Isolated Horner Syndrome as the Sole Presentation of Pediatric Parapharyngeal Rhabdomyosarcoma.

Isolated Horner syndrome (HS) in young children warrants thorough investigation for occult malignancy, despite its rarity as a presenting sign. This report details a unique case of a 30 month-old male presenting with sudden-onset, isolated left HS as the sole manifestation of rhabdomyosarcoma (RMS) in parapharyngeal space. Contrast-enhanced magnetic resonance imaging (MRI) revealed a 2.5 × 3.5 × 5 cm lesion within the left poststyloid parapharyngeal space, displacing the carotid artery anteriorly, and the internal jugular vein posteriorly. Gross total resection via a transcervical approach was performed; intraoperatively, the tumor involved the cervical sympathetic trunk, necessitating its sacrifice, resulting in persistent postoperative HS. Histopathological and immunohistochemical analysis (positive CK, PLAP, CD99) confirmed a diagnosis of embryonal RMS. The child received adjuvant chemoradiotherapy. One-year follow-up with positron emission tomography - computed tomography (PET-CT) showed no evidence of recurrence. This case underscores several critical points: (1) Isolated HS in young children necessitates urgent neuroimaging to exclude malignancy, even in the absence of palpable masses or neurological deficits; (2) The parapharyngeal space, though an exceptionally rare primary site for RMS (constituting a minor subset of head and neck cases), carries significant implications due to its proximity to the sympathetic chain, readily causing HS via mass effect; (3) MRI is pivotal for diagnosis and surgical planning in this anatomically complex region; (4) Multimodal therapy is essential for local control in embryonal RMS; (5) Sacrifice of the sympathetic trunk during resection likely leads to permanent HS, a factor for preoperative counseling. Vigilant long-term follow-up remains crucial.