Congenital high airway obstruction syndrome (CHAOS) is a rare prenatal diagnosis whose true incidence is unknown. It is defined by complete or near-complete obstruction of the upper airway and appears radiographically as echogenic large lungs, flattening or inversion of the diaphragm, and dilation of the tracheobronchial tree. 
Historically, fetal demise among fetuses with CHAOS was unavoidable, but, with the advent of the ex-utero intrapartum treatment (EXIT) procedure (see image below), survival rates have drastically improved.
Albeit a rare prenatal diagnosis, CHAOS is likely more common than reported, as many fetuses with CHAOS die in utero or are stillborn. [2, 3, 4, 5] As the name suggests, CHAOS results from obstruction of the upper airway. Causes include laryngeal atresia, subglottic stenosis, and laryngeal or tracheal rings or agenesis. Laryngeal and tracheal anomalies may be associated with Fraser syndrome, a congenital disorder characterized by ambiguous genitalia, urinary tract anomalies, cryptophthalmos, and syndactyly. 
The pathophysiology of CHAOS results from trapping of pulmonary secretions, which are normally evacuated through the fetal upper airway. Secretions accumulate in the bronchopulmonary tree and cause hyperinflation of the lungs. This, in turn, leads to an increase in the intrathoracic pressure, compromising venous return and cardiac output, potentially resulting in nonimmune fetal hydrops. Interestingly, hydrops is better tolerated among patients with CHAOS than among those with other congenital abnormalities with associated hydrops.
Currently, all viable patients with CHAOS are offered an EXIT delivery. Rare instances of survival without an EXIT delivery have been described among patients with a concurrent tracheoesophageal fistula, providing a passage for evacuation of pulmonary secretions. 
The natural history of CHAOS is grim, with only 1 patient in 48 surviving the disease in the pre-EXIT era.  Besides EXIT deliveries, some fetuses benefit from fetoscopic wire tracheoplasty.
Determining which patients are more likely to benefit from tracheoplasty poses a great challenge, and this procedure is typically reserved for fetuses who present with hydrops early in pregnancy (17-30 weeks’ gestation).
As patients with CHAOS seem to tolerate hydrops better than those with other disease processes, expectant management is generally preferred for those who present later in the third trimester until an EXIT delivery can safely be performed.
In fetuses with hydrops and CHAOS who have very short-segment atresia and who present at 17-30 weeks’ gestation, the authors offer fetoscopy with wire tracheoplasty to decompress the airway. As the literature is confined to a single case report for this procedure, outcomes data are unavailable.
Contraindications can be either maternal or fetal in origin. Maternal contraindications include frequent contractions, membrane rupture, a short cervix, or uncontrolled comorbidities that may predispose the patient to pre-eclampsia or HELLP syndrome. Fetal contraindications include multiple fetal anomalies, chromosomal abnormalities, and anatomic restrictions, including an anterior lying placenta, which may prohibit safe access.
MRI is best used to delineate the length and location of the atretic segment.
Fetal wire tracheoplasty is considered early in pregnancy. The procedure is performed fetoscopically under ultrasound guidance to avoid injury to the placenta. Positioning is critical in order to obtain a direct path into the fetal oropharynx (see image below).
The EXIT procedure requires careful planning with a multidisciplinary team, typically including two pediatric/fetal surgeons, one obstetrician, one neonatologist, one ultrasonographer, and one nurse scrub.
Appropriate anesthesia and tocolytics are important to prevent uterine contractions and placental separation. Uterine atony can lead to diffuse bleeding, and a special uterine stapler (uterine stapler with absorbable staples [US Surgical Corporation, Norwalk, CT]) can reduce the risk of hemorrhage during the EXIT procedure.
Since CHAOS is a very rare disease and viability beyond the immediate neonatal period has become a reality only with the implementation of the EXIT procedure in 1998 and 2000, outcomes data are limited. 
Outcomes may depend on other anomalies, as fetuses with CHAOS frequently have additional congenital abnormalities diagnosed postnatally. In one series, 3 of 4 fetuses with hydrops who underwent fetal intervention had other congenital abnormalities—consisting prune-belly syndrome, imperforate anus, and fragile X syndrome.  In cases of isolated CHAOS without hydrops, outcomes are more favorable and frequently require shorter periods of postnatal ventilator support. 
Despite remarkable progress in CHAOS management, tracheostomy dependence and speech impediments are to be expected.
A diagnosis of CHAOS in the mid-second trimester suggests a poor prognosis, as many progress to in utero fetal demise. Patients who present in the third trimester without signs of hydrops frequently survive until delivery, as they typically have only a partial obstruction.