What key symptom distinguishes orbital cellulitis from preseptal cellulitis?

The key symptom that distinguishes orbital cellulitis from preseptal cellulitis is decreased visual acuity. Orbital cellulitis occurs when the infection spreads to the fat and muscles around the eye, potentially involving the optic nerve and posing a serious risk to vision. This condition can lead to complications such as vision loss, which is often reflected as decreased visual acuity.

In contrast, preseptal cellulitis is an infection of the eyelid and surrounding tissues anterior to the orbital septum, typically not affecting the eye structures deeply enough to cause visual impairment. Patients with preseptal cellulitis may experience eyelid swelling and redness, but they usually maintain normal visual acuity and extraocular movements.

Thus, the presence of decreased visual acuity in a patient raises concern for orbital involvement and indicates a more serious infection requiring urgent intervention.