Purpose or Case Report: A 33-year-old presented for fetal MRI at 22 weeks 2 days gestation following pre-natal ultrasound that demonstrated findings suspicious for congenital high airway obstruction syndrome (CHAOS). Findings were characteristic of CHAOS and included focal narrowing of the cervical trachea, pulmonary hyperexpansion, tracheobronchomegaly, everted hemi-diaphragms, and abdominal free fluid. However, follow-up MRI at 27 weeks 3 days gestation showed decreased pulmonary hyper-expansion and abdominal free fluid. Diagnosis of atypical congenital high airway obstruction (aCHAOS) was suggested. Further improvement was noted on fetal MR at 31 weeks 2 days gestation and the diagnosis of aCHAOS was reinforced.

CHAOS represents a rare, life-threatening fetal airway anomaly characterized by complete or incomplete obstruction of the upper airway. Prenatal imaging findings of classic CHAOS have been well described and include hyperexpanded lungs with everted diaphragms, tracheobronchomegaly, and hydrops. Our experience at quaternary referral centers is that this applies to a particular subset of patients with complete laryngotracheal obstruction without a pathway of fluid egress, and that there is a spectrum of more subtle but equally critical large airway obstructions. This can be in the form of either incomplete large airway obstruction or complete large airway obstruction with a decompression route such as coexisting tracheoesophageal fistula.

Accurate characterization by fetal imaging has increased in importance as treatment options have expanded, including fetal airway management and ex utero intrapartum treatment (EXIT). There is a paucity of literature regarding the characterization of CHAOS variants and their natural history. However, improved understanding is important as it is unknown whether outcomes between classic CHAOS and aCHAOS may differ. Findings on prenatal imaging of aCHAOS are more subtle, and the diagnosis requires nuance and a high index of suspicion. Close follow up is required in fetal life, as a lack of proper preparation risks near-certain neonatal death. Diagnoses that mimic aCHAOS, such as bilateral congenital lung malformation, should be considered and excluded. We present the current case along with companion cases to expand the spectrum of prenatal imaging appearances in congenital large airway obstruction.
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