Standardized radiographic interpretation of thoracic tuberculosis in children

Numerous articles have been published regarding childhood tuberculosis but there is still a lack of standardized approach and terminology to classify the diverse spectrum of manifestations in tuberculosis. It is important to recognize the different clinical and radiographic patterns because management of each condition is varied. A classification of disease manifestation that is common to all will help to facilitate communication and understanding among scientific communities.

Generally the pathological changes in childhood tuberculosis are pauci-bacillary, and thus the diagnosis of intrathoracic tuberculosis depends largely on chest imaging [1]. Pulmonary tuberculosis has been classically classified as primary tuberculosis in children and post-primary tuberculosis in adults. However, because of the changing epidemiology there is a considerable overlap in the radiologic presentations of these entities [2].

In this article we promote a standardized combined clinical and radiographic approach for children suspected of having or diagnosed with childhood tuberculosis. We propose standardized terms to diminish confusion and miscommunication, which can affect management. In addition, we present pitfalls and limitations of imaging in childhood tuberculosis diagnosis. The atypical radiologic patterns seen in immunocompromised children, however, are not discussed in this article.

Intrathoracic tuberculosis has been classified based on clinical, laboratory and radiologic evidence. The child should present at least one sign or symptom suggestive of tuberculosis but without any other plausible etiology. These signs and symptoms include any of (a) persistent cough; (b) weight loss/failure to thrive; (c) persistent unexplained fever, or (d) persistent, unexplained lethargy or reduced activity. The following sections are definitions that have been proposed to indicate the degree of certainty of the diagnosis of tuberculosis.

The major route of Mycobacterium tuberculosis infection is by inhalation [1]. Infection begins when infected droplets are deposited in the terminal airway or alveoli, followed by a localized parenchymal inflammation or pneumonic process called the primary (Ghon) focus. There is then spread via draining lymphatic vessels, usually to the ipsilateral central or regional lymph nodes, which then enlarge. The upper lobes drain to the ipsilateral paratracheal nodes, while the rest of the lung drains to the perihilar nodes. The parenchymal focus and the enlarged lymph nodes are called the primary (Ranke or Ghon) complex [1, 2, 12‐17] (Figs. 1 and 2).

Incubation can be up to 6 weeks from exposure, during which time chest radiographs are normal. After 1–3 months from exposure, hilar or mediastinal adenopathy can be visualized in 50–70% of cases [18‐20]. Primary tuberculosis reflects a patient’s conversion from insensitivity to having the antigens of the tubercle bacilli [2, 14].

Regional (perihilar or paratracheal) lymphadenopathy is the radiologic hallmark of primary infection in childhood [1, 13] (Figs. 3 and 4). Anteroposterior and lateral views are required for optimal lymph node visualization [13, 21], but it can remain difficult to visualize enlarged lymph nodes with certainty [1, 22]. The most common sites of nodal involvement are the right paratracheal and hilar regions [13, 17].

The prevalence of adenopathy decreases with age; it is 100% in children <3 years of age and 88% in older children. The prevalence of parenchymal involvement detectable on radiographs, however, is significantly lower in children <3 years of age (51%) as compared with that in older children (78%) [13].

If the child is immunocompetent, the lesions heal and become dormant while still causing continuous antigenic stimulation for maintenance of hypersensitivity to tuberculous antigen. Thus the tuberculin skin test is positive in 95% of cases. This has been referred to as latent tuberculous infection. The caseating necrosis within the Ghon focus and infected lymph node frequently calcifies [1, 2]. Calcification can occur from 6 months to 4 years after infection, occurring earlier in young children [1]. The parenchymal focus, called pulmonary tuberculomas (Fig. 5), which are identified radiographically, represent sharply defined ovoid granulomas, solitary or multiple, ranging in size from 0.4 cm to 5 cm in diameter [2, 23]. Children with latent tuberculous infection can be treated with a one-drug therapy provided there was no prior treatment.

If immunity is inadequate, the disease progresses either locally or in other parts of the lung or body, with spread of infection via the airways, lymphatics or bloodstream [15, 16]. Clinically active tuberculous disease can develop within 5 years after infection. This is called progressive primary tuberculosis [2, 24].

Progression from infection to disease usually occurs within 1 year after the primary infection in more than 90% of cases. It is bimodal in age distribution, with children younger than 5 years and adolescents being at increased risk [1, 17, 25].

Early disease progression can happen 2–6 months from exposure, when homogeneous consolidation can occur (Fig. 6). Obstructive atelectasis or overinflation can result from compression by an adjacent enlarged node. Distribution is typically on the right side at the level of the right lobar bronchus or bronchus intermedius. Fibrosis and destruction of the lung parenchyma result in traction bronchiectasis and formation of cavities (Fig. 7), respectively [2]. This is known as progressive Ghon focus [18‐20].

Three possible mechanisms are involved in the formation of cavities: (1) progressive primary spread of disease with extensive and bilateral pulmonary cavities, (2) cavities caused by bronchial obstruction by lymph nodes or (3) post-primary tuberculosis showing cavities that are usually single and unilateral in the upper lobe. These have fairly equal incidence [26].

Lymph nodes can continue to enlarge 4–12 months from exposure and can cause progression of the disease affecting the airways, pleura and pericardium, which are discussed in the next sections. On contrast-enhanced CT, involved lymph nodes often measure more than 2 cm and show a very characteristic, but not pathognomonic, rim sign consisting of a low-density center surrounded by a peripheral enhancing rim [17, 22, 27] (Fig. 6). The esophagus, lymphatic duct and phrenic nerve might also be affected, producing tracheoesophageal fistula, chylothorax and diaphragmatic palsy, respectively [1, 18‐20]. Hematogenous miliary dissemination can also occur in this stage.

Post-primary tuberculosis is also known as adult-type, reactivation or secondary tuberculosis and sometimes phthisis [2]. This results from the reactivation of dormant foci. It is further observed in the pediatric age group, mostly in adolescents [2, 13]. It is considered a late disease progression of the primary infection, which can occur 8–24 months from exposure and in children as young as 8 years [18‐20].

The most commonly affected locations are the apical and posterior segments of the upper lobes and the apical segment of the lower lobes because of higher oxygen tension (Fig. 18). Initially there might be cloudy opacification in a segment before coalescence and parenchymal breakdown. Complications include cavitation, bronchogenic spread with bronchopneumonic consolidation, exudative pleuritis, cicatrization atelectasis of the upper lobe with retraction of hilum and formation of traction bronchiectasis [1, 2, 44‐46]. Lymph node enlargement is not common in comparison to primary tuberculosis [18‐20].

Cavitation is radiographically evident in 40% of cases of post-primary disease. The walls of the cavities might appear thin and smooth or thick and nodular. It is difficult to distinguish thin-walled cavities from bullae, cysts or pneumatoceles. Cystic bronchiectasis should also be considered when multiple cavities are present [47] (Figs. 18 and 19).

In 20% of post-primary tuberculosis cases, bronchogenic spread appears on radiographs as multiple, ill-defined micronodules in a segmental or lobar distribution, typically in the lower-lung zones [48]. High-resolution CT, the modality of choice, demonstrates centrilobular nodules ranging 2–4 mm with linear branching opacities (“tree-in-bud” sign) which represent caseous necrosis at and around terminal and respiratory bronchioles [49] (Fig. 20). Complete destruction of the entire lung or a large part of a lung is not uncommon in the end stage of tuberculosis. Secondary pyogenic or fungal infection can occur [47]. Miliary tuberculosis and tuberculomas might also be encountered in post-primary tuberculosis [2, 50].

We adapted the clinical and laboratory components of the classification from Graham et al. [3], Moyo et al. [4] and Triasih [5]. An imaging approach to interpretation is summarized in Fig. 21. The radiologic side of the algorithm is based on the pathological processes that occur in tuberculosis that result in various complications.

Whenever a child comes in for workup for a possible tuberculous infection or disease, it is important to be aware of the terminology and pathological pathways. The diagnosis or impression of the imaging findings should contain the main pathology (i.e. primary tuberculous infection, primary progressive tuberculous disease or post-primary tuberculous disease), followed by the various complications present (e.g., primary progressive tuberculosis with tracheobronchial involvement and pleural disease; Fig. 21).

Chest radiography is the primary screening tool in children suspected of having tuberculosis. It has, however, high intra- and inter-observer variability, with 74% specificity and 39% sensitivity [51] even in the best technical quality radiographs. A normal chest radiograph does not rule out tuberculosis [52]. CT offers excellent anatomical visualization [53], but because of its high cost and the higher radiation exposure, it is reserved for complicated cases [18].

Lymphadenopathy is the most common abnormality noted in children with primary tuberculosis [13], but it is not pathognomonic of tuberculosis because other infectious processes can present with lymphadenopathy. Large pulmonary vessels are sometimes erroneously identified as lymph nodes, leading to over-diagnosis. The inter-observer agreement is low (kappa −0.03 to 0.25) [5]. Knowledge and familiarization of the hilar anatomy are prerequisites prior to interpretation [52].

The abnormalities seen on chest radiographs resolve gradually and can worsen despite clinical improvement. Lymphadenopathy and parenchymal disease without or with calcifications can also persist for many months and even years even after proper treatment. Re-treatment might not be necessary, especially if the child is asymptomatic. Moreover, the calcifications do not equate with healed tuberculosis because these can indicate latency [52].

Radiologic interpretation of pulmonary tuberculosis remains challenging. Classically, tuberculosis is classified as primary and post-primary tuberculosis, but the typical radiologic patterns are now complicated by overlapping imaging characteristics as well as occurrence of atypical features seen in immunocompromised children. It is important to differentiate infection and disease because treatments are different. The proposed standardized clinical and radiographic classification presented in this paper aims to provide helpful guides in the proper nomenclature of suspected tuberculosis patients. The pitfalls and limitations of imaging likewise caution both clinicians and radiologists to avoid erroneous interpretations and over-diagnosis of childhood tuberculosis.