The progression of disease in orbital cellulitis can be rapid and is best described by the Chandler classification system of orbital cellulitis related to sinusitis (Table 22–2).10 In stage 1, the patient presents with inflammatory edema and with findings similar to periorbital cellulitis, such as erythema, warmth, induration, and tenderness over the periorbital tissues. During stage 2, the patient develops true orbital cellulitis with chemosis (edema of the bulbar conjunctiva), proptosis, pain with eye movement, decreased extraocular movements, and decreased visual acuity. It is during this stage that there is extension of disease posterior to the orbital septum. In stage 3, subperiosteal abscess formation occurs, usually between the orbit and the periosteum of the sinuses. In this stage, an increase in extraocular pressure can occur, which may worsen proptosis and vision changes. Complete loss of extraocular movements or ophthalmoplegia is associated with stage 4: development of an orbital abscess, which is a true abscess of the orbital contents. Patients may complain of complete vision loss, and a marked proptosis is noted on examination. Finally in stage 5, the most serious complications occur, including cavernous sinus thrombosis, which may present as headache, bilateral periorbital edema, cranial neuropathies, proptosis, and ophthalmoplegia with extension into the central nervous system.1,10,11 Extension of disease into the central nervous system may also include the development of meningitis or formation of abscesses in the brain and epidural or subdural spaces.