Congenital High Airway Obstruction (CHAOS)

Congenital High Airway Obstruction Syndrome (CHAOS) is an exceedingly rare condition consisting of complete intrinsic obstruction of the fetal upper airway that can be located at the level of the larynx, trachea, or bronchi.¹ As a result of the obstruction, the fetal lungs are completely expanded secondary to retention of bronchial secretions leading to tracheobronchial dilatation and pulmonary developmental impairment.

CHAOS, laryngeal atresia, tracheal obstruction

The incidence of CHAOS is unknown. Only a few cases have been reported in the literature.^{2, 3}  

The pathogenetic mechanism implicated for CHAOS includes intrinsic changes that prevented the normal development of the tracheobronchial tree during embryogenesis.

In addition, other conditions can induce an extrinsic obstruction of the tracheobronchial tree, such as lymphatic congenital malformations, cervical teratoma, or vascular rings, such as double aortic arch or aberrant right subclavian artery.

The histopathologic findings of the fetal airways include the presence of webs, cysts, atresia, stenosis, or agenesis. The obstructed flow of fetal lung fluid leads to intrapulmonary changes including alveolar hyperplasia and accumulation of fluid in the alveoli.⁴

Isolated CHAOS is a sporadic fetal malformation with low risk of chromosomal anomalies⁴ but can be present within a syndrome. A significantly high proportion of cases can be associated with Fraser syndrome, which is an autosomal recessive disease with 25% of recurrence risk characterized by laryngeal atresia secondary to underlying fusion of the false vocal cords, cleft lip/palate, microphthalmia, syndactyly, ear anomalies, ambiguous genitalia, congenital heart disease, and severe oligohydramnios secondary to bilateral renal agenesis.^{5, 6}

CHAOS is characterized by retention of bronchial and alveolar secretions resulting in elevated intratracheal pressure and a significant increment in the amount of fluid within the lungs and tracheobronchial tree leading to severely increased lung volume and tracheal dilatation. The diagnosis is usually made on examination of the fetal thorax, which is typically characterized by hyperexpanded and hyperechoic lungs, flattened diaphragms, and dilated airways below the level of the obstruction.⁷ Secondary to the exceedingly high intrathoracic pressure and the degree of pulmonary expansion, the heart is squeezed into the middle of the fetal mediastinum between the lungs and shows a reduction of the cardiac angle. The sagittal and coronal views of the fetal thorax reveals two pathognomonic signs: flattening or inversion of the diaphragmatic convexity and greatly dilated airways filled with bronchial secretions with an anechoic appearance, which allows observation of the entire tracheobronchial tree as a bronchogram.

Differentiation between laryngeal and tracheal atresia is not feasible on fetal echography because both anomalies have the same imaging findings. In contrast, the unilateral findings of CHAOS highly suggest the diagnosis of bronchial atresia. The lung lesion that has to be considered in the differential diagnosis of CHAOS is the microcystic solid form of CCAM.⁸ In contrast to laryngeal or tracheal congenital obstruction, CCAMs are generally unilateral lesions that are very rarely associated with diaphragm flattening and never with dilated airways. However, congenital bronchial atresia could be easily confused with a large CCAM. Preliminary evidence have shown that bronchial obstruction can be present even in the absence of US dilated bronchi making the diagnosis of bronchial atresia misleading to microcystic type III CCAM.⁹ 

The dilated trachea and bronchi are better displayed with a coronal view of the fetal chest. The diameter can be measured in a coronal or sagittal view and compared with normal reference ranges available in the literature.^{10, 11} In addition, absence of flow in the trachea during fetal breathing or swallowing can be shown using color Doppler ultrasound. Extrapulmonary signs include nonimmune hydrops fetalis and placentomegaly secondary to cardiac failure or impeding venous return, or both, and polyhydramnios secondary to esophageal compression.^{2, 3} 

CHAOS is usually a lethal abnormality, especially in the presence of Fraser syndrome. In the absence of additional congenital malformations, the prognosis depends on the lung size and the presence of hydrops. In the absence of hydrops, some cases might be salvageable with proper perinatal management.⁴

Prenatal diagnosis is clinically relevant in parental counseling and decision making regarding fetal surveillance, planning of fetal intervention, method of delivery, and postnatal management.^{12, 13} When the diagnosis of CHAOS is made, fetal surveillance includes a weekly evaluation of lung sizes, amniotic fluid, and US signs of hydrops. Evaluation of cardiac function parameters such as Doppler of the ductus venosus and tricuspid regurgitation should also be performed to identify the early occurrence of hydrops.

The only available postnatal treatment option to improve survival of fetuses with CHAOS is ex utero intrapartum treatment (EXIT) with tracheal intubation and safe airway control.^18-20 If an endotracheal tube cannot be passed, a tracheostomy could be a reasonable option. Nonetheless, even with the combination of tracheostomy and EXIT, only a few survivors have been reported.^18-20 Small case series assessing the consequences of CHAOS have reported respiratory distress syndrome and capillary leak syndrome during the neonatal period and tracheobronchomalacia and diaphragmatic paralysis (which may be due to chronic diaphragmatic stretch) during the 1st year of life.^{4, 19} Although these reported complications are generally reversible, long-term mechanical ventilation, laryngeal reconstruction by a laryngotracheoplasty procedure, language skills delay, and suboptimal neurodevelopmental outcome have been described late in childhood.^{4, 15, 19} 

1. Hedrick MH, Ferro MM, Filly RA, Flake AW, Harrison MR and Adzick NS. Congenital high airway obstruction syndrome (CHAOS): a potential for perinatal intervention. J Pediatr Surg 1994; 29: 271-274.

2. Gilboa Y, Achiron R, Katorza E and Bronshtein M. Early sonographic diagnosis of congenital high-airway obstruction syndrome. Ultrasound Obstet Gynecol 2009; 33: 731-733.

3. Vidaeff AC, Szmuk P, Mastrobattista JM, Rowe TF and Ghelber O. More or less CHAOS: case report and literature review suggesting the existence of a distinct subtype of congenital high airway obstruction syndrome. Ultrasound Obstet Gynecol 2007; 30: 114-117.

4. Lim FY, Crombleholme TM, Hedrick HL, Flake AW, Johnson MP, Howell LJ and Adzick NS. Congenital high airway obstruction syndrome: natural history and management. J Pediatr Surg 2003; 38: 940-945.

5. Schauer GM, Dunn LK, Godmilow L, Eagle RC, Jr. and Knisely AS. Prenatal diagnosis of Fraser syndrome at 18.5 weeks gestation, with autopsy findings at 19 weeks. Am J Med Genet 1990; 37: 583-591.

6. Berg C, Geipel A, Germer U, Pertersen-Hansen A, Koch-Dorfler M and Gembruch U. Prenatal detection of Fraser syndrome without cryptophthalmos: case report and review of the literature. Ultrasound Obstet Gynecol 2001; 18: 76-80.

7. Onderoglu L, Saygan Karamursel B, Bulun A, Kale G and Tuncbilek E. Prenatal diagnosis of laryngeal atresia. Prenat Diagn 2003; 23: 277-280.

8. Balci S, Altinok G, Ozaltin F, Aktas D, Niron EA and Onol B. Laryngeal atresia presenting as fetal ascites, olygohydramnios and lung appearance mimicking cystic adenomatoid malformation in a 25-week-old fetus with Fraser syndrome. Prenat Diagn 1999; 19: 856-858.

9. Cruz-Martinez R, Mendez A, Perez-Garcilita O, Monroy A, Aguilar-Vidales K, Cruz-Martinez MA and Martinez-Morales C. Fetal bronchoscopy as a useful procedure in a case with prenatal diagnosis of congenital microcystic adenomatoid malformation. Fetal Diagn Ther 2015; 37: 75-80.

10. Kalache KD, Franz M, Chaoui R and Bollmann R. Ultrasound measurements of the diameter of the fetal trachea, larynx and pharynx throughout gestation applicability to prenatal diagnosis of obstructive anomalies of the upper respiratory-digestive tract. Prenat Diagn 1999; 19: 211-218.

11. Richards DS and Farah LA. Sonographic visualization of the fetal upper airway. Ultrasound Obstet Gynecol 1994; 4: 21-23.

12. Watson WJ, Thorp JM, Jr., Miller RC, Chescheir NC, Katz VL and Seeds JW. Prenatal diagnosis of laryngeal atresia. Am J Obstet Gynecol 1990; 163: 1456-1457.

13. Richards DS, Yancey MK, Duff P and Stieg FH. The perinatal management of severe laryngeal stenosis. Obstet Gynecol 1992; 80: 537-540.

14. Kohl T, Van de Vondel P, Stressig R, Wartenberg HC, Heep A, Keiner S, Muller A, Franz A, Frohlich S, Willinek W and Gembruch U. Percutaneous fetoscopic laser decompression of congenital high airway obstruction syndrome (CHAOS) from laryngeal atresia via a single trocar--current technical constraints and potential solutions for future interventions. Fetal Diagn Ther 2009; 25: 67-71.

15. Paek BW, Callen PW, Kitterman J, Feldstein VA, Farrell J, Harrison MR and Albanese CT. Successful fetal intervention for congenital high airway obstruction syndrome. Fetal Diagn Ther 2002; 17: 272-276.

16. Martinez JM, Castanon M, Gomez O, Prat J, Eixarch E, Bennasar M, Puerto B and Gratacos E. Evaluation of fetal vocal cords to select candidates for successful fetoscopic treatment of congenital high airway obstruction syndrome: preliminary case series. Fetal Diagn Ther 2013; 34: 77-84.

17. Martinez JM, Prat J, Gomez O, Crispi F, Bennasar M, Puerto B, Castanon M and Gratacos E. Decompression through tracheobronchial endoscopy of bronchial atresia presenting as massive pulmonary tumor: a new indication for fetoscopic surgery. Fetal Diagn Ther 2013; 33: 69-74.

18. DeCou JM, Jones DC, Jacobs HD and Touloukian RJ. Successful ex utero intrapartum treatment (EXIT) procedure for congenital high airway obstruction syndrome (CHAOS) owing to laryngeal atresia. J Pediatr Surg 1998; 33: 1563-1565.

19. Crombleholme TM, Sylvester K, Flake AW and Adzick NS. Salvage of a fetus with congenital high airway obstruction syndrome by ex utero intrapartum treatment (EXIT) procedure. Fetal Diagn Ther 2000; 15: 280-282.

20. Bui TH, Grunewald C, Frenckner B, Kuylenstierna R, Dahlgren G, Edner A, Granstrom L and Sellden H. Successful EXIT (ex utero intrapartum treatment) procedure in a fetus diagnosed prenatally with congenital high-airway obstruction syndrome due to laryngeal atresia. Eur J Pediatr Surg 2000; 10: 328-333.