The orbital septum is connective tissue from the periosteum of the bony rim that inserts into the tarsal plate, which forms the connective tissue structures of the eyelid. The septum serves as a barrier to the spread of infection, preventing microbes anterior to the septum from spreading posteriorly. Infections posterior to the septum are called orbital cellulitis (or postseptal cellulitis) and those anterior to the septum are called periorbital cellulitis (or preseptal cellulitis).

The most typical presentation of orbital cellulitis is lid swelling. Patients may complain of eye pain or visual disturbances, including blurry vision. The physical examination may be significant for proptosis, poor lid closure, conjunctival injection, restricted ocular movements (ophthalmoplegia), altered pupillary reactivity, pain on movement of the eye, or decreased visual acuity. There may be associated erythema to the bony rim of the orbit where the septum originates. Younger children are less likely to present with ophthalmoplegia or proptosis. The presence of fever, systemic signs, and toxicity is variable.

Orbital cellulitis is secondary to sinusitis in >90% of cases. The ethmoid sinuses are the most commonly involved sinuses, but frontal sinus disease may be present in adolescents. The pathogenesis of orbital cellulitis usually involves the development of subperiosteal abscess and spread of infection from the involved paranasal sinus. Organisms commonly found in orbital cellulitis reflect those found in sinus disease, including Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pyogenes, Staphylococcus aureus, both alpha- and nonhemolytic streptococci,
and anaerobes. Many patients, particularly those older than 9 years, have polymicrobial infections.