmitted?
Spread by respiratory droplets – coughing, sneezing etc
Infectious dose 1-10 bacilli
Contagious, but not easy to acquire infection – prolong exposure facilitates transmission
Describe the pathogenesis of TB
Inhaled aerosols
Engulfed by alveolar macrophages
Local lymph nodes -> primary complex (Ghon’s focus and draining LN)
Either progression to active disease (primary TB) or containment of the infection
Latent infection -> heals or reactivation to form post primary TB
Describe the formation of tubercles in TB
Ingestion of MTB by macrophages causes a granulomatous reaction
Characteristic lesion of TB is a spherical granuloma with central caseation
List the clinical features of TB
Onset of active TB is gradual over weeks or months – tiredness, malaise, weight loss, fever, sweats & cough (may be dry or productive)
May be no clinical signs on examination even when the CXR is abnormal
Describe primary infection TB
First exposure to MTB in susceptible hosts
Ghon’s focus = development of a sub-pleural focus of tubercles
MTB bacilli drain from the primary focus into the hilar lymph nodes -> together these are known as the primary Gohn complex
Most primary infections will heal with/without calcification of the primary complex, but in some the bacteria enters the bloodstream and results in MTB in other parts of lung & body
What is latent infection?
State when MTB can persist within the human host, without causing disease
Person remains well but the potential for reactivation at any site is always present
Reactivation occurs when the patient’s immune mechanisms wane or fail
Characterised by a positive ‘quantiFERON’ test or a positive tuberculin skin test
Describe the tuberculin skin test
Tuberculin is injected intra-dermally
Presence of a skin reaction 48-72 hours later at the site indicates previous exposure to TB and is due to type IV hypersensitivity reaction to proteins
False positives – BCG, non-TB mycobacterium
Describe post primary TB
Disease that arises in a previously infected and sensitised host – typically associated with weakened host resistance
Systemic signs and symptoms: malaise, weight loss, fatigue, low-grade fevers, productive cough & haemoptysis
Cavity formation
Haemorrhage
Spread to involve the rest of the lung
Pleural effusion
Describe miliary pulmonary TB
Occurs when the MTB drainage through the lymphatic system enters the venous blood and circulates back to the lung
Some foci of infection are visible through the lungs (CXR)
Describe extra pulmonary TB
Haematogenous spread of primary TB/reactivation of latent TB in sites other than the lung results in active TB at sites other than the lungs
Sites involved: lymph nodes, bones, CNS, joints, GI tract
Symptoms will depend on the organs seeded
NB: TB causing Addison’s disease is common
Describe the diagnosis for active TB
Acid-fast smear and cultures of sputum
Sputum culture is the most sensitive and specific test & is required to provide drug susceptibility information – may take 1-3 weeks or 4-8 weeks depending on culture medium used
Describe the diagnosis for latent TB
IGRA or tuberculin skin test
A positive IGRA or tuberculin skin test demonstrates T cell mediated immunity to mycobacterial organisms BUT does not differentiate between infection and active disease
Tuberculin skin tests do not differentiate between latent infection & people who have had BCG/infection by atypical mycobacterium
People with suppressed immune systems/active TB may have falsely negative IGRA/skin tests
Describe the IGRA/QuantiFERON test
Based on the ability of some mycobacterium tuberculosis antigens to stimulate host production of interferon gamma
Lymphocytes from the patient’s blood are cultured with these antigens -> if patient has been exposed to TB before, T lymphocytes produced interferon gamma in response
Describe the treatment for TB
Combination of antibiotics over several months
Drugs currently used: rifampicin, isoniazid (INAH), pyrazinamide & ethambutol
All 4 drugs for 2 months followed by rifampicin and INAH for a further 4 months
Why are four drugs used to treat TB?
Wild MTB strain contains a small number of naturally drug resistant organisms arising through spontaneous mutations
Using a single drug allows selection of these resistant strains to emerge
Being resistant to all 4 drugs is unlikely
Why is TB a leading cause of death in people with HIV/AIDS?
CD4 T cells are a key component of the immune response against MTB
HIV-infection-associated CD4 T-cell depletion greatly increases susceptibility to developing TB
NB: may have negative screening test due to co-infection
Describe prevention of TB
Notifiable disease – must notify public health of a case
Infection control
Address risk factors
BCG vaccination – given to babies in high prevalence communities, 70-80% effectiveness in preventing severe childhood TB, little evidence protecting adults
