The Emerging Role of Magnetic Resonance Imaging as a Primary Diagnostic Tool for Paediatric Tracheomalacia

Abstract

Nikolaos-Achilleas Arkoudis

Tracheomalacia is a congenital or acquired condition, which is characterised by tracheal cartilage weakening or softening. This leads to airway collapse, particularly during increased respiratory effort [1]. It has been reported to occur in approximately 1 in 2100 live births [2] and can have significant implications for respiratory health, particularly in vulnerable groups like children and adolescents. A range of symptoms have been reported, depending on disease severity, from stridor to severe respiratory distress [3]. It is notable that diagnosing tracheomalacia can prove challenging, when based on just clinical characteristics, as symptoms may often overlap with those of more common pulmonary diseases, such as allergic asthma [2].

A paper in Acta Paediatrica by Prountzos et al. [4] focuses significant attention on the evolving landscape of tracheomalacia in paediatric patients. It compares the effectiveness of magnetic resonance imaging (MRI) scans for diagnosing tracheomalacia with computed tomography (CT) scans and flexible bronchoscopy. The paper also presents the current state of tracheomalacia diagnosis in children and highlights the absence of a clear, diagnostic algorithm for paediatric patients.

Some context is needed before I discuss this particular paper. There is an understandable necessity for additional tests as part of an appropriate diagnostic approach to tracheomalacia. A combination of clinical assessments and different imaging techniques are usually required to reach the most complete understanding of the condition. Dynamic flexible bronchoscopy, performed during free breathing, seems to be the diagnostic method of choice [3]. However, advances in CT and MRI scans have made tomographic imaging increasingly popular when it comes to the dynamic evaluation of both the upper and lower airways. These imaging techniques are similar to bronchoscopy. They all depend on changes in airway morphology during the respiratory cycle and offer valuable insights into lung parenchyma and mediastinal structures [4]. All these methods rely on changes in the cross-sectional area or anterior–posterior diameter and require a 50% reduction of the tracheal cross-sectional area during expiration [3].

However, flexible bronchoscopy does present several limitations. Although it is valuable for direct visualisation, it is also invasive and risks complications. Moreover, sedation or anaesthesia is required and this carries inherent risks and can affect airway dynamics and assessments in an unpredictable way [5]. Flexible bronchoscopy also relies on the variable skills and experience of the operator and it is difficult to reproduce examinations.

On the other hand, dynamic airway CT has demonstrated excellent specificity and positive predictive values for detecting and categorising tracheomalacia [5]. In addition, it is a non-invasive technique that eliminates the associated risks compared to flexible bronchoscopy and simultaneously visualises other relevant structures, such as lung parenchyma and mediastinum. CT scans are also reproducible, which is valuable for follow-up assessments and enables clinicians to monitor disease progression or treatment responses. However, CT scans involve exposure to ionising radiation [5]. This is a significant concern, particularly for paediatric patients who are more sensitive to radiation and its cumulative effects, especially if they require multiple scans over time [6].

The above context shows that investigating MRI is promising. It can offer non-invasive insights into airway dynamics and cross-sectional area changes without the need for anaesthesia or sedation and without the risks associated with ionising radiation exposure [7].

The paper by Prountzos et al. [4], which is the main focus of this editorial, describes the retrospective evaluation of paediatric patients with suspected tracheomalacia, based on recurrent episodes of a brassy cough. All patients included in the study underwent both a CT and an MRI scan during the same visit, irrespective of the findings, without requiring anaesthesia. The CT scans were performed with a low-dose radiation protocol and dynamic end-inspiratory and forced expiratory acquisition. The evaluations were based on the cross-sectional area ratio reduction between the two scans. The MRI examinations were performed with both static T2-weighted images during end-inspiration and end-expiration and cine sequences that captured airway dynamics across multiple respiratory cycles. The MRI evaluation was also established by estimating the cross-sectional area. After they had undergone their CT and MRI examinations, some patients were also examined using flexible bronchoscopy, with deep sedation, because of a chronic wet cough or to guide clinical management.

The overall evaluation comprised 24 patients who underwent both CT and MRI scans. In addition, 13 of these patients also underwent flexible bronchoscopy. The authors looked at the subset of patients who underwent all three examinations. The MRI scans showed the highest accuracy in detecting tracheomalacia, with a mean expiratory cross-sectional area reduction of 0.53, compared to 0.46 for the CT scans and 0.38 for flexible bronchoscopy (p = 0.002). Post hoc tests revealed significant differences both between the MRI and CT scans (p = 0.008) and between the MRI scans and flexible bronchoscopy (p < 0.001). Moreover, the MRI scans identified the diagnostic threshold of a 50% expiratory reduction in 87.5% of cases, compared to 61.5% for flexible bronchoscopy and 33.3% for the CT scans.

The results of the study seem to suggest that MRI scans can detect tracheomalacia more frequently than other diagnostic methods and that they are more reliable in accurately assessing its severity. These key findings seem promising and could be a valuable addition to tracheomalacia diagnoses. In addition, MRI scans do not generate the ionising radiation involved in CT scans and they do not require the anaesthesia needed for invasive flexible bronchoscopy.

The above outcomes build upon, and support, those of other emerging studies. Cine MRI has already been demonstrated to be a technically viable alternative to CT for evaluating the dynamics of the central airways, both in healthy subjects and tracheomalacia patients [8]. Meanwhile, ultrashort echo-time MRI scans with retrospective respiratory gating are both sensitive and specific for assessing neonatal tracheomalacia in comparison to flexible bronchoscopy [9]. In addition, ultrashort echo-time MRI scans have displayed capabilities for imaging lung parenchyma pathologies at resolutions that are comparable to CT scans [10].

Prountzos et al. [4] decisively suggest that MRI scans should be the first-line diagnostic tool for assessing tracheomalacia in children who just present with a recurrent brassy or barking cough. If the symptoms suggest a potential endobronchial infection, then a more thorough approach with CT scans and flexible bronchoscopy is advised. This is because MRI scans, on their own, may overlook synchronous lower respiratory tract pathology. This step-up approach seems like a viable and logical pathway for tracheomalacia diagnoses. Initiating diagnostic management with a non-invasive, risk-free diagnostic examination, such as MRI scans, has clear advantages for children. It poses no potential harm, can yield beneficial insights and, if decisive, may avoid the need for intrusive and potentially unfavourable procedures. However, this approach may need to be modified, due to the availability of equipment and the level of expertise at different institutions, as with other pathways.

Important limitations of the study that were reported by the authors included the relatively small patient cohort, the retrospective design and the fact that flexible bronchoscopy was not performed on all patients. Considering the above, it seems apparent that future evaluations could benefit from larger, prospective cohorts that provide more robust and generalisable data. This would strengthen the study's findings and contribute to clearer diagnostic guidelines for tracheomalacia. In addition, performing flexible bronchoscopy on all patients would enable a more comprehensive comparison to be carried out across the diagnostic methods. Furthermore, an additional limitation of the study is that it involved relatively older children, for whom sedation or anaesthesia was not required. However, examining younger children, infants and toddlers would necessitate these measures. This is particularly important, as timely diagnosis of tracheomalacia in this age group is critical to prevent prolonged periods of ineffective and unnecessary asthma medication treatment, which these patients often endure.

Overall, the authors highlight the notable absence of a clear, standardised diagnostic algorithm for paediatric patients with tracheomalacia. This lack of a systematic approach can lead to delays in diagnosis and treatment, which ultimately affect patient outcomes. The study by Prountzos et al. [4] provides a framework for clinicians to better understand the advantages and limitations of MRI scans, CT scans and flexible bronchoscopy. It also shows how these different examinations can be included in their diagnostic resources.

To conclude, the study by Prountzos et al. [4] contributes valuable insights into the diagnosis of tracheomalacia. The authors advocate for the development of standardised diagnostic protocols, with a broader adoption of MRI scans in the forefront. Moving forward, it seems sensible to integrate such findings into clinical practice, in order to enhance diagnostic accuracy and care when treating paediatric patients with tracheomalacia.

Nikolaos-Achilleas Arkoudis: conceptualization, project administration, supervision, validation, visualization, writing – original draft, writing – review and editing.

The author declares no conflicts of interest.