What differentiates periorbital cellulitis from orbital cellulitis?

• A. Periorbital cellulitis is posterior to the orbital septum, while orbital cellulitis is anterior to it.
• B. Periorbital cellulitis is anterior to the orbital septum, while orbital cellulitis is posterior to it.
• C. Both lie behind the orbital septum.
• D. Only orbital cellulitis affects the eye.

Answer: (B)

The key idea here is anatomical location relative to the orbital septum. The orbital septum acts as a barrier between the eyelid tissues and the contents of the orbit. Inflammation that is in front of this septum—the eyelids and surrounding periorbital soft tissue—is called periorbital or preseptal cellulitis. It involves the outer structures and typically does not affect the eye itself, so vision is usually preserved and there is no restricted eye movement.

In contrast, inflammation behind the orbital septum involves the orbital contents—fat and extraocular muscles inside the orbit. This orbital cellulitis can cause painful eye movements, limited or painful extraocular movements, proptosis, and potential vision changes, and it carries a higher risk of serious complications requiring urgent intravenous antibiotics and imaging.

So the distinguishing feature is anterior versus posterior to the orbital septum: periorbital cellulitis is anterior, while orbital cellulitis is posterior. The other statements either reverse this relationship or misstate who is involved.