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Congenital High Airway Obstruction (CHAOS) Syndrome – A Rare Case Presentation

Dr. Vivek Chail, Dr. Dinakara Prithviraj, Dr.Shahina Anjum, Dr.Santosh KV, Dr. Manjunath, Dr.Mousumi Kar, Dr. Swagata M, Dr.Obaid Zafar, Dr.Aravind N,
Neonatal ICU, Dept of Pediatrics, Dept of Radiology, Dept of Pathology. Vydehi institute of medical Sciences  and research centre.Whitefield, Bengaluru.  Phone no:  080-28413385-89 Ext 273. 
Email drdinakar.nishanth@gmail.com,  vchail@gmail.com

Congenital chest malformations are rare, and may involve the lung parenchyma, bronchi, arterial supply, and venous drainage. They include congenital cystic adenomatoid malformation (CCAM), congenital diaphragmatic hernia (CDH), bronchopulmonary sequestration (BPS), congenital hydrothorax, and congenital lobar em­physema. Rarer entities include congeni­tal high airway obstruction syndrome (CHAOS), congenital bronchogenic cyst, bronchial atresia, pulmonary arteriovenous malformation (PAVM), congenital pulmonary lymphangiectasia, pulmo­nary hypoplasia-aplasia, mediastinal teratoma, and mediastinal lymphangioma. Sometimes there may be compromise in normal pulmonary development, causing hypoplastic lung formation. Pulmonary abnormalities are not mutually exclusive, since abnormalities frequently occur together as hybrid conditions.
To evaluate congenital chest malformations, all thoracic structures must be systematically an­alyzed to define the origin of the malformation. Prenatal sonography remains the primary imag­ing modality for evaluating the fetus, because it is safe, largely accessible, reproducible and inexpensive.

Congenital High Airway Obstruction Syndrome

Congenital high airway obstruction syndrome (CHAOS) was defined by Hedrick et al in 1994 as upper airway obstruction that is diagnosed in utero by ultrasound, with concomitant findings of large echogenic lungs, flattened or inverted diaphragms, dilated airways distal to the obstruction, and fetal ascites or hydrops. CHAOS is a rare abnormality with a poor prog­nosis and is caused by complete or near-complete obstruction of the fetal airway, leading to the trapping of lung fluids, hyperplasia of pulmo­nary alveoli, and tracheal dilatation. Fetuses with CHAOS may develop hydrops due to car­diac compression and obstructed venous return.

Airway abnormalities and lesions presenting as congenital high airway obstruction syndrome at birth are Laryngeal atresia, Laryngeal stenosis, Laryngeal cyst, Laryngeal web, Tracheal stenosis and Tracheal atresia.

Only 52 cases have been reported with 22 of these cases reported since 1989; though the true incidence may be higher than suggested by these case reports. A genetic cause or predisposition for CHAOS has not been determined. Vanhaesebrouck et al in 2006 reported an unique family with autosomal dominant inheritance of CHAOS and variable expression in the affected father and two affected children. The father had a history of being treated for “chronic croup” by tracheal cannulation. The father underwent an indirect laryngoscopy, after his child’s presentation with CHAOS, which revealed a partial subglottic webbing suggesting that the father was also mildly affected by CHAOS. They concluded that minor expression in one of the parents may be an important indicator for genetic counseling in CHAOS and management of future pregnancies.

In the human, the first signs of laryngeal development are apparent in the fourth gestational week, with the development of a laryngotracheal groove on the ventral surface of the caudal end of the pharynx. This groove progressively deepens to form a diverticulum anterior to the pharynx. The distal portion of this diverticulum will eventually enlarge to form a lung bud. Proximally, the lateral walls of the diverticulum invaginate to form tracheoesophageal folds, which eventually fuse in the midline. This fusion forms a tracheoesophageal septum, which separates the primitive airway from the pharynx and esophagus. The cartilaginous structures of the larynx are derived from the fourth and sixth pairs of branchial arches and begin their development in the fifth week of gestation. Arytenoid swellings form at the laryngeal inlet, narrowing the slitlike aperture. The embryological development of the glottis has been a subject of controversy in the past. Today, there is agreement that the glottic epithelium proliferates, rapidly forming a temporary occlusion of the laryngeal lumen. This plug is recanalized by the 10th week of fetal life. Throughout this phase of laryngeal development, a posterior communication between the primitive trachea and the foregut remains patent as the pharyngoglottic duct. In 1965, Smith and Bain reported 9 of their own cases of laryngeal atresia and reviewed 21 more from the literature. Using these cases, the authors categorized laryngeal atresia into 3 types, depending on the location of the deformity. Later work by Zaw-Tun suggested that these 3 types of atresia may represent arrests at specific periods of development. Therefore, CHAOS has 3 possible presentations: (1) complete laryngeal atresia without an esophageal fistula, (2) complete laryngeal atresia with a TEF, and (3) nearcomplete high upper airway obstruction.

Antenatal Detection
The findings observed on prenatal ultrasound are thought to be a result of upper airway obstruction, which prevents the normal flow of fetal lung fluid into the amniotic fluid. The lungs therefore expand and produce a flattening of the diaphragm and appear hyperechogenic on ultrasound; if the lung fields expand to the point of producing esophageal compression, polyhydramnios may occur as a result of impaired swallowing of amniotic fluid.
The fundamental pathophysiology of CHAOS is altered when there is either an incomplete upper airway obstruction or a tracheoesophageal fistula (TEF), as the fetal lung fluid now has a way to flow from the pulmonary system, and the lung fields therefore do not expand outward. In utero ultrasound may therefore not diagnose the impending danger due to the upper airway obstruction.
Type 3 CCAM (congenital cystic adenomatoid malformation) is one of the differential diagnosis for CHAOS.


Associated Anomalies
The most frequently associated syndrome observed with CHAOS is Fraser syndrome which is characterized by malformations of the larynx, cryptophthalmos, syndactyly, genitourinary tract, craniofacial dysmorphism, orofacial clefting, mental retardation, and musculoskeletal anomalies. Other syndromes that have been reported in association with CHAOS include: Short-rib polydactyly syndrome, Cri-du-Chat syndrome and Velo-cardio-facial syndrome. Recently CHAOS has been described as part of a newly proposed association, TACRD (Tracheal Agenesis, complex congenital Cardiac abnormalities, Radial ray defects, and Duodenal atresia) pattern.


In our case we had a 26 years old pregnant lady with 22 weeks history of amenorrohea who had come for a routine antenatal scan. She was the mother of a 2 year old child which was born normally and was doing well. On antenatal ultrasound of the present pregnancy, the fetus was found to be of approximately 20 weeks gestation. The fetus was found to have large hyperechoic lungs, flattened diaphragm and hydrops which form a triad in the detection of CHAOS. The patient and her husband were counseled regarding the risks involved on progession of the fetus till birth. The parents agreed to terminate the pregnancy voluntarily.
A dead male fetus was delivered and sent for autopsy subsequently. An interesting finding noted during dissection of neck and thoracic organs was an abrupt ending of the trachea approximately 11 mm caudal to the lower end of larynx.

Newer Challenges

Prenatally, fetoscopic tracheoscopy has been performed to delineate and treat the cause of obstruction.1 A favorable outcome following inutero ultrasound-guided decompression of the fetal trachea was recently reported in an infant with CHAOS from laryngeal atresia. At delivery, the management of prenatally diagnosed CHAOS requires the presence of a multidisciplinary team including: neonatologists, pediatric           otorhinolaryngologist, and pediatric surgeons. The EXIT procedure (ex utero intrapartum treatment), which was first developed for reversing tracheal occlusion in fetuses with severe congenital diaphragmatic hernia, offers the advantage of ensuring uteroplacental gas exchange while on placental support and has resulted in favorable outcome in some cases of prenatally diagnosed CHAOS.

CCAM volume ratio=
[mass lgth(in cm)xmass ht(in cm)xmass wdth(in cm)x0.52]
Fetal HC(in cm)
CCAM volume ratio identifies foetues at high risk for developing hydrops. CVR > 1.6 is associated with an 80% risk of developing hydrops. Neonatal survival approaches 100% in the absence of hydrops.


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