TUBERCULOSIS Flashcards
What is the organism that most commonly causes tuberculosis (TB)?
Mycoplasma tuberculosis is a aerobic bacteria with an unusual waxy coating on its cell surface. This renders it impervious to Gram staining, although it is considered Gram positive.
Where in the lungs does the bacterium usually implant itself and why?
In the upper lobes because this is where the oxygen partial pressure is the highest.
Do most patients with TB display symptoms?
No. Most cases of TB are latent. The infection is controlled by the cell mediated immune system. Tissue will heal through scar formation and eventual calcification, which may be seen on a chest x-ray.
Which group of patients is most often affected by primary pulmonary TB?
Immunocompromised patients. It is the leading cause of death in patients with HIV.
What is the UK incidence of TB?
8,200 per year. It results in 350 deaths per year in the UK.
Explain briefly how M. tuberculosis causes disease.
The organism finds its way into an alveolar macrophage through phagocytosis. M. tuberculosis then replicates within the cell. Granulomas are then formed as other macrophages, T and B lymphocytes and fibroblasts aggregate around the infected macrophage. The bacteria can remain dormant in the granuloma or can spread to other parts of the body via the lymphatic system and then the blood stream. Tubercles form around the granuloma and necrosis will happen at the centre of the tubercle.
How is the necrosis that occurs in tuberculosis described, describing its texture and appearance?
Caseous necrosis named because it seems to have the texture of soft white cheese.
What is the primary site of infection in the lung known as?
The Ghon focus
Where might M. tuberculosis spread to?
Almost anywhere. Peripheral lymph nodes, the kidneys, the brain and bones. It rarely affects the heart, skeletal muscle, the pancreas or the thyroid.
What is a Ghon complex?
When tubercle bacilli drain into the local lymph nodes, the lesion in the lung plus the granulomas in the lymph node all form the ghon complex.
How are M. tuberculosis organisms identified in histology?
Ziehl-Neelsen staining. This picks up acid fast bacteria.
What is secondary pulmonary tuberculosis?
The form of disease that arises in a previously sensitised host. Usually arises from reactivation of dormant primary lesions.
What is cavitating tuberculosis?
This is when the apical lesion enlarges significantly through necrosis. There is an expansion of the area of caseation creating a ragged edge to the lung.
There is erosion of the vessels which leads to haemoptysis. This means that likelihood of infection spreading is very high.
Patient is coughing up necrotic tissue so infection can spread to upper airway or other lung.
What is miliary tuberculosis?
Form of tuberculosis characterised by wide dissemination into the body and by the tiny size of the lesions (1-5 mm). These foci of consolidation may expand and coalesce. The organism drains into the lymphatics and then into the blood stream. It can affect any number of organs, including the lung, liver, spleen, bone marrow, meninges, fallopian tubes and epididymis.
What is open tuberculosis?
This is the rapid spread of the organism through the airways. It is also called Tuberculous Bronchopneumonia. There is extensive parenchymal inflammation.
What is Pott’s disease?
This is an isolated infection of TB in the spine. It results from a single episode of haematogenous spread.
How might someone with suspected tuberculosis present?
Remember that with a primary infection of TB there can be very few signs or symptoms. Patients may also present with:
cough, sputum production, malaise, weight loss, night sweats, pleurisy, haemoptysis, pleural effusion, nail clubbing.
What is the name of the test that can detect whether the body has previously come across TB antigen?
Mantoux test or Tuberculin skin test or Pirquet test
What is involved in the Mantoux test and what is considered a positive result?
TB antigen is injected intradermally and the cell mediated response is recorded at 48-72 hours. The response is based on the diameter of the induration that has formed at the sight of injection.
An outright positive result is when the diameter is 15 mm or more. However, a result of 10 mm or more should be considered positive in those with risk factors, such as injection drug users, and children under 4. A result of 5 mm or more should be considered positive in those who have come in contact with a TB patient and those who are HIV positive.
What might lead to a false positive result on the Mantoux test?
Someone who has had the BCG vaccination may react positively to the Mantoux test. However, the size of the induration is likely to be smaller than 15 mm. Interferon-gamma testing should be considered following a positive result in someone who has had the vaccine.
What might lead to a false negative result on the Mantoux test? (Name 5)
Several conditions can suppress the reaction to the Mantoux test:
Infectious mononucleosis Live virus vaccine Sarcoidosis Hodgkin's disease Corticosteroid therapy Malnutrition Immunologically compromised
What investigations would be needed to help diagnose active TB having performed the Mantoux test?
A chest x-ray would reveal consolidation and possible marked cavities (tubercle), fibrosis and calcification.
3 or more sputum samples are needed and sent for MC&S (microscopy, culture and sensitivity). Ziehl-Neelsen staining will reveal acid-fast bacilli. If sputum cannot be obtained spontaneously, bronchoscopy and lavage may be required.
What are the drugs used to treat tuberculosis? Can you name the standard doses of each one and the length of time the patient would normally stay on each drug?
Rifampicin 600-900 mg PO
Isoniazid 15 mg/kg (max 900 mg) PO
Pyrazinamide 2.5 g PO
Ethambutol 30 mg/kg PO
All 3 times a week for 8 weeks minimum.
What are the main side effects of rifampicin?
Rifampicin can be hepatotoxic. There will be small rises in AST (aspartate transaminase) but any rise in bilirubin should prompt the doctor to stop therapy immediately.
Reduction in platelet count.
Orange discolouration of urine
Rifampicin induces P450 enzymes therefore there will interaction with other medication. The Pill becomes inactive.
What are the main side effects of isoniazid?
Rise is liver function tests.
Reduction in white cell count.
Isoniazid can cause neuropathy and should be stopped at the first sign.
What should be given instead of isoniazid when there are signs of neuropathy? Can you give the dose and route of administration?
Pyridoxine 50 mg/8 hr PO
