Congenital high airway obstruction syndrome (CHAOS) is a prenatally diagnosed clinical syndrome manifested by the presence of extremely large echogenic lungs, flattened or inverted diaphragms, a dilated tracheobronchial tree, ascites, and other manifestations of nonimmune hydrops due to complete obstruction of the fetal airway (Hedrick et al., 1994). No fetus diagnosed prenatally with CHAOS associated with hydrops and complete airway obstruction has survived without intervention. The exception to this occurs when there is spontaneous perforation of the laryngeal or tracheal atresia, which may occur in up to one-third of cases and results in resolution of the hydrops.

The airway obstruction in CHAOS may be due to one of several causes, including laryngeal atresia, tracheal atresia, or laryngeal cyst, but the fetal clinical presentation is the same. Three types of laryngeal atresia are recognized: type I, in which the supraglottic and infraglottic parts of the larynx are atretic; type II, in which the atresia is infraglottic; and type III, which is glottic. A wide range of anomalies can be seen in association with laryngeal atresia (Table 30-1). Some fetuses, however, have atresia as an isolated anomaly (Fox and Crocker, 1964).

Congenital obstruction of the fetal airway resulting in CHAOS was initially thought to be extremely rare (Hedrick et al., 1994). Only 55 cases of this syndrome have been reported since 1989 (Wigglesworth et al., 1987; Delechotte et al., 1988; Silver et al., 1988; Tournier et al., 1988; Arizawa et al., 1989; Fang et al., 1989; Scurry et al., 1989; Didier et al., 1990; Schauer et al., 1990; Watson et al., 1990; Richards et al., 1992; Weston et al., 1992; Hedrick et al., 1994). However, this syndrome may be more common than generally appreciated because many of the affected fetuses die in utero or are stillborn (Fox and Crocker, 1964; Smith and Bain, 1965; Cohen, 1971; Wigglesworth et al., 1987; Fang et al., 1989; Scurry et al., 1989; Schauer et al., 1990; Watson et al., 1990; Richards et al., 1992). The true incidence of this syndrome is unknown.

The sonographic findings of CHAOS are due to complete obstruction of the upper airway. The lungs normally produce fluid that leaves the trachea at a rate estimated at 4 mL per kilogram of body weight per day as a result of fetal breathing movements. Complete obstruction of the upper trachea or larynx results in elevated intratracheal pressure and distention of the tracheobronchial tree due to accumulation of fetal lung fluid. The lungs become distended and appear sonographically to be extremely echogenic and diffusely enlarged (Tournier et al., 1988; Watson et al., 1990; Richards et al., 1992; Weston et al., 1992; Hedrick et al., 1994). The diaphragm becomes inverted and the mediastinal structures compressed (Figure 30-1). The heart may appear elongated, with a shift in its axis, and the chambers small and compressed by the large lungs (Figure 30-2). The dilation of the tracheobronchial tree can be traced up to the larynx or level of the tracheal obstruction. In a rare form of CHAOS, there may be complete absence of the trachea (Vaikunth et al., 2009). In advanced cases, signs of nonimmune hydrops, including ascites, placentomegaly, and anasarca may be seen as a result of compromised venous return to the heart. Polyhydramnios may also be observed secondary to esophageal compression. The fetus with CHAOS may also exhibit qualitatively abnormally vigorous breathing movements. Baarsma et al. (1993) observed that a fetus with complete laryngeal atresia exhibited high-amplitude vigorous jerky breathing movements as it tried in vain to move tracheal fluid through an atretic larynx.

The cause of CHAOS may vary from laryngeal or tracheal obstruction or stenosis to an intraluminal web or cyst (Figure 30-3). However, it may be difficult, if not impossible, to distinguish these conditions sonographically. Recently, the use of half-Fourier acquisition single shot turbo spin echo sequence (HASTE) and echoplanar magnetic resonance imaging (MRI) has aided in the diagnosis of CHAOS. In the report by Crombleholme et al. (2000), fetal MRI was helpful in excluding bilateral cystic adenomatoid malformation as well as determining the level of obstruction (Figure 30-4).

The main diagnosis mistaken for CHAOS is bilateral cystic adenomatoid malformation (see Chapter 35). This is an even rarer finding than CHAOS, as cystic adenomatoid malformation is usually lobar, not involving the entire lung, and less than 2% of cases are bilateral. Cases of CHAOS are commonly misdiagnosed as bilateral CCAMS and this should raise suspicion that the diagnosis may, in fact, be CHAOS. An important distinguishing feature is the compressed rim of normal lung that can usually be seen in cystic adenomatoid malformation but not in CHAOS. The uniformly echogenic lungs, which are massively enlarged and associated with flattened or inverted diaphragms, a compressed mediastinum, and dilated trachea and mainstem bronchi allow CHAOS to be distinguished from cystic adenomatoid malformation.