Discussion
Non-infectious pediatric central airway obstruction is usually due to malacia and stenosis (7,8). The estimated incidence of congenital TM is approximately 1:2100 children being the most common congenital tracheal anomaly (9). Albeit most patients with TM or BM may outgrow their disease, a distinct group may exhibit life-threatening symptoms such as apnoeic spells or inability to extubate the airway. In this settingt, surgical or endoscopical treatment is mandatory (10).
First experience with PDO airway stents in children was reported by Vondrys et al. in 2011 (11). Albeit increasing interest with stenting in the pediatric age group, most of the publications have been case reports or short series of patients (12-15). In 2016 we reported our initial experience with BD-PDO intraluminal airway stents in children (4). These stents were effective when dealing with stenosis or malacia with fewer complications than those exhibited by metallic or plastic stents (5). We have continued using PDO stents, when indicated, and gradually replacing other types of stents. Parallel to our clinical experience, our group has accomplished experimental studies addressing the biologic behavior of PDO stents in the rabbit trachea (16).
In a recent article, Minen et al (17) reported the largest group of pediatric patients with airway PDO stents belonging to a single institution. Interestingly, their experience was very similar to ours: 33 patients and 55 stents in an 8 year period of time. They addressed efficacy and safety of PDO stents and specifically focused on stent-related data (size, time to degrade, and stent related complications). According to their protocol, they intentionally downsized the stents in order to reduce granulation tissue which may be caused by excessive stent pressure on airway mucosa and cartilage (17). This is a very relevant issue because they did not describe major granulation tissue needing bronchoscopic removal in their series. Although we did not deliberately downsize our PDO stents, stent dimensions in our cohort were very similar to theirs (median values, 7 mm x 20 mm tracheal and 5 x 25mm left bronchial ). Conversely, our patients were significantly younger (median age, 4 months vs 13.1 m) so probably this implies that our stents were relativeley larger compared to theirs. Additionally, we observed that patients with complete or partial clinical improvement had statistically significant smaller stents than those with no improvement. Although stent size and radial force may play a role in granulation tissue formation, and ultimately a better outcome, there are probably other factors involved (18). Zhang et al (19) studied the role of tracheal wall injury in the development of benign airway stenosis in rabbits. They demonstrated that cartilage injury was the key factor of airway stenosis and that acute injury of the mucosa alone was unlikely to cause it. We have investigated the biologic effects caused by successive placement of PDO stents ( up to 3) in the rabbit trachea (16). According to our data, consecutive stenting did not show a statistically significant increase in tracheal wall collagen and cartilage structure was not modified in those rabbits with one or more PDO stents compared to the non-stented tracheal sections. In this study 4 rabbits out of 21 (19%) showed severe granulation tissue soon after the first PDO stent placement (<7 weeks) and they were excluded from the final analysis. This clinical behavior was completely different from the observed in the other 16 animals who achieved the anticipated survival time in the study design (14, 28 and 42 weeks). Although these experimental data must be interpreted with caution and conversion to clinical grounds must not be considered linear, they give relevant information regarding tissue tolerance of PDO stents. Albeit we do not know the biological rationale, we presume that there must be some kind of individual intolerance to PDO stents in certain subjects. This could apply too for pediatric patients though we have observed good tolerance with mild granulation tissue in the majority of our patients but approximately one third exhibited this stent-related complication. To exemplify this point, one patient received 9 consecutive PDO stents, in a 35 months period, showing excellent tissue tolerance with only mild granulation tissue although stent size was gradually increased (5-9 mm diameter, 15-35 mm length). Conversely, another developed severe granulation tissue, and eventually tracheal stenosis, with the first PDO stent ( 6x20 mm).
Congenital airway malacia, either primary or secondary, is the most frequent indication for BD stenting (7,17,20). Most of these patients exhibit severe associated anomalies, mainly cardiovascular and syndromic, that have a relevant impact in their clinical status so it is sometimes difficult to establish the precise contribution of airway malacia to the clinical situation. The rationale for using BD stents in this particular scenario relies on what is called the ”proof of principle “ (10). This means that restoration of patency improves clinical status before attempting surgery or permament stenting. BD stents may also play a role as a “bridge” treatment with stabilisation of the airway to allow spontaneous resolution of malacia or definitive surgical correction (4,17,20).
PDO stent placement with RB and fluoroscopic control has proved to be a safe and straight forward procedure. Some authors advocate stent implantation thru an endotracheal tube with fluoroscopy and broncograms (11,17). We consider that both techniques are sound and selection depends on institutional preferences and local availability. In our center, the procedure is done by general pediatric surgeons with broad experience in bronchoscopy and airway surgery. In our experience, placing a PDO stent in the airway is easier than implantation of other types of stents (5).
Limitations of our study include its retrospective design and the cohort size which precludes more thorough statistical analyses. Additionally, it includes a diverse sample of central airway obstructive diseases associated to other severe congenital anomalies and genetic syndromes that have a relevant impact in the clinical situation.
In conclusión, BD-PDO stents are a safe and effective tool when dealing with severe tracheobronchial obstruction in children. They can be even life-saving in certain critical scenarios where other therapeutic measures have failed or are contraindicated. Albeit stent-related complications seem to be fewer than with other type of devices, granulation tissue formation continues to be a relevant matter of concern. This issue together with increased degradation times deserve further research.