A) DISEASES CAUSING AIRWAY LUMINAL NARROWING

    1. DIFFUSE AIRWAY LUMINAL NARROWING

- Tracheobronchomalacia.

Excessive collapse of the trachea and bronchi on expiration in which their transverse area gets reduced, due to the presence of reduced or abnormal cartilaginous tissue. This entity can be congenital, which is the most frequent congenital pathology of the airway; or acquired, due to long-term intubation with mechanical ventilation, COPD, relapsing polychondritis or sarcoidosis.

Currently, a collapse greater than a 70% of the tracheal lumen is very suggestive of this pathology, as it is a "crescent shape" disposition of the lumen. It also associates with an increased transverse lumen diameter during inspiration.

- Saber-sheath trachea.

Coronal narrowing of the intrathoracic portion of the trachea, getting a transverse diameter inferior to 2/3 of the anteroposterior diameter. It is frequently associated with COPD, especially in males (Fig. 1).

- Congenital tracheal stenosis.

Infrequent entity presenting the first year of life, caused by the existence of a complete cartilaginous ring and the absence of the posterior membrane. The tracheal lumen has an “O” shape, with complete rings but without parietal thickening.

    2. AIRWAY LUMINAL NARROWING AND PARIETAL THICKENING

- Postintubation/post tracheostomy stenosis.

It is produced in the area where the balloon of the intubation tube gets inflated or in the tracheostomy stoma. CT MPR are essential for surgical or interventionist planning.

The acute stenosis is due to the edema or the granulation tissue with normal tracheal cartilage. In a chronic stage, the cartilage and the posterior membrane get also deformed and the stenosis turns into an "hourglass" shape (Fig. 2).

- Tracheobronchial infections.

Tuberculosis is the most frequent infectious disease causing airway stenosis, caused by a granulomatous affection of the trachea or by the compression of external adenopathies. The main left bronchus is the most frequently affected segment. In the acute phase irregular stenosis can be seen, just as a parietal thickening and enhancement. In the chronic phase, the thickening is concentric and the narrowing, regular and affecting a large segment.

Fungal infections are mostly seen in immunosuppressed patients, for example, tracheobronchial aspergillosis (Fig. 3).

- Relapsing polychondritis.

Infrequent autoimmune process which destroys cartilage, being able to affect the tracheobronchial tree (50%), with recurrent inflammation episodes. On TC we can see a smooth parietal thicken of the anterior and lateral walls of the trachea, being its posterior wall respected; and a short focal stenosis, due to the fibrosis from the cartilage destruction.  

The trachea can remain flaccid conditioning a tracheomalacia. It is important to assess this condition with a dynamic CT in forced expiration, since it involves a worse prognosis. It is also frequent to detect air trapping.

- Tracheobronchopathia osteochondroplastica (TBO).

Unusual and idiopathic airway disease which also respects the posterior membranous wall. It consists of a proliferation of submucosal osteocartilaginous nodules in tracheobronchial walls.

On CT a nodular thickening of the cartilage and multiple calcified nodules poking through the anterior and lateral wall into the tracheal lumen are seen. It usually affects to the inferior 2/3 of the trachea and can extend to the main bronchi or even further (Fig. 4). Sometimes, it can condition such severe stenosis that it can be difficult to assess whether the posterior membrane is affected or not.

- Granulomatosis with polyangiitis (Wegener’s disease).

This is a granulomatous and necrotizing systemic vasculitis which can affect the upper airway, lungs and kidneys.

The thoracic affection is usually presented as lung nodules or masses, but can also affect the trachea (14%) and main bronchi (22%). The focal affection is the most frequent. The circumferential, nodular or smooth, wall thickening can distinguish it from the TBO and the relapsing polychondritis. This thickening can end up in airway stenosis.

- Amyloidosis.

A diffusely thickened trachea with irregular parietal nodules is found, generally circumferential and with stenosis associated. The nodules can often calcify or ossify but, contrary to TBO, the posterior membrane can be affected. The combination of large stenosed segment and a high attenuation wall are suggestive findings of amyloidosis. Finding a submucosal and focal lesion simulating an endobronchial neoplasm is less frequent.

- Sarcoidosis.

The larynx and subglottis are affected in 1-3% of the patients, being exceptional for the trachea to be the initial affection. The tracheal affection can be produced by the presence of a granulomatous parietal lesion, shown as a focal or diffuse nodularity; or by extrinsic compression of the trachea by adenopathies or mediastinal fibrosis.

- Central airway neoplasms.

Less than 10% of the airway tumors are benign neoplasms (papilloma, submucosal adenoma from the mucous glands and mesenchymal tumors, such as the hamartoma). On CT they reveal smooth polypoid lesions (sometimes sessile or pediculated), which grow into the tracheal lumen, generally smaller than 2 cm and well circumscribed by the tracheal cartilage (Fig. 5).

Primary malignant tumors from the tracheobronchial tree are less than 1% of the thoracic tumors. More frequent ones are epidermoid and cystic adenoid carcinoma. Mucoepidermoid and carcinoid tumors are less common. 

Metastases usually come from direct invasion, principally by thyroid, esophagus, larynx and lung. Hematogen metastases are rare and usually come from melanoma, breast, colon and genitourinary tumors.

B) DISEASES CAUSING AIRWAY LUMINAL WIDENING

- Tracheomegaly and bronchomegaly.

A wide range of causes can produce tracheal widening, including long-term ventilation, radiation, infections and pulmonary fibrosis. Acquired tracheomegaly occur as a result of airway damage and posterior remodeling.

- Tracheobronchomegaly (Mounier-Kuhn syndrome).

It is a rare congenital entity characterized by a tracheomegaly and central bronchiectasis secondary to the elastic fibers and smooth muscle layer atrophy of the trachea or central bronchi.

It usually appears in 30-50 year-old-male adults, as repeated infections which worsen the bronchial damage and can lead to bronchiectasis and chronic cough.

Chest X-ray can be sufficient for the diagnosis. The findings are the following (Fig. 6 and Fig. 7).

1. Tracheal diameter >3 cm or main bronchi >2.4 (right one) or 2.3 cm (left one).

2. Tracheal wall slimming with a diverticulum formation between the tracheal rings associated.

3. Tracheobronchial collapse in expiration.

The differential diagnosis must be done with the Ehlers-Danlos syndrome, the generalized elastosis, mechanic ventilation in premature and tracheomalacia.

- Williams-Campbell syndrome.

This syndrome is due to a deficiency of the 5-6º order bronchial cartilage which causes subsegmental bronchiectasis, usually with a cystic morphology.

Contrary to Mounier-Kuhn syndrome, it does not affect the trachea or central bronchi.

C) LARGE AND SMALL AIRWAY WIDENING: BRONCHIECTASIS

Bronchiectasis consists of a permanent, focal or diffuse widening of the tracheobronchial tree. The causes are multiple, although the more common are the infections (bacterias and mycobacteria, mostly). 40% of the cases do not have a clearly identified cause.

Depending on the severity of the dilatation and their morphology they can be divided in cylindrical (uniform dilatation with parallel walls), varicose or cystic (usually exceeding 2 cm, with a branched disposition, sometimes leading to a honeycomb pattern).

Although this classification shows an excellent correlation with the CT morphology, it has a poor diagnostic utility due to the coexistence of more than one type or the association of each one to more than one illness. In the clinical practice, the most important thing is to determinate their extension and distribution, as surgery cannot be conducted in those patients with a multisegmental affection.

The chest X-ray generally represents the initial study in those patients with respiratory symptoms. The findings are usually mild and unspecific unless the advanced cases and the diagnosis can be suggestive in only 40% of the cases.

Tram-tracks opacities are seen in patients with cylindrical bronchiectasis, although it is a finding that can also be seen if there is a parietal thickness of the bronchi, without dilatation (Fig. 8).

Mucus plugs into the dilated bronchi can be seen as oval opacities in “glove finger” or branched opacities in “glove hand” from the hilum towards the periphery. On HRCT these mucus plugs are seen as tubular or branched opacities in the axial images (Fig. 9).

Cystic bronchiectasis show multiple small cystic spaces with or without an air-fluid level, which are usually similar in all of them as the cystic spaces are communicated (Fig. 10).

On CT, some direct signs are described:

1. Internal diameter of the bronchus > pulmonary arterial branch partner (bronchoarterial index). It can also be seen in asthmatic patients, in high altitude inhabitants and in some normal persons.

2. Alteration of the bronchial contour.

Signet-ring sign: dilated bronchus seen transversally together with the artery.

Tram-tracks opacities: parallel bronchial walls seen longitudinally.

Pearl-necklace image: longitudinal sight of the varicose bronchiectasis.

Cystic bunch in cystic bronchiectasis.

3. Absence of distal bronchial narrowing in at least a 2 cm segment from a bronchial ramification point.

4. Visualization of the distal airway in less than 1 cm from the costal pleural and/or in contact with the mediastinal pleura. This is suggestive of bronchiolectasis.

We can also see indirect signs such as bronchial wall thickening, mucus plug, small airway affection (tree-in-bud opacities, centrilobular nodules, mosaic perfusion and/or air trapping in expiration) or systemic bronchial arteries hypertrophy in the bronchiectasis surroundings.

Bronchiectasis distribution is usually of great importance. In Fig. 11 and Fig. 12 we enclose two tables explaining the differential diagnosis between bronchiectasis depending on their location, correlated with the clinical history. 

 Differential diagnosis of bronchiectasis should also be done with other entities: 

- Reversible bronchial dilatations inside a pneumonic consolidation. 

- Traction bronchiectasis. Peripheral bronchiectasis, caused by pulmonary fibrosis, which distorts the parenchyma and dilates and fix bronchi present in many interstitial pulmonary diseases, connective tissue disease or sarcoidosis (Fig. 16). 

- Diseases with pulmonary cysts. Langerhans cells’ histiocytosis or Pneumocystis Jiroveci infection, for example. (Fig. 17).

- Laryngotracheal papillomatosis. It is characterized by the presence of multiple solid and cystic nodules (1-3 cm), with a central or perihilar distribution.

- Congenital bronchial atresia. Dilated and mucus plugged bronchi, distal to the atretic segment. It associates a hyperlucent and hypoperfused pulmonary lung segment (Fig. 18 y Fig. 19). 

 - Asthma. Mild and cylindrical bronchiectasis associated with mucus plag. ABPA can be combined with this entity.

Hereunder, there are two specific causes of bronchiectasis which will be further explained.

 1. CYSTIC FIBROSIS (CF)

This pathology is caused by a recessive autosomal genetic disorder which conditions an abnormality in the regulation of the chlorine membrane transportation. This abnormality conduces to a low-water content of the airway mucus, which causes a diminishing of its clearance, mucus clogging and an increase of bacterial infections. 

The inflammation of the bronchial wall can progress to bronchiectasis. They can present in childhood, with a variable gravity spectrum.

The earlier radiological signs are unspecific: focal atelectasis, recurrent pneumonia or bronchial wall thickening; but the worsening of the radiological findings is inexorable.

Chest X-rays may demonstrate perihilar diffuse opacities (peribronchial inflammation and bronchiectasis), glove-fingers opacities, pulmonary consolidations, focal atelectasis, hilar engorgement (by pulmonary hypertension or adenopathies) and air trapping with increased retrosternal space and diaphragm flattening. These findings are more frequently seen in upper lobes.

In order to evaluate the illness evolution, grading systems are established, as the Brasfield one, based on chest X-ray findings: 1. Hyperinsufflation, 2. Linear densities, 3. Nodular cystic lesions, 4. General severity, and 5. Big lesions (pneumonia, atelectasis).

CT can reveal morphological abnormalities in the early asymptomatic phase, when the chest X-ray and the pulmonary function are normal. The image findings are the following:

- Bronchiectasis.

In every case, the affection includes peripheral bronchi and the upper-lobes ones. They can show any morphology (varicose and cystic are seen in 1/3 of the cases), affect every pulmonary lobe (although initially the affection is usually located in the upper right lobe) and are present in every advanced cystic fibrosis case. The best correlation between the clinical situation and the radiological findings is established with the bronchiectasis (Fig. 20).

- Bronchial wall thickening.

In patients with mild CF, the thickening of the proximal segment of the upper right lobar bronchus is the earlier visible affection.

- Mucus plug (25-50% of the cases).

- Other findings are tree-in-bud opacities, atelectasis and consolidations, mosaic pattern and air trapping in expiration, and pulmonary cavities secondary to fibrotic abscesses, especially in the apex.

- Dilatation of the pulmonary arteries and cor pulmonale by pulmonary hypertension are poor prognostic factors.

2. ALLERGIC BRONCHOPULMONARY ASPERGILLOSIS (ABPA)

This entity reflects a hypersensibility reaction to Aspergillus and is characteristically associated with eosinophilia, asthmatic symptoms and central bronchiectasis, usually combined with mucus lag, atelectasis and consolidations.

It is usually presented in young adults with a clinical history of asthma or atopy and in the 2-10% of CF patients. 

On chest X-rays bronchiectasis are usually centrally located. They can be rounded, oval or branched. Mucus plug can condition glove-finger or glove-hand opacities (Fig. 21). In acute exacerbations, patched consolidations are often seen, similar to those of eosinophilic pneumonia.

CT can show (Fig. 22).

- Central bronchiectasis, generally varicose or cylindrical.

- Endobronchial mucus plag. In 25% of the cases, the mucus plug has a higher density of soft tissues (> 100%), which is very suggestive of ABPA.

- Bronchial wall thickening, seen in upper lobes.

- Dilated airway can contain air-fluid levels or aspergillomas.

- Consolidations, collapse, cavitation areas or bullas.