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Congenital high airway obstruction syndrome (CHAOS) is a condition diagnosed in the fetus and characterized by several abnormalities due to the airway being completely obstructed. The obstruction may be due to a cyst or web in the larynx (the voice box in the upper part of airway, below the throat) or to atresia (complete blockage) of the larynx or of the trachea (the continuation of the airway below the larynx and before dividing into the bronchial trees within the lungs).
Regardless of the cause, the results are massive enlargement of the lungs and dilation of the bronchial trees, a flattened or inverted diaphragm (the partition between the chest and abdominal areas), a large amount of fluid in the abdomen (ascites), and other signs of in utero heart failure (nonimmune hydrops or extensive swelling), all due to obstruction of the fetal airway.
Most fetuses diagnosed with CHAOS associated with hydrops do not survive, unless there is spontaneous perforation of the atresia that is causing the airway obstruction. This spontaneous perforation may occur in up to one third of cases and halt the progression in hydrops.
CHAOS is detected by ultrasound. The sonogram produced by ultrasound can show the effects of obstruction of the upper airway. Fluid normally produced by the lungs is trapped by the obstruction and accumulates in the trachea and the bronchial trees, which appear distended. The lungs also become distended and appear extremely large. The diaphragm, which usually curves upward, may be inverted and curves downward. The heart may appear elongated and the chambers of the heart small and compressed. The fetus may also exhibit abnormal or jerky breathing as the fetus tries to breath against the blocked airway.
Magnetic Resonance Imaging (MRI) is often useful in determining the extent of the obstruction and ruling out other conditions.
In addition to the sonogram, a fetus diagnosed with CHAOS should have fetal echocardiography because of the possibility of structural heart disease. The fetus should be followed closely for early signs of hydrops. A karyotype analysis (the study of the chromosomes of the cells) should also be obtained to rule out serious genetic abnormalities that might preclude fetal intervention. Most cases of CHAOS occur as sporadic, isolated malformations without a known risk of recurrence. In cases of CHAOS associated with a condition known as Fraser syndrome, there is a 25% chance of recurrence. Consultations with a medical geneticist and with a pediatric surgeon are advised.
Delivery should be planned in a tertiary care center experienced with the EXIT (ex utero intrapartum treatment) procedure.
The EXIT procedure allows the head and chest of the fetus to be delivered through an incision in the mother’s uterus, but the fetus to remain attached by the umbilical cord. This preserves the circulation between the mother and the fetus and allows stable exchange for up to 60 to 90 minutes while the airway is secured. The procedure is performed under deep anesthesia, which allows the uterus to relax and preserves placental function.
During the EXIT procedure, a lighted instrument known as an endoscope is used to evaluate the larynx (laryngoscopy). An endoscope may also be used to evaluate the bronchial tubes (bronchoscopy) and to relieve airway obstruction due to a simple cyst or web in the larynx. If a simple web or cyst is not the cause of the obstruction, surgery should be done to create an opening in the trachea (tracheostomy).Once the airway is secured, the cord umbilical cord is clamped and the infant is handed over to the neonatologist. After delivery of the infant and placenta, vigorous massage of the uterus stimulates contraction and minimizes blood loss in the mother.
The first successful delivery and long-term survival of a fetus with CHAOS due to complete tracheal atresia was reported in 2000. The EXIT procedure was performed at 31 weeks gestation and a tracheostomy performed. The newborn did well, requiring ventilation because of dysfunction of the diaphragm. The child is developing normally at 10 years of age.
Without medical intervention, the expected outcomes for fetuses diagnosed with CHAOS can be divided into thirds.
One third of the fetuses will develop progressive nonimmune hydrops and die in utero.
One third of the fetuses will have spontaneous perforation through the tracheal or laryngeal atresia, or less commonly, experience decompression through a fistula (abnormal passageway) in the area of the trachea and esophagus (the tube that branches off the trachea and carries food to the stomach).
One third will have stable hydrops, tolerating it reasonably well until 30 to 32 weeks of gestation, when preterm labor or fetal distress will likely develop. At that point, an EXIT procedure can be performed.
A fetus diagnosed in the third trimester with CHAOS but without associated anomalies or hydrops likely has incomplete airway obstruction and will do well until delivery by the EXIT procedure. Laryngoscopy, bronchoscopy and tracheostomy will be required immediately before delivery, so the fetus needs to be managed by the EXIT procedure to secure the airway while being maintained on placental support.
There is little information regarding the long-term outcome for children affected with CHAOS. In cases of simple laryngeal cyst or web, normal laryngeal function can be anticipated. The outcome in cases of more significant laryngeal malformation is less certain regarding voice and the ability to have the tracheostomy closed. Because infants with CHAOS have not survived until recently, our information on long-term outcome is limited.
In select cases, obstruction of the larynx or trachea can be treated through a fetoscope, a lighted instrument for examining the fetus in utero. Surgery in a 25-week gestation fetus demonstrated the feasibility of treating CHAOS in utero. In a similar case, a laryngoscope was used to pass a guideline across tracheal atresia in a fetus, converting a complete airway obstruction to a partial airway obstruction. In both instances, an EXIT procedure would still be necessary, but resolving the hydrops reduces the medical interventions needed when premature infants are delivered and makes it easier to manage them medically.
If fetal surgery is performed, a bronchoscopy should be performed first, since the obstruction may be due to a simple laryngeal web or cyst, which can be popped with the bronchoscope. For more significant obstruction, laser perforation of the laryngeal or tracheal atresia may be done. Fetal tracheostomy may need to be performed if the CHAOS is due to thick plaque formed by cartilage blocking the airway.
There are still many unanswered questions about the long-term effects of in utero tracheostomy on lung development. While tracheostomy reversed the hydrops in a fetal lamb model, evidence suggests that in some cases, hydrops can be tolerated for prolonged periods. If close monitoring is available, intervention can be deferred until after 30 weeks of gestation, when an EXIT procedure can be considered with acceptable risks of premature birth.
If the EXIT procedure is performed, the infant should have an airway in place before being handed over to the neonatologist. A geneticist should examine the baby for possible anomalies or syndromes that can be associated with CHAOS. If a karyotype was not obtained during pregnancy, blood for the analysis should be taken at birth.
Depending on physical findings, the physician may recommend x-rays of the spine, sonogram of the genitourinary region, and an echocardiograph to evaluate the structure and function of the heart. The possibility of an opening between trachea and esophagus must be evaluated and resolved in all patients with CHAOS.
Careful fluid management is required for newborns with ascites (abnormal accumulation of fluid) and capillary leak syndrome (a condition in which fluid and proteins leak out of tiny blood vessels and flow into surrounding tissues). The tracheal tissue may be softened because of chronic obstruction and prone to collapse (tracheomalacia) and need pressure ventilation to remain open. The massive size of the lungs in utero may have stretched and damaged the diaphragm, but progressive improvement can be anticipated during the first five months of life. The infant can be weaned from mechanical ventilation as soon as function of the diaphragm is restored and reconstruction of the trachea can be considered at 6 to 12 months of age.
The most important aspect of the care of the newborn diagnosed with CHAOS is securing and maintaining the airway. The initial evaluation should be directed toward diagnosing the level of obstruction to allow planning of definitive surgical repair. It is important to understand that, depending on the nature of the malformation, perfect laryngeal reconstruction and adequate speech may not always be possible.
Because of the unusual nature of CHAOS, these newborns are best managed at centers experienced in EXIT procedures and with the neonatal care of infants with CHAOS and the complex airway reconstructive surgery that will be required.
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