A 25-year-old man sustains a blow to the orbit from a squash ball. He develops diplopia on upward gaze, enophthalmos, and infraorbital paraesthesia. CT orbit shows a blowout fracture of the orbital floor with herniation of inferior rectus muscle and periorbital fat into the maxillary sinus. What is the mechanism of the diplopia?
- A Direct contusion of the oculomotor nerve (CN III)
- B Haemorrhage within the inferior rectus muscle causing myositis
- C Entrapment or restriction of the inferior rectus muscle within the fracture ✓
- D Sympathetic ganglion disruption causing superior oblique palsy
Explanation
In orbital floor blowout fractures, the sudden pressure rise within the orbit from the blunt impact force transmits to the thin orbital floor (lamina papyracea), which fractures, allowing herniation of orbital fat and the inferior rectus muscle into the maxillary sinus. Entrapment of the inferior rectus (or its fascial attachments) prevents elevation (upward gaze) and sometimes depression, producing diplopia on upward gaze — a 'tethering' restriction. The forced duction test is positive (inability to passively elevate the eye). Infraorbital paraesthesia results from injury to the infraorbital nerve traversing the orbital floor. CN III contusion would cause multiple muscle palsies and pupil involvement. Sympathetic ganglion disruption causes Horner syndrome.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.