ERS statement on tracheomalacia and bronchomalacia in children

Introduction

Tracheomalacia (TM) is a condition of excessive tracheal collapsibility, due either to disproportionate laxity of the posterior wall (pars membranacea) or compromised cartilage integrity. As a result, the anterior and posterior walls appose, reducing the tracheal lumen opening and creating a shape abnormality during bronchoscopy [1, 2]. TM may be localised or generalised [2, 3]. If the main bronchi are also affected the condition is called tracheobronchomalacia (TBM). The term bronchomalacia (BM) is used when the excessive collapsibility is restricted to one or both of the mainstem bronchi and/or their divisions at the lobar or segmental level [3, 4]. Cases of isolated BM as well as extrathoracic or cervical TM are relatively rare [5, 6].

Malacia is defined in this European Respiratory Society (ERS) Task Force report as an arbitrary >50% expiratory reduction in the cross-sectional luminal area during quiet respiration [7–13]. There is no universally agreed “gold standard” diagnostic test, although flexible bronchoscopy is the most commonly used modality by respiratory paediatricians. The degree of TM/TBM can be assessed either bronchoscopically or radiologically. There is also no universally accepted classification of severity. In clinical practice, the anatomical changes are arbitrarily described as mild (50–75% reduction), moderate (75–90% reduction) or severe (>90% reduction), most often on the subjective visual inspection at bronchoscopy [14]. This is a purely descriptive system of classification that does not reflect clinical severity since the degree of lumen occlusion is not associated with disease morbidity (see the later section on clinical signs and symptoms).

The expiratory recoil pressure of the chest wall leads to a dynamic increase in intrathoracic pressure, which is transmitted to the airways. If the large intrathoracic airways are normal, changes in calibre are negligible but occur, for example, with coughing. If there is malacia, the tracheal/bronchial walls collapse with partial or complete occlusion of the lumen [15, 16], in particular if expiratory efforts are increased due to airflow obstruction. The adult literature distinguishes between collapse of the pars membranacea and the cartilaginous wall. Some use the term hyperdynamic airway collapse to describe the excessive protrusion of the posterior trachealis muscle into the central airway lumen during expiration, and reserve the terms TM and TBM for the collapsibility of central airways due to the loss of structural integrity of the affected cartilaginous rings [17–19]. Because of the intrinsic softness of the paediatric tracheal cartilages, this distinction is less clear in the newborn, infant and young child. For the purposes of this report, TM and TBM will also encompass hyperdynamic airway collapse.

This ERS Task Force report on paediatric TBM reviews the current literature in children, describing the evidence for diagnosis, clinical impact and therapeutic options, and the impact on families and patients, with suggestions for future research.

Conditions associated with TBM

There is no generally accepted paediatric classification of the causes of airway malacia. The Task Force used a division into those conditions with an intrinsic alteration of airway cartilage (primary or congenital) and those where the cartilage was embryologically normal but developmentally malformed because of pressure on the airway wall from outside (secondary) or acquired from airway luminal disease such as chronic infection.

Pragmatically, it may also be helpful to distinguish conditions where TBM is clinically the main problem and those in which, while still a factor, there are either more important complex extrapulmonary comorbidities such as cardiovascular abnormalities or pulmonary parenchymal disease. An example of the latter would be bronchopulmonary dysplasia, in which TBM worsens the prognosis but is far from the only abnormality [20].

The numerous causes of TBM are summarised in table 1. Congenital airway malacia is part of many rare syndromes. TM can be found in association with chromosomal defect syndromes, mucopolysaccharidoses and inherited connective tissue disorders [21]. In addition, TM has been described in ∼5% of children with achondroplasia [22]. Some conditions are associated with a discrete area of malacia. For example, children with tracheo-oesophageal fistula (TOF) typically have a short segment of TM post-operatively [18, 23, 24]; post-operative repair of vascular rings can leave a defined, short tracheomalacic defect.

One study reported malacia (including laryngomalacia, a condition beyond the remit of this Task Force report) in 299 out of 885 bronchoscopies [3]. 41 had cardiovascular abnormalities, 29 had been treated for TOF, nine had congenital lobar emphysema and 24 were syndromic. Boogaard et al. [25] found BM in 160 out of 512 paediatric flexible bronchoscopies, 136 cases being primary. 67 out of 86 children with primary malacia in whom bronchoalveolar lavage was obtained had a positive bacterial culture; lavage neutrophil counts were not reported. Whether infection was secondary to malacia, or the converse, and the relationship between malacia and persistent bacterial bronchitis is also unclear.

It has been suggested that infant wheeze may be related to TBM [26] as a developmental phenomenon with spontaneous recovery, but the contribution of malacia is rarely determined in clinical practice.

Clinical symptoms and signs

The type and onset of symptoms depend on length, site and severity of the malacic segment. The Task Force members were unable to find any consistent correlation between anatomical severity and clinical features in the literature.

Symptoms may be persistent or intermittent and of varying severity [6, 27]. If the extrathoracic trachea is malacic there may be stridor; if intrathoracic, a monophonic expiratory wheeze is common [6, 28]. If the child develops a respiratory infection, there may be a barking cough, prolonged resolution of cough, expiratory wheeze or croup-like symptoms. Older patients often complain that complete exhalation is difficult [18].

In more symptomatic cases, stridor or wheezing are persistent, respiratory infections are frequent and respiratory distress may occur. Wheezing in children with malacia is typically centrally located, low pitched and monophonic [29], and distinct from the diffuse, high-pitched and musical wheezing in asthma. Moreover, in patients with malacia, wheezing remains unchanged or even worsens after bronchodilator inhalation [15]. Importantly, TM/BM should always be considered in the differential diagnosis of infants and pre-school children with “atypical wheeze” (e.g. infants who are never completely symptom-free or infants with frequently recurring wheeze). The “bagpipe sign”, an expiratory sibilant sound that persists after the end of visible expiration, may also be present [6]. Intermittent compression of a malacic trachea during bolus progression in the oesophagus can cause desaturation, leading to poor feeding and, consequently, poor weight gain.

In severe cases, airway obstruction with cyanosis, inspiratory and expiratory stridor during tidal breathing, apnoea, and even cardiac arrest or sudden infant death may occur [6, 27, 30–32]. In the most severe cases, airway obstruction can only be resolved with intubation [6, 27] and successful extubation can be challenging [33]. Tracheal obstruction can cause “dying spells” (also called “apnoeic spells”, “reflex apnoea”, “death attacks” or “blue spells”). These events are possibly elicited by a reflex triggered by secretions or when a bolus of food in the oesophagus compresses the trachea or the presence of increased intrathoracic pressure from a Valsalva effect.

Severe malacia is typically evident clinically from birth, but many children with TM or BM do not show any symptoms before age 2–3 months [6, 27, 30]. Boogaard et al. [25] described symptoms in 96 outpatients with primary airway malacia and without comorbidities. Cough was found in 83% of children (night-time cough 42%; productive cough 60%; exercise-induced cough 35%; characteristic barking cough 43%), recurrent lower airway infections in 63%, dyspnoea in 59%, recurrent wheeze in 49%, recurrent rattling in 48%, reduced exercise tolerance in 35%, symptoms of gastro-oesophageal reflux (GOR) in 26%, retractions in 19%, stridor in 18% and funnel chest in 10% [25]. However, in many patients symptom onset is insidious and for some the diagnosis is only made later in life [25], even in the elderly [34].

Table 2 summarises symptoms and signs of TM/BM. A barking or brassy cough is most commonly reported. Intra- and interobserver clinician agreement for brassy cough was very good (κ=0.79, 95% CI 0.73–0.86) when undertaken by respiratory specialists, and the sensitivity and specificity of brassy cough (compared with TM seen at flexible bronchoscopy) were 0.57 and 0.81, respectively [35]. The brassy cough is caused by vibration due to the mechanical juxtaposition of the anterior and posterior walls of the trachea [27], which causes an irritable focus that stimulates further cough [36]. In a meta-analysis including five studies with 455 patients in whom bronchoscopy was performed because of recurrent croup-like symptoms, TM was found in 4.6% of children [37].

In children with TM/BM, both airway closure during cough and ineffective cough due to an underlying condition can cause impaired clearance of secretion, leading to recurrent and/or prolonged respiratory infections [6, 27]. Santiago-Burruchaga et al. [38] demonstrated airway malacia in 52% of 62 children in whom bronchoscopy was performed because of recurrent lower respiratory tract infections. Airway malacia is a frequent bronchoscopic finding in children with recurrent respiratory symptoms [3, 39, 40] and in children with protracted bacterial bronchitis [41], although which is causal and which is secondary is often unclear.

In children with TM/BM, symptoms can be aggravated by any conditions requiring increased respiratory efforts, such as exercise, coughing, crying, feeding, Valsalva manoeuvres, forced expiration or lying supine. All these activities cause increased intrathoracic pressure that worsens airway collapse [6, 18]. Placing an infant in the prone position may open the airway because gravity pulls the mediastinal structures anteriorly, thus alleviating symptoms [15, 42].

The role of pulmonary function testing in diagnosing TBM

11 studies address the role of pulmonary function tests (PFTs) in diagnosing TM/TBM, but all are small and only two [25, 45] reported ≥20 children. Many different PFTs were undertaken: spirometry, maximal flow at functional residual capacity (V′_maxFRC), FRC, peak expiratory flow, mid expiratory flow, tidal expiratory flow, airway resistance, flow–volume loop description and airway hyperresponsiveness. None of the studies used newer techniques such as oscillometry and multiple breath washout.

Many but not all studies showed that some children had expiratory airway obstruction. PFTs cannot be used to diagnose TBM, but an obstructive airway pattern is supportive evidence [46, 47]. Early-phase plateauing of the expiratory limb has been described. A plateau of both inspiratory and expiratory limbs of the flow–volume loop is more likely due to fixed obstruction, unless there is both intra- and extrathoracic TM [48]. Increased thoracic gas volume was also documented in one study [49]. Flow limitation during V′_maxFRC is neither sensitive nor specific for malacia, but flow limitation during tidal breathing is highly predictive and 100% specific [50].

One study [46] found airway hyperreactivity to mannitol in two out of 15 (7%) children, the significance of which is unclear. There was no significant effect of β₂-agonists on spirometry [25]. Indeed, β₂-agonists actually reduced V′_maxFRC by 31.6% in three children [51].

The studies are for the most part small and none related PFTs to severity of TBM or even defined how the diagnosis was made. Hence, we could not calculate the sensitivity and specificity of any PFT abnormality for TBM. PFTs may or may not be abnormal in children with TBM. Limited data exist on whether or not currently available PFTs can be used to diagnose TBM. Until further data are available, we are unable to quantify a precision of an estimate.

The role of imaging to diagnose TBM

The Task Force members could find no evidence to support the use of plain radiographs to diagnose TBM [52].

Multidetector computed tomography

Multidetector computed tomography (MDCT) provides new diagnostic options (figure 1). Paired end-inspiratory and end-expiratory MDCT or paired end-inspiratory and dynamic expiratory MDCT are both reliable techniques [7–9, 57]. Children age <5 years generally require intubation and controlled ventilation technique [10, 58], which will influence airway dynamics. In some institutions a securely positioned face mask is used instead of intubation, especially when a tracheal stenosis is suspected above the thoracic inlet level.

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FIGURE 1

Paired end-inspiratory and end-expiratory multidetector computed tomography scans of the chest: axial scan at a) end-inspiration and b) end-expiration phase and sagittal multiplanar reconstruction at c) end-inspiration and d) end-expiration phase showing significant expiratory reduction in the cross-sectional luminal area of the trachea. The appearance is consistent with tracheomalacia.

Intravenous contrast injection is only mandatory when looking for underlying compressive causes such as vascular abnormalities or mediastinal masses [10]. The reported overall diagnostic accuracy of paired airway MDCT compared with laryngoscopy/bronchoscopy is 91% [9]. However, patients included in that study suffered from very severe TBM in whom surgery was required, so this was a biased population and the accuracy of the MDCT scan may have been overestimated [9]. Free-breathing cine-MDCT is an alternative technique in young children as it does not require general anaesthesia and controlled ventilation, and radiation exposure is low (mean effective dose <2 mSv, with the minimum ever reported 0.19–0.8 mSv) [11–13]. Reported sensitivity (96.3%) and specificity (97.2%) are high compared with bronchoscopic evaluation [11]. Virtual bronchoscopy is not very sensitive (sensitivity <75%) in detecting TM [59]. Only one study compared virtual bronchoscopy with flexible bronchoscopy, reporting a sensitivity of 54.1% and specificity of 87.5% for TM and 45.2% and 95.5% for BM [60].

MDCT has the advantage that it is a quick, less invasive technique that allows simultaneous assessment of any mediastinal, vascular and lung pathologies, as well as visualisation of airways distal to the obstruction [61]. The major disadvantage is the radiation exposure, which increases with the paired technique. This can be partially overcome by using reduced-dose techniques in the expiratory scan [62]. Another concern is the need for sedation and intubation in younger children, which can distort the airway and change the tracheal dynamics [7, 9, 11, 12, 28]. This problem can be partially solved by using free-breathing cine-MDCT [11–13].

Tracheobronchography

Tracheobronchography performed with low volumes of nonionic water-soluble contrast is safe [67], and useful in the evaluation of TBM because of its high spatial and temporal resolution (figure 2) [68]. Many centres continue to use tracheobronchography [12, 67, 69–71], often in combination with flexible bronchoscopy [68]. Free breathing, as with many of the imaging techniques, is required for diagnostic accuracy [72].

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FIGURE 2

a) Bronchography image showing diffuse tracheobronchomalacia in a 7-month-old girl with 22q11 deletion, right aortic arch and an aberrant left subclavian artery. The tracheostomy tube has been withdrawn into the upper trachea. b) Bronchography image showing apposition of the anterior and posterior wall of the trachea (white arrow highlighting darker “smudge” effect) in a 1-year-old girl with tracheomalacia related to vascular compression. c) A 5-month-old girl with left pulmonary artery and vein hypoplasia with severe bronchomalacia of the left main bronchus. Bronchography image showing complete collapse of the left main bronchus (white arrow). d) Patient in (c) where continuous positive expiratory pressure (CPAP) is now applied. Bronchography image showing some opening of the left main bronchus with additional CPAP (white arrow).

The role of bronchoscopy to diagnose and grade TBM

We reviewed 27 papers on the role of bronchoscopy in the diagnostic work-up of TBM [7, 55, 59, 60]. The Task Force members use flexible bronchoscopy in a spontaneous breathing child as a gold standard for the diagnosis of TM and BM. Rigid bronchoscopy plays a role but may splint the airway and is not as useful as flexible bronchoscopy in the evaluation of the airway dynamics. However, the limitations of flexible bronchoscopy must be appreciated. First, the bronchoscope occludes a significant part of the airway, likely raising airway pressure and reducing the chances of detecting malacia. Second, even for experienced bronchoscopists, assessment of changes in the lumen is subjective [5, 6, 16, 27, 29, 53, 72–74]. Third, there are also specific problems linked with the optical attributes of the instrument, i.e. the distortion of the image due to curvature and orientation of the lens [75]. Furthermore, the bronchoscopic diagnosis of TM and BM can be difficult in children because of the small size of the bronchial tree and the rapid respiratory rate. There are no studies comparing flexible and rigid bronchoscopy.

Good anaesthetic technique is essential in order not to mask (too deep) or exaggerate (too light with severe coughing) TM and BM, but the Task Force found a paucity of data in the literature on anaesthetic practice [76]. There are case reports of airway collapse with anaesthesia in patients with either symptomatic or asymptomatic TM [77–79]. Because general anaesthesia leads to increased collapsibility of the upper airways the same might be true of the lower airways [53, 80–82]. The topic of the most suitable drugs for anaesthesia in bronchoscopy remains an area for future research.

In their bronchoscopy practice, the Task Force members divide the trachea into three arbitrary regions to describe the site and extent of TM: from the cricoid to the thoracic inlet, from the thoracic inlet to the mid portion of the intrathoracic trachea and from there to the carina. Examples of malacia are portrayed in figure 3.

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FIGURE 3

Bronchoscopic images. a) Severe malacia affecting the carina and opening of both right and left bronchi. b) Bronchoscopy image showing focal tracheomalacia and predominantly posterior membrane collapse in a 15-year-old boy with tracheo-oesophageal fistula and oesophageal atresia repaired at birth. c) Severe malacia affecting the left main bonchus.

Four papers attempt to refine the role of flexible bronchoscopy to quantify the degree of malacia. Masters et al. [75] and Masters [83] used digital video to capture and quantitate the images with high intra- and interobserver agreement. The authors demonstrated that neither the site nor the severity of malacia correlate with the clinical symptoms or severity. Okazaki et al. [84] quantitated the static pressure–area relationship of the trachea under general anaesthesia and paralysis. Finally, Loring et al. [85] evaluated central airway narrowing in adults by a “shape index” based on images taken during bronchoscopy and plotted against the transtracheal pressure.

Medical therapies in the management of TBM

The following medical therapies for TBM are considered in the literature.

The role of respiratory physiotherapy

Respiratory physiotherapy is commonly used in the treatment of children with TM or TBM, aiming to enhance mucociliary clearance [44, 89]. Our review of the literature did not identify any studies investigating the effectiveness of physiotherapy for patients with TM. Moreover, we have not found any studies on the role of airway clearance techniques in conjunction with mucolytics. Positive expiratory pressure (PEP) is often used as an airway clearance technique in clinical practice. In infants with TM, continuous positive airway pressure (CPAP) increases maximal expiratory flow by raising FRC [47]. One study reports that a PEP of 5–10 cmH₂O increases the peak cough expiratory flow of children with clinically diagnosed TBM after TOF repair [95]. However, the authors of the study note that an increase of PEP above 15 cmH₂O may have a negative effect, suggesting there should be close monitoring of PEP or use of a threshold expiratory pressure device. It is unclear if PEP devices used for airway clearance prevent or reduce the impact of lower respiratory tract infections. Children with BM may experience exercise limitation [44], but we did not find any studies on exercise rehabilitation.

Surgery including stenting for TBM

Surgery may be necessary in severe TBM with acute life-threatening events (“apnoeic spells”), cyanosis, feeding difficulties, inability to extubate the airway and recurrent pneumonia [89]. A detailed diagnostic work-up informs planning for the most appropriate operative technique. Surgical and endoscopic options include tracheostomy, aortopexy, tracheal resection, tracheopexy (anterior or posterior), internal stenting and external airway splinting [27]. Intra-operative flexible bronchoscopy may be helpful in guiding the surgeon during some of these procedures [27, 96–98].

Internal stenting for TBM

Internal stenting is an attractive concept, but several practical problems limit the use of this technique in children [112]. The indications vary considerably, but in general a multidisciplinary team and an individualised approach to each patient are emphasised [113, 114]. Most centres reserve the use of internal stents for children who have no curative surgical options and where tracheostomy is not appropriate [113–116]. Other centres consider stent implantation to be a valid alternative to tracheostomy [117]. Stent insertion is usually followed by an immediate improvement in the patient's clinical condition [118], although this may be transient.

Various stent types with different physical characteristics are available (table 3) [114]. Silicone stents [119] and self-expanding plastic stents [120] have rarely been used in children with TBM. Encrustation with mucus, migration and the development of granulation tissue or mucosal hyperplasia at the ends of the stent are problems [112, 116, 117, 119, 121]. Balloon-expandable metals stents may be difficult to retrieve after being in situ for longer than a few weeks and may fracture or (rarely) cause vascular erosion [112–115, 117]. They may be dilated as the child grows (figure 4) [112, 114]. Uncovered self-expanding metal stents are less likely to fracture or cause vascular erosion, but cannot be dilated and are very difficult to remove [114, 116]. Covered self-expanding metal stents are retrievable, but suffer from the same problems as silicone stents [112, 114]. Recently, bioabsorbable airway stents have been used in selected children with malacia (figure 5) [117, 122]. Realistic goals for the use of absorbable stents include “proof of principle” that restoration of patency improves clinical status (e.g. by making the patient independent of invasive ventilation) before attempting surgery or permanent stenting [112, 113, 115] and stabilisation of the airway to allow spontaneous resolution of malacia [115].

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TABLE 3

Internal airway stents commonly used in children

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FIGURE 4

A self-expanding stent inserted into the left main bronchus of the same child illustrated in figure 2c and d with immediate relief of symptoms.

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FIGURE 5

Biodegradable stents. a) A biodegradable stent. b) Stent within introducer. c) Bronchoscopic image of stent recently deployed. d) Bronchoscopic image of biodegradable stent after 8 weeks placement with evidence of partial reabsorption showing an open lumen and mild granulation tissue formation.

Extensive tracheobronchial stenting for diffuse TBM is not appropriate [112, 114], and it has been previously recommended to avoid a combination of stenting and tracheostomy [112, 113, 118]. Aortopexy is almost universally preferred over stenting for TM associated with oesophageal atresia [116, 123]. Metal stents [124], or serial stenting with absorbable stents [115], could be used as an alternative to aortopexy, but this concept has not been widely accepted.

Complications of metal stents include airway obstruction due to granulation tissue and mucus plugging, which is aggravated by the variable degree of impairment of mucociliary clearance caused by the stent [112, 116, 125]. Granulation tissue is usually managed by repeat bronchoscopic laser treatment, endoscopic removal or crushing with balloons [112, 126]. Fatal complications, including severe airway haemorrhage and pneumonia, may occur, but are uncommon when stents are used appropriately [118, 127]. Fully epithelialised metal stents are usually considered permanent, as they are difficult to remove safely and the long-term results of permanent stenting are acceptable [112, 115, 117, 128].

Ventilatory pressure support in TBM

There is a pathophysiological rationale for pressure support in paediatric TBM. The variable dynamic deformity throughout the airway results in variable airflow velocities and time constants of respiration. Pressure support must overcome these forces and concomitantly allow enough downstream expiratory gas flow to ensure respiratory stability. Computer modelling reveals a complex relationship between length of malacia, diameter of the tracheal ring, site of malacia and tissue type generally to predict the tipping point to airway collapse/closure or snap point [132–134]. These mathematical modelling studies provide some insight into the use of pressure support, but greater insight might come from 3D modelling of the airway lesions [135].

TBM has been managed with all forms of noninvasive pressure support (CPAP, bilevel airway pressure, high-flow drivers and full ventilation) [21, 136–140]. CPAP is the most widely used. Bilevel airway pressure is rarely used because there is no advantage in the vast majority of patients (other perhaps than in those who require a very high distending CPAP pressure) and poor synchronisation may be a problem [137]. Our literature search found no prospective or randomised studies that help inform decisions regarding when and how to use these approaches. Comorbidities also may affect decision making.

Clinical studies have shown that CPAP improved gas flows at FRC without changing the appearance of the flow–volume loop and, most importantly, flows were also still appreciably decreased compared with controls, particularly at lower lung volumes [47, 141]. Bronchoscopy or imaging can help titrate CPAP levels [138].

Currently there are no proven management algorithms. Pressure support should be considered in acute severe life-threatening events, although maintaining a child on noninvasive ventilation 24 h a day without a tracheostomy is likely to prove impractical. Pressure support is usually considered in any patient with recurrent acute or chronic respiratory failure and sleep disordered breathing. Speculatively, it may also play a role in some patients with TBM associated with recurrent persistent bacterial bronchitis or recurrent pneumonia and poor growth where alternative surgical or medical therapies have failed [142].

Application of noninvasive pressure support is dependent on informed parental agreement for the intervention, type of device, interface interaction between device and patient, operator experience, and availability of sleep monitoring systems. These interventions may need to be established over a number of monitoring sessions with careful follow-up. Protocols are usually determined on an individual basis. Weaning protocols take into account the possible natural history of improvement [2, 21, 47, 139, 140]. Just how long these modes of pressure support are required for on a daily basis or cumulatively over a longer time period is also not known and likely to be individualised.

A tracheostomy for the delivery of positive pressure is reserved for more severe cases in whom other approaches have failed or where pressure support is required for most of the 24 h day [21, 43, 139, 140, 143].

Parent and patient perspective

There are few published studies that specifically address this topic [144–146]. Grey literature searches identified discussion forums, blogs and news articles where parents, carers and patients shared experiences and sought advice. Key concerns for parents and carers are described in the following, with selected illustrative quotes from parents and carers in box 1.

Areas for future endeavour

For any future research into this topic of malacia, it is important to have a working definition of TM, BM and TBM. All Task Force members perform flexible bronchoscopy under carefully regulated anaesthetic conditions with airways undistorted by endotracheal tubes or laryngeal masks, wherever possible, in order to get the best anatomical assessment of malacia during free breathing and forced expiratory manoeuvres such as coughing. In experienced hands, tracheobronchography provides invaluable information.

The Task Force members grade the degree of malacia as:

1. Normal: collapse up to 50% of the lumen.

2. Mild: loss of cross-sectional area (which may be asymmetrical) between 50% and 75%.

3. Moderate: loss of cross-sectional area between 75% and 90%.

4. Severe: >90% loss of cross-sectional area.

The reliance of subjective visualisation may be replaced by enhanced digital quantification in the future.

The site and length of the affected area can also be assessed bronchoscopically. Contrast-enhanced tracheobronchography provides useful dynamic information as airway collapse is dependent on the properties of the airway and transmural airway pressure.

Using these defined criteria, standardised data collection in a prospective manner by a network of interested parties could establish much needed information on a number of key areas:

1. The natural history of this condition from premature infancy to mature adolescents.

2. The role and impact of current surgical interventions, such as the anterior and posterior pexy procedures, and the impact of these procedures on long-term growth and outcomes.

3. Studies into the use of antibiotic therapy in the prevention and treatment of infection in TBM.

4. Increasing awareness and establishing markers for the role of malacia in refractory respiratory illness, thereby reducing delay in the diagnosis and unnecessary therapies.

5. Registry data to establish the role of biodegradable stents in the management of TBM and the histological changes that may happen in the tracheal wall as part of the reabsorption process.

6. Clarity on the apparent disconnect between the extent and degree of malacia and the clinical presentation.

7. Long-term outcomes, including the management of realistic expectations around full recovery.

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