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.