Hamad Hejazi ¹, David Farrell ²

1. Newcastle upon Tyne Hospitals NHS Foundation Trust, England, UK
2. Southern Health and Social Care Trust, Northern Ireland, UK

Introduction

The orbit is a small, crowded space holding the globe, optic nerve, extraocular muscles, fat and vessels, bordered by the paranasal sinuses and the anterior cranial fossa. Orbital problems such as a swollen red eye, proptosis, double vision or facial trauma are often first seen by non-ophthalmologists in the emergency department or on the wards. Choosing the right scan, and requesting it well, speeds diagnosis and avoids unnecessary radiation. This is a guide aimed at junior doctors to help select the appropriate modality based on suspected pathology.

The three modalities

• CT: fast and available around the clock. Best for bone (fractures, decompression planning), the paranasal sinuses, calcification, foreign bodies and acute abscess. It uses ionising radiation, which matters for the lens and for children. IV contrast is added when infection is suspected.
• MRI: superior soft-tissue contrast for the optic nerve, extraocular muscles, orbital apex, cavernous sinus and brain. There is no ionising radiation, but scans are slower, less available out of hours, and children may need sedation. Screen for implants and any possible metallic intraocular foreign body before scanning.
• Ultrasound: B-scan assesses the globe (retinal detachment, vitreous haemorrhage, intraocular foreign body) and is usually performed by ophthalmology. It cannot assess the orbital apex or intracranial structures. Never press on an eye that may be ruptured.

Key points

• Choose the scan by the clinical question: CT for bone, sinuses, acute infection and trauma; MRI for soft tissue, the optic nerve and intracranial extension; ultrasound for the globe.
• In suspected orbital cellulitis, imaging must never delay antibiotics or ophthalmic review.
• A good request states the question, the key findings (vision, pupils, eye movements), the urgency and the safety information.
• Reduced vision, a relative afferent pupillary defect (RAPD), reduced colour vision, restricted or painful eye movements and neurological signs are red flags.

Choosing by presentation

Presentation	First-line imaging	Consider adding	Key point
Suspected orbital cellulitis	Contrast CT orbits and sinuses	MRI if intracranial extension or cavernous sinus thrombosis is suspected	Start IV antibiotics and call ophthalmology and ENT; do not wait for the scan
Thyroid eye disease (atypical or sight-threatening)	CT or MRI orbits	MRI to assess disease activity	Most cases are diagnosed clinically; image when unilateral, rapidly progressive or optic neuropathy is suspected
Orbital trauma	Thin-slice CT orbits	CT head if indicated	Suspected open globe: shield, no pressure, urgent ophthalmology
Unexplained visual loss or optic neuropathy	MRI orbits and brain with contrast	CT if bone or calcification is the question	Discuss with ophthalmology first

Orbital cellulitis

This is a clinical diagnosis. Image when there are red flags, when vision is affected, or when the patient fails to improve on treatment. CT identifies subperiosteal and orbital abscesses and the sinus source. The Chandler classification grades the spectrum from pre-septal cellulitis to cavernous sinus thrombosis (1,2).

Thyroid eye disease

CT typically shows enlarged muscle bellies that spare the tendons, with crowding at the orbital apex. Dysthyroid optic neuropathy (reduced vision or colour vision, RAPD, disc swelling) is sight-threatening and needs urgent specialist review (3).

Orbital trauma

Look for diplopia with restricted upgaze or downgaze, enophthalmos and infraorbital numbness. In children, a trapdoor (‘white-eyed’) fracture can trap muscle and cause pain, nausea, vomiting and bradycardia. This is an emergency requiring prompt surgical review.

Writing a good request

1. The question: for example ‘?subperiosteal abscess’ rather than ‘swollen eye’.
2. The findings: visual acuity, colour vision, pupils and RAPD, eye movements, proptosis and any systemic upset.
3. The urgency, and who to contact with the result.
4. Safety information: renal function and contrast allergy, pregnancy, MRI screening including any metal in or around the eye, and whether the patient can lie flat and stay still.

Red flags: escalate immediately

Reduced visual acuity or colour vision, RAPD, ophthalmoplegia or pain on eye movement, marked proptosis, optic disc swelling, severe headache, drowsiness, fever with neurological signs, or suspected globe rupture. Do not delay treatment or ophthalmic review while waiting for imaging.

Conclusion

The right orbital scan depends on the question being asked. Knowing what each modality does best, requesting it with the key clinical details, and recognising the red flags that cannot wait will make you a safer and more effective first responder for orbital emergencies.

References

1. Chandler JR, Langenbrunner DJ, Stevens ER. The pathogenesis of orbital complications in acute sinusitis. Laryngoscope. 1970;80(9):1414-28.
2. Tsirouki T, Dastiridou AI, Ibanez Flores N, et al. Orbital cellulitis. Surv Ophthalmol. 2018;63(4):534-53.
3. Bartalena L, Kahaly GJ, Baldeschi L, et al. The 2021 European Group on Graves’ Orbitopathy (EUGOGO) clinical practice guidelines for the medical management of Graves’ orbitopathy. Eur J Endocrinol. 2021;185(4):G43-67.