Pulmonary
Tuberculosis
Worldwide, tuberculosis (TB) affects
10 million people and causes 2 million deaths each year. In developed countries
it is uncommon, affecting approximately 1 per 10 000 population. Pulmonary TB
is most common in Asian, Chinese and West Indian people. Airborne transmission and close contact spread the disease. Those who are elderly,
malnourished or immunosuppressed (HIV infection, diabetes mellitus,
corticosteroid therapy, alcoholism, intercurrent lymphoma) are more susceptible.
Improved housing and nutrition reduce incidence.
Pathogenesis
Primary pulmonary TB is caused by the acid-fast bacillus Mycobacterium tuberculosis. The inhaled bacillus infects well-ventilated,
poorly perfused upper lung lobes subpleurally. A granuloma forms (Fig.
38a) known as the Ghon focus, and with the enlarged hilar lymph node
draining the affected lung is known as the 'primary complex' (Fig. 38a).
This occurs over 3-8 weeks, and is accompanied by development of an inflammator
reaction to injection of tubercular protein (tuberculin) into the skin,
which can be used as a diagnostic test (Mantoux or Heaf test).
Complete healing usually follows, with fibrosi and calcificatio of the Ghon
focus and immunity to further infection.
Post-primary pulmonary TB occurs if the Ghon focus fails to heal due to poor
host defences, or following reactivation. It is potentially fatal. Local
dissemination causes tuberculous pneumonia and pleural effusions.
Bloodborne spread may affect the meninges or individual organs. In a few cases,
widespread infection involves many tissues (miliary TB).
Clinical features
Primary pulmonary TB usually occurs at an early age. Often asymptomatic
with no clinical signs, it may cause a mild febrile illness, erythema
nodosum (painful, indurated shin lesions) and small pleural effusions.
Bronchial compression by lymphadenopathy may cause wheeze and occasionally
lobar collapse followed by late bronchiectasis (Chapter 34).
Post-primary TB develops over months, with malaise, anorexia, weight
loss, night sweats and a productive cough. Breathlessness, chest pain,
haemoptysis and cervical lymphadenopathy may occur. Clinical signs of pneumonia
and pleural effusion are common, whereas lupus vulgaris (an indolent skin
infection) is less frequent. Miliary TB presents with a non-specifi
pyrexial illness, malaise and weight loss. Sparse clinical signs include
hepatomegaly and choroidal tubercles in the retina.
Investigation
Blood tests may detect anaemia, decreased sodium and increased
calcium.
Mantoux test: strongly positive in post-primary pulmonary TB (>5
mm skin induration with 10 units of intradermal tuberculin; read at 3 days).
Often negative in miliary TB (reduced host response) and HIV (reduced cellular
immunity).
Heaf test (screening test; now less commonly used): a ring
of six pinpricks is made through a tuberculin solution on the forearm. No
response at 4-7 days (grade 0) demonstrates lack of immunity; 4-6 discrete
nodules (grade 1) or a ring formed by coalition of all pinpricks (grade 2)
indicates immunity. A single nodule formed by infillin of the ring (grade 3)
represents recent contact or early tuberculous infection, and a nodule of more
than 5-7 mm with surface vesicles or ulceration (grade 4) suggests infection.
Microbiology: the acid-fast bacilli may be detected in sputum or
lung washings using Ziehl-Neelsen stain. However, bacilli are slow growing,
and culture and drug sensitivities take 4-6 weeks. Bone marrow or cerebrospinal
f uid (CSF) culture may confir the diagnosis of miliary TB.
Histopathology: pleural aspiration with biopsy confirm TB in ap-
proximately 90% of patients with pleural effusions. Liver biopsy will isolate
miliary TB in approximately 60% of cases.
Chest radiography (Fig. 38b): upper lobe shadowing is suggestive.
Apical cavities, pleural effusions and pneumothoraxes may occur. In miliary TB,
widespread small nodules (2-3 mm diameter) are diffusely spread throughout the
lungs (miliary shadowing), and are easily missed.
Drug therapy
Prognosis is good if the patient is
not immunocompromised. Good nutrition, reduced alcohol consumption and compliance
with drug therapy are important factors. Uncomplicated pulmonary TB is
treated for 6 months. Initially, at least three drugs are used to prevent
development of resistant strains. The recommended regimen is rifampicin,
pyrazinamide and isoniazid for 2 months, followed by rifampicin and isoniazid
for 4 months. Additional pyridoxine prevents isoniazid-induced peripheral
neuropathy. Liver function should be monitored, as rifampicin and pyrazinamide
can cause liver dysfunction. If drug resistance is suspected (TB recurrence in
a non-compliant patient) then a four-drug regimen (adding ethambutol) may be
initiated. When culture results are available, alternative drugs replace those
to which the mycobacterium is not sensitive. Ethambutol (monitor colour vision
for optic neuritis), streptomycin (monitor plasma levels to avoid hearing
impairment) or ciprofloxaci may be used. In severe pulmonary TB,
corticosteroids occasionally improve results.
In some organs (e.g. bone), TB is
treated for longer, often with additional drugs. In meningeal or cerebral TB, a
four-drug regimen for 12 months with additional steroids is recommended, to
ensure adequate brain penetration and to prevent cranial nerve compression by
meningeal scarring.
Complications
Reactivation of old tuberculous scars
may occur when a patient is immunocompromised (Fig. 38c). Chemoprophylaxis with
isoniazid is often given before immunosuppressive treatment (chemotherapy,
organ transplantation). Bronchiectasis and lung cavities with secondary fungal
infections (mycetoma), cranial nerve lesions and renal tract obstructions may
develop due to scarring associated with healing after TB. Non-compliance or
inadequate treatment results in multiresistant strains of mycobacteria that may
be very difficul to eradicate. Compulsory supervision and isolation of these
patients may be required.
Prevention and contact tracing
Vaccination of non-immune subjects with
BCG (bacille Calmette-Gue'rin), a non-virulent strain of
bovine TB, produces immunity and reduces the risk of pulmonary TB by
70%. Community health services must
be notiļ¬ed when a patient is
diagnosed with TB, to
trace contacts and
prevent spread. Contacts are
screened with a Mantoux
test. If this
suggests a risk
of infection, then
chest and appropriate follow-up are arranged.
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