Horner Syndrome Secondary to Rapid Enlargement of a Mediastinal Lymphatic Malformation.

Horner syndrome is a clinical triad of ptosis, miosis, and anhidrosis, which commonly prompts urgent neuroimaging. The differential diagnosis of new-onset Horner syndrome in children includes neurological emergencies. We report here a case of a 13-month-old male infant with acute-onset Horner syndrome, subsequently found to have a large mediastinal lymphatic malformation compressing the trachea and neck vasculature.