Orbit and Lacrimal System
This section covers the orbit and lacrimal system: proptosis, orbital infection, thyroid eye disease, and tearing disorders. The exam tests urgent differentiation and imaging choices.
Core rules and concepts
- Proptosis is measured with a Hertel exophthalmometer; axial imaging with CT or MRI defines the compartment and extent.
- Orbital cellulitis causes proptosis, painful ophthalmoplegia, and fever; it is an emergency requiring intravenous antibiotics and sinus evaluation.
- Preseptal cellulitis is limited to the lids, without proptosis or motility restriction; it can often be treated orally in mild cases.
- Thyroid eye disease causes lid retraction, proptosis, restrictive myopathy, and optic neuropathy; urgent orbital decompression or steroids are used for compressive optic neuropathy.
- Nasolacrimal duct obstruction causes epiphora and dacryocystitis; dacryocystorhinostomy bypasses the obstructed duct.
Worked example
A 7-year-old has fever, eyelid swelling, proptosis, and painful eye movement after a week of sinusitis. CT shows ethmoid opacification and a subperiosteal orbital collection. The diagnosis is orbital cellulitis with abscess. Treatment is admission, intravenous antibiotics covering sinus pathogens, and urgent ENT review. Drainage is considered if the abscess is large or vision is threatened. The pupil and colour vision are monitored because optic neuropathy can occur.
Traps that cost marks
Calling all lid swelling preseptal cellulitis misses proptosis and ophthalmoplegia. Do not prescribe oral antibiotics for orbital cellulitis with visual signs. In thyroid eye disease, smoking worsens prognosis and should be addressed. Probing is not the first step for adult epiphora without assessing the lacrimal outflow system.