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cellulitis, orbital and periorbital
These terms refer to bacterial infection of the tissue layers under the skin (cellulitis) around the eye - periorbital cellulitis - and sometimes in the deeper structures around and behind the eyeball - orbital cellulitis.
The terminology of orbital and periorbital has been clarified by the more specific reclassification of these infections into pre-septal and post-septal categories, based on where around the eye they occur. The septum is a seal that circles the globe of the eye, separating the mucous membranes of the lids and the schlera (white covering of the eyeball) from the deeper attachments of the eye muscles. The orbital septum is a watershed divide that determines which way the veins drain - directly back to the body circulation through facial veins or into the brain area (intracranial cavity). Because of anatomic factors discussed below, cellulitis of the eye region is referred to as either pre-septal or post-septal.
So-called preseptal (or periorbital - "around the eye") cellulitis is a bacterial inflammation of the tissues around around the eye and eyelid. This infection produces marked swelling and tender redness around the eye, but there are no signs of limitation of movement of the eye. Preseptal or periorbital cellulitis is much less serious than its postseptal counterpart, and is much more common.
True orbital cellulitis (post-septal, behind the septum) is a very dangerous infection because germs have direct access to the brain through the veins that drain the eye region. Entry of bacteria into the brain and spinal fluid system would result in dangerous bacterial meningitis. Ophthalmoplegia - limitation of the movements of the eye - is the frightening hallmark of this infection.
Because of the danger of developing into widespread blood-born or central nervous system infections, infections around the eye are often treated intravenously in the hospital. True orbital cellulitis requires very aggressive intravenous antibiotic therapy, and generally speaking, only the milder periorbital (preseptal) infections are treated orally.
The spectrum of causative germs for orbital and preseptal cellulitis has changed radically in recent years. Orbital cellulitis infections used to be classically caused by Hemophilus influenza type B, as a complication of a sinus infection with the Hemophilus bacterium, whereas preseptal or periorbital cellulitis was classically a strep or staph infection as a complication of a bug bite. Hemophilus infection has disappeared with the H. influenza vaccine (HiB). Now the predominant causative germs for both pre- and post-septal cellulitis are Streptococcus and Staphylococcus species.