A full-term male neonate with perinatal depression presents with a palpable left neck mass at 17 days of age with ultrasonographic imaging shown in Fig 1.

The infant was encephalopathic and met the criteria for therapeutic hypothermia, so he was transferred to another NICU. Cooling was initiated 6 hours after birth and continued for 72 hours. Empiric treatment with ampicillin and cefotaxime was initiated for suspected sepsis, given the initial presentation, and continued for 7 days; the blood culture was sterile. The infant remained intubated for 3 days. His postnatal course was complicated by intermittent hypoglycemia (unknown etiology) and persistent thrombocytopenia (thought related to myelosuppression), which eventually resolved without intervention before discharge.

The infant’s physical examination findings at 17 days of age were notable for hypertonicity of his left upper and lower extremity, which resolved with passive movement, improved Moro response, and appropriate rooting response. At that time, he...

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