Subglottic stenosis (SGS) is a narrowing of the subglottic airway (see the image below). The subglottis is defined as the airway directly below the level of the vocal folds and encased in the cricoid cartilage. The subglottic airway is the narrowest area of the airway because it is encased in a complete, nonexpandable, and nonpliable ring. In contrast, the trachea has a posterior membranous section, and the larynx has a posterior muscular section. The term SGS implies a narrowing of the subglottic airway that is either congenital or acquired in etiology, though the term is applied to both congenital lesions of the cricoid ring and acquired SGS.
Acquired SGS is the most common acquired anomaly of the larynx in children and the most common abnormality requiring tracheotomy in children younger than 1 year. Correction of this abnormality requires expanding the lumen of the cricoid area to increase airflow and decrease obstructive breathing. Surgical correction of SGS has been performed with various techniques over the years.
Early in the 20th century, acquired SGS was usually related to trauma or infection from syphilis, tuberculosis, typhoid fever, or diphtheria. Often, the treatment itself for SGS at this time, a tracheostomy, led to further damage to the airway
The incidence of acquired SGS increased in the late 1960s through the 1970s, after McDonald and Stocks introduced long-term intubation as a treatment method for neonates in need of prolonged ventilation.  The increased incidence of SGS focused new attention on the pediatric larynx, as well as the need for development of novel treatment modalities.
Approaches to operative treatment
Surgery without cartilage expansion
In 1971, Rethi and Rhan described a procedure for vertical division of the posterior lamina of the cricoid cartilage with Aboulker stent placement. A metal tracheotomy tube was attached to the Aboulker stent with wires, and the anterior cartilaginous incision was closed.
In 1974, Evanston and Todd described success with a castellated incision of the anterior cricoid cartilage and upper trachea, which was sewn open, and a stent made of a rolled silicone sheet was placed in it for 6 weeks.
In 1980, Cotton and Seid described a procedure in which tracheotomy is avoided called the anterior cricoid split (ACS).  The procedure was designed for use in neonates (usually, those born prematurely) with anterior SGS or SGS who had airway distress after extubation. The cricoid ring was divided anteriorly and a laryngofissure was created in an attempt to expand the airway without a tracheotomy.
Holinger et al also described success with this procedure in 1987. 
Surgery with cartilage-grafting reconstruction
In 1974, Fearon and Cotton described the successful use of cartilage grafts to enlarge the subglottic lumen in African green monkeys and in children with severe laryngotracheal stenosis.  All augmentation materials were evaluated, including thyroid cartilage, septal cartilage, auricular cartilage, costal cartilage, hyoid bone, and sternocleidomastoid myocutaneous flaps. After significant work, it appeared that costal cartilage grafts had the highest success rate.
In the 1980s, Cotton reported his experience with laryngeal expansion with cartilage grafting. [5, 6, 7, 2] His success rates depended on degree of stenosis. More severe forms of stenosis required multiple surgical procedures. Cotton used the Aboulker stent.
In 1991, Seid et al described a form of single-stage laryngotracheal reconstruction in which cartilage was placed anteriorly to expand the subglottis and upper trachea to avoid a tracheotomy. 
In 1992, Cotton et al described a four-quadrant cricoid split, along with anterior and posterior grafting. 
In 1993, Zalzal reported 90% decannulation with any degree of SGS with his first surgical procedure.  Zalzal customized the reconstruction on an individual basis, and most patients received Aboulker stents for stabilization.
In 1993, Monnier described partial cricotracheal resection with primary anastomoses for severe SGS because grade III and grade IV SGS (ie, severe SGS) often requires multiple (three or four) surgical augmentations for decannulation. 
In 1997, Stern described his experience with the procedure, reporting a decannulation rate higher than 90% for primary and rescue cricotracheal resection. 
The exact pathophysiology of congenital SGS is unknown, though there is a known association of SGS with syndromes such as Down syndrome.
Acquired SGS, on the other hand, is most freqently associated with intubation or airway trauma. Mechanical trauma from an endotracheal tube, as it passes through or remains for long periods in the narrowed neonatal and subglottic airway, can lead to mucosal edema and hyperemia. These conditions then can progress to pressure necrosis of the mucosa. These changes have been observed within a few hours of intubation and may progress to expose the perichondrium of the cricoid cartilage. Infection of the perichondrium can result in a subglottic scar.
This series of events can be hastened if an oversized endotracheal tube is used. Always check for an air leak after placing an endotracheal tube because of the risk of necrosis of the mucosa, even in short surgical procedures. Usually, the pressure of the air leak should be less than 20 cm H2O, so that no additional pressure necrosis occurs in the mucosa of the subglottis.
The exact etiology of congenital SGS is unknown.
The etiology of acquired SGS is related to trauma of the subglottic mucosa. Injury can be caused by infection or mechanical trauma, usually from endotracheal intubation but also from blunt, penetrating, or other trauma. Historically, acquired SGS has been related to infections such as tuberculosis and diphtheria. Over the past 40 years, the condition has typically been related to mechanical trauma.
Factors implicated in the development of SGS include the size of the endotracheal tube relative to the child's larynx, the duration of intubation, the motion of the tube, and repeated intubations. Additional factors that affect wound healing include systemic illness, malnutrition, anemia, and hypoxia. 
Local bacterial infection may play an important role in the development of SGS. Gastroesophageal reflux (GER) may play an adjuvant role in the development of SGS because it causes the subglottis to be continually bathed in acid, which irritates and inflames the area and prevents it from correctly healing. A systemic or gastrointestinal (GI) allergy may cause the airway to be more reactive, creating a greater chance of developing stenosis.
United States statistics
No known frequency has been reported for congenital SGS; the incidence of acquired SGS has greatly decreased over the past 40 years. In the late 1960s, when endotracheal intubation and long-term ventilation for premature infants began, the incidence of acquired SGS was as high as 24% in patients who required such care. In the 1970s and 1980s, estimates of the incidence of SGS were 1-8%.
In 2000, Choi reported that the incidence of SGS had remained constant at the Children's National Medical Center in Washington DC, accounting for approximately 1-2% of the children who had graduated from the neonatal intensive care unit (NICU). 
Walner reported that, among 504 neonates who were admitted to the level III NICU at the University of Chicago in 1997, 281 were intubated for an average of 11 days; over a 3-year period, no patients developed SGS. 
International frequency is the same as that of the United States.
In 1996, a report from France also described no incidence of SGS in the neonatal population who were intubated with very small endotracheal tubes (2.5 mm internal diameter) in attempts to prevent trauma to the airway.
Age-, sex-, and race-related demographics
SGS is observed more often in premature infants because they may require mechanical ventilation for other system or pulmonary problems secondary to their prematurity. The mechanical ventilation can result in airway trauma and, potentially, SGS.
Equal sex distribution is noted. No racial predilection is noted.
The symptoms and prognosis of SGS can be highly variable, mostly depending on the severity of the SGS. Difficulty in breathing and exercise intolerance can occur with mild, moderate, or severe SGS. In moderate-to-severe cases of SGS, respiratory insufficiency can lead to failure to thrive, acute life threatening events (ALTE), and even death.
The outcome of laryngotracheal reconstruction depends on its grade and the procedure performed. Most authors report success rates of 80-90% when the patient has undergone successful preoperative evaluation and when the appropriate operation has been performed (see the images below). The presence of acute or chronic respiratory illness, GER, or a reactive larynx may decrease the success rate. Choi and Zalzal showed that age can affect success rates; scars are more likely to recur in children younger than 2 years than in others.  Success is more likely with localized SGS than with extensive SGS. 
Zalzal noted that in any child with voice abnormalities before surgery, those abnormalities persisted after surgery.  Subglottic pressure is required to produce a strong voice. If the narrowed subglottic airway is expanded, subglottic airflow and pressure increase, and the voice usually is stronger (see the images below). Voice therapy may help relieve nonsevere glottic stenosis over time.
The voice of a patient with SGS, especially those who require reconstruction, may never return to its preoperative state, because the following are possible:
Imperfect closure of a laryngofissure through the anterior commissure
Potential vocal cord weakness or tension caused by other laryngeal pathologic conditions
Because reconstructive techniques have improved over the past 20 years, the focus of attention in patients with SGS who require reconstruction has switched from decannulation to decannulation with improved voice outcome.
Avelino et al reported a 100% success rate with balloon laryngoplasty in children with acute subglottic stenosis, though they found the procedure to be less successful in chronic subglottic stenosis.  Factors that predicted a good outcome were as follows:
Less severe grades of stenosis
Younger patient age
Absence of tracheotomy
Teaching parents life-saving maneuvers for a child with a tracheotomy or airway stent following laryngotracheal reconstruction is critical. Also teach parents cardiopulmonary resuscitation (CPR) before their child leaves the hospital.
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