In most cases,
a child is infected with Mycobacterium tuberculosis after an adolescent or adult in their close environment (usually the household) with active tuberculosis expels bacilli into the shared air.
As the distribution of inhaled droplet nuclei is determined by the ventilatory pattern and volumes of the various lung lobes,
the site of implantation preferentially occurs in the middle and lower lung zones,
although any lobe may be affected.
Development of specific immunity is usually adequate to limit further multiplication of the bacilli,
the host remains asymptomatic and the lesions heal.
Some of the bacilli remain dormant and viable for many years,
and this condition is referred to as latent TB infection.
Risk factor for progression from infection to active TB are co-infection with HIV andM tuberculosis,
conditions that are associated with defects in T-lymphocyte and/or macrophage function,
such as malnutrition,
drug and alcohol abuse,
and corticosteroid or other immunosuppressive therapy.
Patients who develop disease after initial exposure are considered to have primary TB.
Postprimary TB occurs in a person who has previously been infected and has retained a degree of acquired immunity.
It can result from endogenous reactivation or,
less commonly,
exogenous reinfection.
Miliary spread of TB may also occur during either primary or postprimary stages of disease.
It happens mostly within 3 months of infection and that is why some authors refer to it as early postprimary tuberculosis while other authors refer to it as a complication of a primary disease.
It results when a focal collection of tubercle bacilli discharges into a blood or lymph vessel,
releasing a large number of viable bacilli that embolize to capillary beds in multiple organs.The lung is the most commonly involved organ.
TB symptoms in children are very vague and common to symptoms and signs seen in children with other chronic disease: chronic cough ( an unremitting cough that is not improving and has been present for more than 21 days),
weight loss or failure to thrive,
fever lasting many days and repeated respiratory tract infection.
Specific clinical manifestations of TB are influenced by the age and immune status of the infected person.
Because of the narrower diameter of their airways,
younger children are more likely to have respiratory symptoms which include cough and wheezing or rales over the involved region.
Tuberculin skin test identifies children infected with TB but not necessarily those with active disease.
The test can be positive in children who are asymptomatic (TB infection) as well as those with active disease (TB disease).
TB blood test,
also called an Interferon Gamma Release Assay or IGRA is a relatively new diagnostic tool for TB.
There are two kinds of TB blood tests: Quantiferon-TB and T-SPOT-TB.
They should not replace the existing standard diagnostic methods for the diagnosis of active TB and negative IGRA result does not exclude active TB disease.
It is always preferable to make a bacteriological diagnosis using whatever specimens and laboratory methods are available.
Bacteriological methods most commonly used in diagnosing pulmonary TB are examination of sputum smears for tubercle bacilli and culture of bacilli from sputum specimens and gastric aspirates.
Radiographically,
primary pulmonary tuberculosis manifests as parenchymal disease and lymphadenopathy.
The primary focus is often so small that it is not visible and only the accompanying mediastinal lymph gland enlargement is seen.
It can occur as dense homogenous parenchymal consolidation in any lobe,
predominantly in middle and lower lobes.
Lymphadenopathy is typically unilateral and right sided as the right hilar and right paratracheal nodes drain the right lung and lower half of the left lung.
TB lymph nodes resolve very slowly even with appropriate TB therapy and can be visible on the radiograph even after completion of successful therapy.
If the enlargement of lymph nodes progresses it can cause narrowing,
obstructing or ulcerating into airways that manifests as unilateral hyperinflation,
lobar or segmental collapse,
TB expansive pneumonia and tuberculous bronchopneumonia.
If the alveolar infiltration breaks through to the pleural space,
it provokes a hypersensitivity response in the pleura and a large pleural effusion.
Miliary disease is manifested as a diffuse nodular pattern on chest x ray,
with nodule size 1 – 2 mm in diameter.
In postprimary TB which is mostly seen in adolescents,
the earliest radiological finding is the development of patchy,
ill-defined segmental,
lobar or confluent consolidation with the development of cavities later in time.
It usually affects the apical and posterior segments of the upper lobe or the superior segment of the lower lobe.
Upper lobe involvement with cavitation and the absence of lymphadenopathy are helpful in distinguishing postprimary TB from primary TB.
A normal frontal and lateral chest radiograph are essential to rule out active TB.
The lateral view helps us to distinguish other central shadows from lymph nodes which are spherical and can be frequently seen on both views.
Computed tomography is not indicated in the evaluation of an asymptomatic child with a normal chest radiograph and a positive tuberculin skin test but in cases when the radiograph is equivocal or when other causes of disease are to be evaluated.
A chest CT scan with contrast is particularly sensitive for detection of adenopathy.
Lymph nodes,
calcifications and small cavitations that are not evident on plain films can be present on CT.
The associated complications of TB,
such as the erosion of vessels and rupture into the pleural space are also better defined with CT scanning than with radiography.