Frontal pneumosinus dilatans as a cause of delayed post-traumatic proptosis in a patient with chronic facial nerve palsy

Pneumosinus dilatans (PSD) is a rare expansile disorder characterized by enlargement of an aerated paranasal sinus beyond its normal anatomic boundaries while preserving bony integrity. We report a 26-year-old man with progressive unilateral proptosis presenting 18 years after severe craniofacial trauma complicated by permanent sixth and seventh cranial nerve palsies. Examination demonstrated 5 mm axial left proptosis with paralytic lagophthalmos, lower eyelid retraction, and exposure keratopathy. Computed tomography revealed marked left frontal sinus enlargement (62.7% volumetric increase) with intact bony walls, consistent with frontal PSD. This case demonstrates the importance of objective exophthalmometry and orbital imaging in distinguishing true space-occupying sinus-related proptosis from smaller anterior globe displacement and periocular changes associated with facial nerve palsy alone. Management consisted of orbital decompression combined with upper and lower eyelid rehabilitation. Hertel asymmetry improved from 5 mm to 1 mm with satisfactory functional and cosmetic outcomes. The case emphasizes that careful imaging is essential when evaluating globe prominence in the setting of facial asymmetry.