Orbital cellulitis  is inflammation of eye tissues behind the orbital septum. It is most commonly caused by an acute spread of infection into the eye socket from either the adjacent sinuses or through the blood. It may also occur after trauma. When it affects the rear of the eye, it is known as retro-orbital cellulitis. It should not be confused with periorbital cellulitis, which refers to cellulitis anterior to the septum. Without proper treatment, orbital cellulitis may lead to serious consequences, including permanent loss of vision or even death. Signs and symptoms
Orbital cellulitis commonly presents with painful eye movement, sudden vision loss, chemosis, bulging of the infected eye, and limited eye movement. Along with these symptoms, patients typically have redness and swelling of the eyelid, pain, discharge, inability to open the eye, occasional fever and lethargy. Complications
Complications include hearing loss, blood infection, meningitis, cavernous sinus thrombosis, cerebral abscess, and blindness. It is possible that children experience more severe complications due to their immature immune system and because they have thinner orbital bones, which makes the infection easier to spread. Causes
Orbital cellulitis occurs commonly from bacterial infection spread via the paranasal sinuses, usually from a previous sinus infection. Other ways in which orbital cellulitis may occur are from blood stream infections or from eyelid skin infections. Upper respiratory infection, sinus infection, trauma to the eye, ocular or periocular infection, and systemic infection all increase ones risk of orbital cellulitis.
Staphylococcus aureus, Haemophilus influenzae B, Moraxella catarrhalis, Streptococcus pneumoniae, and beta-hemolytic streptococci are bacteria that can be responsible for orbital cellulitis. Staphylococcus aureus is a gram-positive bacterium, which is the most common cause of staphylococcal infections. Staphylococcus aureus infection can spread from the skin to the orbit. This organism is able to produce toxins which promotes its virulence, leading to the inflammatory response seen in orbital cellulitis. Staphylococcus infections are identified by a cluster arrangement on gram stain. Staphylococcus aureus forms large yellow colonies when cultured (which is distinct from other Staph infections such as Staphylococcus epidermidis, which forms white colonies). Streptococcus pneumoniae is also a gram-positive bacterium responsible for orbital cellulitis due to its ability to infect the sinuses. Streptococcal bacteria can invade surrounding tissues, causing the inflammatory response seen in orbital cellulitis (similar to Staphyloccoccus aureus). Streptococcal infections are identified on culture by their formation of pairs or chains. Streptococcus pneumoniae produce green (alpha) hemolysis, or partial reduction of red blood cell hemoglobin. Risk Factors
Risk factors for the development of orbital cellulitis include, but are not limited to:
Recent upper respiratory illness
Sinus infection
Younger age
Retained foreign bodies within the orbit
Trauma
Immunosuppression
Systemic infection
Dental infection

Diagnosis
Early diagnosis of orbital cellulitis is urgent, and it involves a complete and thorough physical examination. Common presenting signs include: a protruding eye (proptosis), eyelid edema (swelling), eye pain, vision loss, inability to move the eye completely (ophthalmoplegia), and fever.