Rapidly Progressive Orbital Mucopyocele Mimicking Pre-septal Cellulitis: A Diagnostic Challenge With Sight-Threatening Implications

We present the case of a male patient in his 60s referred by his GP with mild, painless swelling of the left eyelid, initially suggestive of pre-septal cellulitis. Within 24 hours, the condition progressed rapidly to acute proptosis with marked lower eyelid ectropion. Examination revealed complete ophthalmoplegia with limitation in all axes of extraocular movement. CT imaging demonstrated a left frontal-anterior ethmoid mucopyocele with erosion of the frontal and ethmoidal walls, extending into the extraconal space of the left orbit and abutting the medial rectus at its scleral insertion. The patient underwent urgent endoscopic sinus surgery, including left frontal sinus trephination and lateral canthotomy for infection drainage. Rapid symptomatic improvement followed. Microbiological analysis confirmed <i>Haemophilus influenzae</i> as the causative organism. The patient was commenced on outpatient intravenous antibiotics, with ophthalmology follow-up arranged for lower eyelid repair. This case highlights the potential for mucopyocele to mimic pre-septal cellulitis and the risks of delayed imaging, which may lead to serious, vision-threatening complications.