Subglottic Stenosis
Although congenital subglottic stenosis is rare, acquired subglottic stenosis is common in neonates with BPD. The cricoid is the narrowest portion of the pediatric airway; as a result the cricoid is the most likely area to be damaged in neonates requiring intubation.(44) Multiple factors predispose children to developing subglottic stenosis including duration of intubation, multiple intubation attempts, traumatic intubation, nasal vs oral intubation, endotracheal tube composition, and inadequate sedation.(45-48) However, the most important factor is the relative size of the endotracheal tube to the patient’s airway.(46) The endotracheal tube exerts pressure of the airway mucosa that can exceed capillary filling pressure, particularly with cuffed endotracheal tubes. (49, 50) Within two hours, pathologic changes of the airway mucosa related to intubation can occur, and the alterations of the airway mucosa progress with longer periods of intubation.(51, 52) Damage to the airway related to endotracheal intubation can eventually lead to tissue necrosis and scar formation that manifests as subglottic stenosis
Post-intubation subglottic stenosis develops in 0.9-8.3% of intubated neonates; however, because of the small airway size, prolonged intubation, and multiple intubation attempts, the risk of subglottic stenosis in BPD is likely higher than other neonatal populations. (48, 53) As with TBM, the diagnosis of subglottic stenosis depends on imaging techniques that expose neonates to the ionizing radiation and/or direct visualization with bronchoscopy; thus, the true incidence of subglottic stenosis in BPD is unknown. Subglottic stenosis can be suspected based on airway plain films or computed tomography; (54, 55) however, definitive diagnosis is made by bronchoscopy. Subglottic stenosis is classified using the Myer-Cotton grading scale, which determines the largest endotracheal tube that permits an air leak at 20 cm H2O. The severity of subglottic stenosis is defined as grade 1 (<50% narrowed), grade 2 (51-70% narrowed), grade 3 (71-99%narrowed), and grade 4 (no detectable lumen) (Fig.3B-E).(56)
As the severity of subglottic narrowing increases, airway resistance and respiratory work increases exponentially. Clearly, the extent of the stenosis is the primary driver, but the length of the stenotic segment and the location of the stenosis in relationship with the glottis also impacts airway resistance. (57, 58) The narrowing and increased airway resistance most commonly manifests with biphasic stridor and increased respiratory effort. Indeed, neonates with subglottic stenosis may not tolerate extubation and are at high risk of undergoing tracheotomy. (20, 59)
Management of subglottic stenosis in neonates with BPD should focus on prevention. Recent efforts have increased the use of nasal CPAP at birth rather than intubation to minimize the risk of the development of BPD.(60, 61) If neonates are adequately supported non-invasively, acquired subglottic stenosis does not develop. Non-invasive positive airway pressure can also be utilized to minimize the risk of extubation failure, thereby reducing the risk of airway trauma related to multiple intubations.(62) In the event that endotracheal intubation is necessary, an endotracheal tube that leaks at less than 20-25 cm H2O minimizes the risk of developing subglottic injury;(63) unfortunately, this may not be feasible to adequately support gas exchange and respiratory comfort in patients with particularly severe lung disease. For patients who require prolonged intubation, adequate sedation that minimizes agitation may also limit the development of subglottic stenosis. (47)
If a neonate with BPD does develop subglottic stenosis treatment options include non-operative measures, endoscopic intervention, open airway surgery, and tracheotomy. Non-operative treatments include downsizing to a smaller endotracheal tube that permits an air-leak at 20-25 cm H20 combined with topical steroid and antibiotic drops delivered via the endotracheal tube. An oral endotracheal tube can be replaced with an nasal tube to minimize mucosal trauma related to movement of the tube along the axis of the airway.(44) Even if non-operative measures do not prevent the need for surgical management, reduction of airway edema and inflammation may aid operative intervention.
For patients with grade 1 or grade 2 subglottic stenosis, balloon dilation is the mainstay of endoscopic intervention. Balloon dilation is a minimally invasive technique that involves inflating a high pressure, non-compliant airway balloon in the narrowed segment of the airway and is generally safe and well-tolerated; however, multiple dilations are often needed for a successful outcome. (64, 65) While endoscopic balloon dilation is successful at avoiding tracheotomy in a majority of pediatric patients with mild subglottic stenosis, balloon dilation is frequently inadequate for more severe stenoses.(64-66) Infants that are born premature or have multiple medical comorbidities appear to be at increased risk of failed endoscopic balloon dilation and are more likely to need invasive surgical interventions.(66)
Surgical options available for the management of subglottic stenosis in neonates with BPD include cricoid split, laryngotracheal reconstruction with cartilage grafting, and tracheotomy. Anterior cricoid split was first described in 1980 for aiding extubation in premature infants with isolated subglottic stenosis. The operation involves a small neck incision over the cricoid and a vertical incision in the anterior airway from the lower thyroid cartilage to the upper two tracheal rings. The endotracheal tube is left in place, and the airway is allowed to heal by secondary intention. In highly selective patients, anterior cricoid split is successful in facilitating extubation.(67) In more severe cases, modifications to this technique include the placement of a cartilage graft using thyroid ala or costal cartilage to close the airway and a posterior cricoid split with or without a posterior cartilage graft.(68, 69) In the event that a cartilage graft is used, the operation is referred to as laryngotracheal reconstruction, which is highly successful for appropriately selected patients, even with severe subglottic stenosis.(69, 70) For infants that are likely to need prolonged mechanical ventilation for parenchymal lung disease or those who are not candidates for airway reconstruction, tracheotomy can be pursued to bypass the stenotic segment.